Earth
Science
Glossary
absolute humidity
The mass of water vapor in a given volume of air.
It represents the density of water vapor in the air.
A-horizon
The uppermost soil horizon (topsoil) made up of a
mixture of organic material (up to 30% humus) and minerals. The
A-horizon becomes coarser as you move downward due to the water
washing through the soil and carrying away the fine particles.
absolute vorticity
See vorticity
absolute zero
A temperature reading of -273° C, -460° F, or 0 K.
Theoretically, there is no molecular motion at this temperature.
absolutely stable air
An atmospheric condition that exists when the
environmental lapse rate is less than the moist adiabatic rate. This
results in a lifted parcel of air being warmer than the air around
it.
absorption
Absorption of light refers to the removal of
photons from a light stream. The energy of the photon is absorbed by
the molecule and converted to heat.
absorption coefficient
A measure of the amount of radiant energy,
incident normal to a planar surface, that is absorbed per unit
distance or unit mass of a substance.
abyssal plain
Flat areas of the ocean floor lying at a depth of
3000 to 6000 metres. The abyssal plain has a slope of less than
1:1000 and is generally covered in a thick layer of sediment.
accreted (planet)
The process where small particles and gases in the
solar nebula came together to form larger bodies, eventually of
planetary size.
accretion
The growth of a precipitation particle by the
collision of an ice crystal or snowflake with a supercooled liquid
droplet that freezes upon impact.
accumulation (glacial)
All processes, which include snowfall,
condensation, avalanching, snow transport by wind, and freezing of
liquid water, that add snow or ice to a glacier, floating ice, or
snow cover. The term also includes the amount of snow or other solid
precipitation added to a glacier or snowfield by these processes.
acid deposition
The depositing of acidic particles (usually
sulfuric acid and nitric acid) at the earth's surface. Acid
deposition occurs in dry form (dry deposition) or wet form (wet
deposition). Acid rain and acid precipitation often denote wet
deposition. (See Acid rain.)
acid fog
See acid rain.
acid rain
Cloud droplets or raindrops combining with gaseous
pollutants, such as oxides of sulfur and nitrogen, to make falling
rain (or snow) acidic - pH less than 5.6. If fog droplets combine
with such pollutants it becomes acid fog. It is caused by emissions
of sulphur dioxide and nitrogen oxides. Although natural sources of
sulphur oxides and nitrogen oxides do exist, more than 90% of the
sulphur and 95% of the nitrogen emissions occurring in eastern North
America are of human origin. These primary air pollutants arise from
the use of coal in the production of electricity, from base-metal
smelting, and from fuel combustion in vehicles. Once released into
the atmosphere, they can be converted chemically into such secondary
pollutants as nitric acid and sulfuric acid, both of which dissolve
easily in water. The resulting acidic water droplets can be carried
long distances by prevailing winds, returning to Earth as acid rain,
snow, or fog.
acoustic thermography
The use of sound waves to measure seawater
temperature.
actual vapor pressure
See vapor pressure.
adiabatic process
A process that takes place without a transfer of
heat between the system (such as an air parcel) and its
surroundings. In an adiabatic process, compression always results in
warming, and expansion results in cooling. It is a thermodynamic
change of state of a system such that no heat or mass is transferred
across the boundaries of the system.
advection
The predominately horizontal large-scale movement
of air that causes changes in temperature or other physical
properties. In oceanography, advection is the horizontal or vertical
flow of seawater as a current.
advection fog
Occurs when warm, moist air moves over a cold
surface and the air cools to below its dew point.
aerosol
In atmospheric science, tiny suspended solid
particles (dust, smoke, etc.) or liquid droplets that enter the
atmosphere from either natural or human (anthropogenic) sources,
such as the burning of fossil fuels. In general it is a suspension
in which the dispersion medium is a gas (usually air) and the
dispersed (colloidal) phase are liquid or solid particles (e.g.
mist, haze, smoke).The particulate material, other than water or
ice, in the atmosphere ranges in size from approximately 10-3 to
larger than 102 ?m in radius. Aerosols are important in the
atmosphere as nuclei for the condensation of water droplets and ice
crystals, as participants in various chemical cycles, and as
absorbers and scatterers of solar radiation, thereby influencing the
radiation budget of the earth-atmosphere system, which in turn
influences the climate on the surface of the Earth.
aerovane
A wind instrument that indicates or records both
wind speed and wind direction.
AFOS
(Meteorology) Acronym for Automation of Field
Operations and Services, an electronic-computerized system that
displays weather information on TV-type consoles.
agglomeration
(Meteorology) The process by which precipitation
particles grow larger by collision or contact with cloud particles
or other precipitation particles.
air density
See density.
air glow
A faint glow of light emitted by excited gases in
the upper atmosphere. Air glow is much fainter than the aurora.
air mass
A large body of air that has similar horizontal
temperature and moisture characteristics.An air mass, by definition,
is a large dome of air which has similar horizontal temperature and
moisture characteristics. Often, a front separates two different air
masses. Fronts are very narrow zones of transition. In other words,
temperatures can change dramatically with short horizontal distances
near fronts. Fronts are usually anywhere from 10 kilometers to
hundreds of kilometers wide, while air masses can be thousands of
kilometers wide.
air mass weather
A persistent type of weather that may last for
several days (up to a week or more). It occurs when an area comes
under the influence of a particular air mass.
air parcel
See parcel of air.
air pressure
or atmospheric pressure. Air pressure is the force
exerted on a surface by the weight of the air above it. The
internationally recognized unit for measuring this pressure is the
kilopascal. In the U.S.A. usually expressed in millibars (mb) or
inches of mercury (Hg).
airborne fraction
The portion of CO2 released from all
energy consumption and land use activities that remains in the
atmosphere as opposed to the amounts absorbed by plants and oceans.
How the world's total carbon is partitioned among the oceanic,
terrestrial, and atmospheric pools is determined by complex
biogeochemical and climatological interactions.
airborne particulates
Total suspended matter found in the atmosphere as
solid pieces or liquid droplets. Airborne particulates include
windblown dust, emissions from industrial processes, smoke from the
burning of wood and coal, and the exhaust of motor vehicles. See
aerosols.
air-mass thunderstorm
A thunderstorm produced by local convection within
an unstable air mass.
Aitken nuclei
See condensation nuclei.
albedo
The fraction of the total solar radiation incident
on a body that is reflected by it.
Aleutian low
The subpolar low-pressure area that is centered
near the Aleutian Islands on charts that show mean sea level
pressure.
algae
Any photosynthetic member of the kingdom Protista.
A member of the plant divisions Rhodophyta (red algae), Chlorophyta
(green Algae), or Phaeophyta (brown algae).
alluvial fan
The mass of sediment deposited in a cone shape
where a stream widens or there is a sharp decrease in gradient.
alluvium
The sediment (sand, mud etc.) left by flowing
water.
altimeter
An instrument that indicates the altitude of an
object above a fixed level. Pressure altimeters use an aneroid
barometer with a scale graduated in altitude instead of pressure.
altocumulus
A middle cloud, usually white or gray. Often
occurs in layers or patches with wavy, rounded masses or rolls.
altocumulus castellanus
An altocumulus cloud showing vertical development.
Individual cloud elements have tower-like tops, often in the shape
of tiny castles.
altostratus
A middle cloud composed of gray or bluish sheets
or layers of uniform appearance. In the thinner regions,the sun or
moon usually appears dimly visible.
amniotic
possessing an amnion. The amnion is a membrane
lining the sac that encloses the embryos of reptiles, birds and
mammals. The amniotic sac contains amniotic fluid.
amphibole
A common dark rock-forming silicate mineral
present in many igneous and metamorphic rocks. Examples are
hornblende and anthophyllite (asbestos).
analogue method of forecasting
A forecast made by comparison of past large-scale
synoptic weather patterns that resemble a given (usually current)
situation in its essential characteristics.
analysis (weather)
The drawing and interpretation of the patterns of
various weather elements on a surface or upper-air chart.
andesite
A dark-colored fine-grained rock of volcanic
origin composed mainly of plagioclase feldspar (andesine) and one or
more of pyroxene, hornblende or biotite. Andesite is named after the
Andes mountains in South America.
anemometer
An instrument to measure wind speed. Wind
directions are always reported as the direction winds are coming
from a southerly wind pushes air from the south to the north.
aneroid barometer
An instrument designed to measure atmospheric
pressure. It contains no liquid.
angular momentum
The product of an object's mass, speed, and radial
distance of rotation.
animal adaptation
Animal Adaptation - Animals camouflage. Through
millions of years of evolution, animals have camouflaged themselves
by adapting to the look and feel of their environment. One such
adaptation is protective coloration where species use their
adaptations to take on the color, shade and even texture of their
immediate surroundings to camouflage or conceal themselves from
predators and prey.
annual range of temperature
The difference between the warmest and coldest
months at any given location.
anthropogenic
Man made. Usually used in the context of emissions
that are produced as the result of human activities.
anticline
A fold in rock that resembles an arch (generally
convex upwards). The rocks in the core of an anticline are generally
the oldest.
anticyclone
(high-pressure area) An atmospheric high-pressure
closed circulation with clockwise rotation in the Northern
Hemisphere, counterclockwise in the Southern Hemisphere, and
undefined at the Equator.
anticyclonic rotation
Rotation in the opposite sense as the Earth's
rotation, i.e., clockwise in the Northern Hemisphere as would be
seen from above. The opposite of cyclonic rotation.
aphanitic
The texture of an igneous rock is said to be
aphanitic when the crystalline components are not distinguishable to
the naked eye. See phaneritic.
apparent pole wandering
The apparent movement of the magnetic poles with
respect to the continents.
aquifer
A permeable zone below the Earth’s surface which
stores groundwater and allows it to move easily.
Archaean
This is the earlier part of Precambrian time.
Arctic haze
A persistent winter diffuse layer in the Arctic
atmosphere whose origin may be related to long-range transport of
midlatitude continental man-made pollutants.
arcus cloud
See roll cloud.
arid climate
An extremely dry climate - drier than the
semi-arid climate. Often referred to as a "true desert" climate.
arroyos
A small deep flat-floored channel or gully formed
by an intermittent stream, usually with vertical or steeply cut
banks, that is usually dry. The term "arroyos" derives from the
Spanish for "stream, brook; gutter, watercourse of a street" and is
applied in arid and semi-arid regions of the South-Western US.
ASOS
(Meteorology) Acronym for Automated Surface
Observing Systems. A system designed to provide continuous
information of wind, temperature, pressure cloud base height, and
runway visibility of selected airports.
asthenosphere
The upper mantle zone directly below the
lithosphere from about 70-200 km. The material in this zone is
thought to be soft and yielding to plastic flow. Magmas can be
generated here.
atmosphere
The envelope of gases that surround a planet and
are held to it by the planet's gravitational attraction. The Earth's
atmosphere is mainly nitrogen and oxygen and is subdivided into four
sections: the troposphere- from the earth's surface to an altitude
of about 10 km; the stratosphere from 10 km to 50 km; the mesosphere
from 50 km to 80 km; and the thermosphere- beyond 80 km.
atmosphere
The gaseous mass or envelope surrounding a
celestial body, especially the one surrounding the earth, and
retained by the celestial body's gravitational field. The air or
climate in a specific place.
Also a standard unit of pressure equal to the air pressure at sea
level. It equals the amount of pressure that will support a column
of mercury 760 millimeters high at 0 degrees Celsius under standard
gravity, or 14.7 pounds per square inch [1 atmosphere] (atm.) =
1.013 bar = 1.013 x 105 Pa = 1.013 x 105 N m-2
= 14.7 lbs in-2.
atmospheric greenhouse effect
The warming of an atmosphere by its absorbing and
reemitting infrared radiation while allowing shortwave radiation to
pass on through. The gases mainly responsible for the earth's
atmospheric greenhouse effect are water vapor and carbon dioxide.
Also called the greenhouse effect.
atmospheric models
Simulation of the atmosphere's behavior by
mathematical equations or by physical models.
atmospheric stagnation
A condition of light winds and poor vertical
mixing that can lead to a high concentration of pollutants. Air
stagnations are most often associated with fair weather, an
inversion, and the sinking air of a high-pressure area.
atmospheric turbulence
A state of the flow of air in which apparently
random irregularities occur in the air's instantaneous velocities,
often producing major deformations of the flow.
atmospheric window
The wavelength range between 8 and 11 p.m. in
which little absorption of infrared radiation takes place.
attenuation
Any process in which the rate of flow of a beam of
energy decreases (mainly due to absorption or scattering) with
increasing distance from the energy source.
aurora
Glowing light display in the nighttime sky caused
by excited gases in the upper atmosphere giving off light. In the
Northern Hemisphere it is called the aurora borealis (northern
lights); in the Southern Hemisphere,the aurora australis (southern
lights).
aurora borealis
Also known as the northern lights The luminous,
radiant emission from the upper atmosphere over middle and high
latitudes, and centred around the earth's magnetic poles. These
silent fireworks are often seen on clear winter nights in a variety
of shapes and colours.
automated weather station
An unmanned station with various sensors that
measure weather elements such as temperature/wind/pressure and
transmit these readings for use by meteorologists.
autumnal equinox
The equinox at which the sun approaches the
Southern Hemisphere and passes directly over the equator. Occurs
around September 23.
AWIPS
(Meteorology) Acronym for Advanced Weather
Interactive Processing System. New computerized system that
integrates and processes date received at a Weather Forecasting
Office from NEXRAD, ASOS; and analysis and guidance products
prepared by NMC.
B-horizon
The soil horizon below the A-horizon where mineral
and organic material is deposited that has been leached from the
A-horizon. The B-horizon is also known as the "zone of accumulation"
and is often rich in clay and red-brown in color due to the presence
of limonite and hematite.
back-door cold front
A cold front moving south or southwest along the
Atlantic seaboard of the United States.
backing winds
Winds which shift in a counterclockwise direction
with time at a given location (e.g. from southerly to
southeasterly), or change direction in a counterclockwise sense with
height (e.g. westerly at the surface but becoming more southerly
aloft). The opposite of veering winds.
ball lightning
A rare form of lightning that may consist of a
reddish, luminous ball of electricity or charged air.
banner cloud
A cloud extending downward from an isolated
mountain peak, often on an otherwise cloud-free day.
baroclinic (atmosphere)
The state of the atmosphere where surfaces of
constant pressure intersect surfaces of constant density. On an
isobaric chart, isotherms cross the contour lines, and temperature
advection exists.
baroclinic instability
A type of instability arising from a meridional
(north to south) temperature gradient, a strong vertical wind speed
shear, temperature advection, and divergence in the flow aloft. Many
mid-latitude cyclones develop as a result of this instability.
baroclinic model
A model of atmospheric circulation that, in
contrast with barotropic models, does not constrain
constant-pressure surfaces to coincide with constant-density
surfaces.
baroclinic zone
A region in which a temperature gradient exists on
a constant pressure surface. Baroclinic zones are favored areas for
strengthening and weakening systems.
barograph
A recording barometer.
barometer
An instrument used to measure air pressure. The
international standard of measurement is the kilopascal although
millibars and inches of mercury are also commonly used.
barotropic (atmosphere)
A condition in the atmosphere where surfaces of
constant destiny parallel surfaces of constant pressure.
basalt
A fine-grained igneous rock composed of
calcium-rich plagioclase feldspar and pyroxene, with lesser amounts
of olivine and amphibole. Basalt is the most common type of volcanic
rock in the earth's crust, and makes up the upper layers of the
oceanic crust.
bathymetry
The science of measuring ocean depths to determine
the topography of the sea floor.
Beaufort scale
A scale assigned to wind force.
beds
(Geology) Layering of sheet-like units, the
smallest distinguishable division within the classification of
layered sedimentary rocks.
Bergeron process
See Ice crystal process.
Bermuda high
See Subtropical high.
billow clouds
Broad, nearly parallel lines of clouds oriented at
right angles to the wind.
bimetallic thermometer
A temperature-measuring device usually consisting
of two dissimilar metals that expand and contract differentially as
the temperature changes.
biogeochemical cycle
The chemical interactions among the atmosphere,
biosphere, hydrosphere, and lithosphere.
biological productivity
The amount of organic matter, carbon, or energy
content that is accumulated during a given time period.
biomass
The total dry organic matter or stored energy
content of living organisms that is present at a specific time in a
defined unit (community, ecosystem, crop, etc.) of the Earth's
surface.
biosphere
The portion of Earth and its atmosphere that can
support life. The part (reservoir) of the global carbon cycle that
includes living organisms (plants and animals) and life- derived
organic matter (litter, detritus). The terrestrial biosphere
includes the living biota (plants and animals) and the litter and
soil organic matter on land, and the marine biosphere includes the
biota and detritus in the oceans. All area occupied by living
organisms includes areas of the lithosphere, hydrosphere and
atmosphere.
biota
The animal and plant (fauna and flora) life of a
given area.
biotite
A widely distributed and important rock-forming
mineral of the mica group that is generally dark in color. Found in
rocks such as granite and diorite.
bitumen
Dark, naturally occurring solid or semisolid
substances composed mainly of a mixture of hydrocarbons with little
oxygen, nitrogen, or sulfur.
black body
A hypothetical object that absorbs all of the
radiation that strikes it. It also emits radiation at a maximum rate
for its given temperature.
blizzard
A severe storm lasting four or more hours. It is
characterized by low temperatures, strong winds and poor visibility
due to blowing snow. True blizzard conditions are most common on the
prairies of Canada and the United States. Blizzards are a rare
occurrence on the west coast and in Atlantic Canada. The Environment
Canada has listed the following general criteria for issuing
blizzard warnings in Canada: visibility less than 1 kilometer, wind
speeds greater than 40 kilometers per hour, high windchill values.
block faulting
A type of normal faulting in which the crust is
divided into structural or fault blocks of different elevations and
orientations.
boiling point
The temperature a substance changes from the
liquid state to a gaseous state.
bolide
An exploding or exploded meteor or meteorite; a
detonating fireball.
Boulder winds
Fast-flowing, local downslope winds that may
attain speeds of 100 knots or more. They are especially strong along
the eastern foothills of the Rocky Mountains near Boulder, Colorado.
boundary current
Surface currents found on the eastern and western
margins of the Atlantic and Pacific Oceans.
boundary layer
(General) a layer adjacent to a bounding surface.
(Meteorology) Specifically, the term most often refers to the
planetary boundary layer, which is the layer within which the
effects of friction are significant. For the earth, this layer is
considered to be roughly the lowest one or two kilometers of the
atmosphere. It is within this layer that temperatures are most
strongly affected by daytime insolation and nighttime radiational
cooling, and winds are affected by friction with the earth's
surface. The effects of friction die out gradually with height, so
the "top" of this layer cannot be defined exactly. There is a thin
layer immediately above the earth's surface known as the surface
boundary layer (or simply the surface layer). This layer is only a
part of the planetary boundary layer, and represents the layer
within which friction effects are more or less constant throughout
(as opposed to decreasing with height, as they do above it). The
surface boundary layer is roughly 10 meters thick, but again the
exact depth is indeterminate. Like friction, the effects of
insolation and radiational cooling are strongest within this layer.
Brocken bow
A bright ring of light seen around the shadow of
an observer's head as the observer peers into a cloud or fog bank.
Formed by diffraction of light.
buffer factor (Revelle factor)
The ratio of the instantaneous fractional change
in the partial pressure of CO2 (pCO2) exerted
by seawater to the fractional change in total CO2
dissolved in the ocean waters. The buffer factor relates the partial
pressure of CO2 in the ocean to the total ocean CO2
concentration at constant temperature, alkalinity and salinity. The
Revelle factor is a useful parameter for examining the distribution
of CO2 between the atmosphere and the ocean, and measures
in part the amount of CO2 that can be dissolved in the
mixed surface layer.
Buys-ballot's law
A law describing the
relationship between the wind direction and the pressure
distribution. In the Northern Hemisphere, if you stand with your
back to the surface wind, then turn clockwise about 30o, lower
pressure will be to your left. In the Southern Hemisphere, stand
with your back to the surface wind, then turn counterclockwise about
30 degrees, lower pressure will be to your right.
C-horizon
The soil layer beneath the B-horizon. The
C-horizon is composed of only partially weathered material and for
the most part retains its original appearance. It contains little
organic material and grades downward into fresh, unweathered
bedrock.
calcareous
Said of a substance that contains calcium
carbonate. When applied to a rock name it implies that as much as
50% of the rock is calcium carbonate.
calcite
A common rock-forming mineral, CaCO3.
It is usually white, colorless or light shades of gray, yellow and
blue. Calcite is a principal constituent of limestone.
California current
A strong, dry, northerly wind that blows in late
spring, summer, and early fall in northern and central California.
Its warmth and dryness are due to downslope compressional heating.
Cambrian
This is the earliest period of the Paleozoic era
(590-458 million years ago).
cap cloud
See Pileus cloud.
carbon cycle
The balance of the exchanges (incomes and losses)
of carbon between the carbon reservoirs or between one specific loop
(e.g., atmosphere-biosphere) of the carbon cycle. An examination of
the carbon budget of a pool or reservoir can provide information
about whether the pool or reservoir is functioning as a source or
sink for CO2. All parts (reservoirs) and fluxes of carbon; usually
thought of as a series of the four main reservoirs of carbon
interconnected by pathways of exchange. The four reservoirs, regions
of the Earth in which carbon behaves in a systematic manner, are the
atmosphere, terrestrial biosphere (usually includes fresh water
systems), oceans, and sediments (includes fossil fuels). Each of
these global reservoirs may be subdivided into smaller pools ranging
in size from individual communities or ecosystems to the total of
all living organisms (biota). Carbon exchanges from reservoir to
reservoir by various chemical, physical, geological, and biological
processes.
carbon dioxide (CO2)
A colorless, odorless gas whose concentration is
about 0.035 percent (350 ppm) in a volume of air near sea level. It
is a selective absorber of infrared radiation and, consequently, it
is important in the earth's atmospheric greenhouse effect. Solid CO2
is called dry ice.
carbon dioxide fertilization
Enhancement of plant growth or of the net primary
production by CO2 enrichment that could occur in natural
or agricultural systems as a result of an increase in the
atmospheric concentration of CO2.
carbon dioxide reference gas
A mixture of a known quantity of CO2-in
air or CO2-in-N2 used to calibrate carbon
dioxide analyzers.
carbon flux
The rate of exchange of carbon between pools
(reservoirs).
carbon isotope ratio
Ratio of carbon-12 (12C) to either of
the other, less common, carbon isotopes, carbon-13 (13C)
or carbon-14 (14C).
carbon pool
The reservoir containing carbon as a principal
element in the geochemical cycle.
carbon sink
A pool (reservoir) that absorbs or takes up
released carbon from another part of the carbon cycle. For example,
if the net exchange between the biosphere and the atmosphere is
toward the atmosphere, the biosphere is the source, and the
atmosphere is the sink.
carbon source
A pool (reservoir) that releases carbon to another
part of the carbon cycle.
carbon-based resources
The recoverable fossil fuel (coal, gas, crude
oils, oil shale, and tar sands) and biomass that can be used in fuel
production and consumption.
carbonaceous
A rock or sediment that is rich in carbon is said
to be carbonaceous.
Carboniferous
The Mississippian and Pennsylvanian periods
combined, ranging from 360-286 million years ago.
Cb
(Meteorology) Symbol for cumulonimbus cloud,
characterized by strong vertical development in the form of
mountains or huge towers topped at least partially by a smooth,
flat, often fibrous anvil shaped top. Also known as a "thunderhead."
ceiling
The height of the lowest layer of clouds when the
weather reports describe the sky as broken or overcast.
ceiling balloon
A small balloon used to determine the height of
the cloud base. The height is computed from the balloon's ascent
rate and the time required for its disappearance into the cloud.
ceilometer
An instrument that automatically records cloud
height.
Celsius scale
A temperature scale where zero is assigned to the
temperature where water freezes and 100 to the temperature where
water boils (at sea level).
Cenozoic
This is an era of geologic time, from the
beginning of the Tertiary period to present. Termed the era of
recent life.
centripetal acceleration
The inward-directed acceleration on a particle
moving in a curved path.
centripetal force
The radial force required to keep an object moving
in a circular path. It is directed toward the center of the curved
path.
chemical cycle
The constant reorganizing of chemicals within our
ecosystem.
Chinook
Chinooks occur when a mountain range is exposed to
a strong winds blowing at right angles, or near right angles to the
direction of the mountain ridge. Moist air is forced up the
mountains bringing both cloud and precipitation to the windward
side. The descending air then becomes warmer and drier as it is
forced down the leeward side of the mountains. The relatively warm,
dry gusty winds that occasionally occur to the leeward side of
mountain ranges around the world are known by many names. In Canada
and the northern United States, they are referred to as Chinooks. In
the southern states, they are known as Santa Ana and in parts of
Europe, foehn winds.
Chinook wall cloud
A bank of clouds over the Rocky Mountains that
signifies the approach of a chinook.
Chinook wind
A warm, dry wind on the eastern side of the Rocky
Mountains. In the Alps, this wind is called a foehn.
chlorinity
The amount of chlorine in water by weight (with
all bromides and oxides converted to chloride).
chlorofluorocarbons
A family of inert nontoxic and easily liquified
chemicals used in refrigeration, air conditioning, packaging, and
insulation or as solvents or aerosol propellants. Because they are
not destroyed in the lower atmosphere, they drift into the upper
atmosphere where their chlorine components destroy ozone.
chloroplasts
The organelle in plant cells where photosynthesis
occurs. Chloroplasts contain chlorophyll a and a variety of other
pigments.
cirrocumulus
A high cloud that appears as a white patch of
clouds without shadows. It consists of very small elements in the
form of grains or ripples.
cirrus
High-level clouds (5,000 metres or more), composed
of ice crystals and appearing in the form of white, delicate
filaments or white or mostly white patches or narrow bands. Cirrus
clouds typically have a fibrous or hair-like appearance, and often
are semi-transparent. Thunderstorm anvils are a form of cirrus
cloud, but most cirrus clouds are not associated with thunderstorms.
clay
A rock or mineral fragment that is smaller than a
very fine silt grain (less than 4 micrometers in diameter). Clay
minerals are a group of fine-grained crystalline hydrous silicates
formed by weathering of minerals such as feldspar, pyroxene or
amphibole.
clear air turbulence (CAT)
Turbulence encountered by aircraft flying through
cloudless skies. Thermals, wind shear, and jet streams can each be a
factor in producing CAT.
clear ice
A layer of ice that appears transparent because of
its homogeneous structure and small number and size of air pockets.
climate
The prevalent or characteristic meteorological
conditions, and their extremes, of any place or region. Climate is
the statistical collection and representation of the weather
conditions for a specified area during a specified time interval,
usually decades, together with a description of the state of the
external system or boundary conditions. The properties that
characterize the climate are thermal (temperatures of the surface
air, water, land, and ice), kinetic (wind and ocean currents,
together with associated vertical motions and the motions of air
masses, aqueous humidity, cloudiness and cloud water content,
groundwater, lake lands, and water content of snow on land and sea
ice), and static (pressure and density of the atmosphere and ocean,
composition of the dry air, salinity of the oceans, and the
geometric boundaries and physical constants of the system). These
properties are interconnected by the various physical processes such
as precipitation, evaporation, infrared radiation, convection,
advection, and turbulence.
climate change
The long-term fluctuations in temperature,
precipitation, wind, and all other aspects of the Earth's climate.
External processes, such as solar-irradiance variations, variations
of the Earth's orbital parameters (eccentricity, precession, and
inclination), lithosphere motions, and volcanic activity, are
factors in climatic variation. Internal variations of the climate
system also produce fluctuations of sufficient magnitude and
variability to explain observed climate change through the feedback
processes interrelating the components of the climate system.
Climate change refers to the change in the average weather in a
region over a long period of time, usually years or longer (decades,
centuries, millenia, etc.).
climate sensitivity
The magnitude of a climatic response to a
perturbing influence. In mathematical modeling of the climate, the
difference between simulations as a function of change in a given
parameter.
climate signal
A statistically significant difference between the
control and disturbed (see climate sensitivity) simulations of a
climate model.
climate system
The five physical components (atmosphere,
hydrosphere, cryosphere, lithosphere, and biosphere) that are
responsible for the climate and its variations.
climate variation
The change in one or more climatic variables over
a specified time.
climatic analog
A past climate situation in which changes similar
to the present occurred. Used in making climatic projections.
climatic anomaly
The deviation of a particular climatic variable
from the mean or normal over a specified time.
climatic controls
The relatively permanent factors that govern the
general nature of the climate of a region.
climatic optimum
The period in history from about 7000 to about
5000 years ago during which surface air temperatures were warmer
than at present in nearly all regions of the world. In the Arctic
region, the temperature rose many degrees, and in temperate regions,
the increase was 1.0 degrees-1.7 degrees C. In this period, glaciers
and ice sheets receded greatly, and the melt-water raised sea level
by about 3 meters.
climatological forecast
A weather forecast, usually a month or more in the
future, which is based upon the climate of a region rather than upon
current weather conditions.
cloud
A visible cluster of tiny water and/or ice
particles in the atmosphere. Clouds may be classified on their
visible appearance, height, or form.
cloud albedo
Reflectivity that varies from less than 10 to more
than 90% of the insolation and depends on drop sizes, liquid water
content, water vapor content, thickness of the cloud, and the sun's
zenith angle. The smaller the drops and the greater the liquid water
content, the greater the cloud albedo, if all other factors are the
same.
cloud feedback
The coupling between cloudiness and surface air
temperature in which a change in surface temperature could lead to a
change in clouds, which could then amplify or diminish the initial
temperature perturbation. For example, an increase in surface air
temperature could increase the evaporation; this in turn might
increase the extent of cloud cover. Increased cloud cover would
reduce the solar radiation reaching the Earth's surface, thereby
lowering the surface temperature. This is an example of negative
feedback and does not include the effects of lonwave radiation or
the advection in the oceans and the atmosphere, which must also be
considered in the overall relationship of the climate system.
cloud formations
This CD has several photos and descriptions of the
various cloud types.
cloud seeding
The introduction of artificial substances (usually
silver iodide or dry ice) into a cloud for the purpose of either
modifying its development or increasing its precipitation.
cloud streets
Lines or rows of cumuliform clouds.
cloudburst
Any sudden and heavy rain shower.
coalescence
The merging of cloud droplets into a single large
droplet.
cohesion
The attraction between molecules of a single
substance.
cold advection
Transport of cold air into a region by horizontal
winds.
cold clouds
See supercooled cloud.
cold fog
See Supercooled cloud.
cold front
The leading edge of a cooler airmass.
cold occlusion
See occluded front.
cold pool
A region of relatively cold air, represented on a
weather map analysis as a relative minimum in temperature surrounded
by closed isotherms. Cold pools aloft represent regions of
relatively low stability, while surface-based cold pools are regions
of relatively stable air.
cold wave
A rapid fall in temperature within 24 hours that
often requires increased protection for agriculture, industry,
commerce, and human activities.
cold-air funnel
A funnel cloud or (rarely) a small, relatively
weak tornado that can develop from a small shower or thunderstorm
when the air aloft is unusually cold (hence the name). They are much
less violent than other types of tornadoes.
comma cloud
A synoptic scale cloud pattern with a
characteristic comma-like shape, often seen on satellite photographs
associated with large and intense low-pressure systems.
computer enhancement
A process where the temperatures of radiating
surfaces are assigned different shades of gray (or different colors)
on an infrared picture. This allows specific features to be more
clearly delineated.
condensation
The physical process by which vapour becomes
liquid or solid; the opposite of evaporation.
condensation nuclei
Tiny particles upon whose surfaces condensation of
water vapor begins in the atmosphere. Small nuclei less than 0.2 m m
in radius are called Aitken nuclei; those with radii between 0.2 and
1 m m are large nuclei, while giant nuclei have radii larger than 1
m m.
conditionally unstable air
An atmospheric condition that exists when the
environmental lapse rate is less than the dry adiabatic rate but
greater than the moist adiabatic rate. Also called conditional
instability.
conduction
The transfer of heat by molecular activity from
one substance to another, or through a substance. Transfer is always
from warmer to colder regions.
conductivity
The ability of a substance to conduct electricity
(electrical conductivity) or heat (thermal conductivity). Electrical
conductivity measures how easily a current flows through a substance
and is the opposite of reciprocal of electrical resistivity.
constant pressure chart
A chart showing variables, such as temperature and
wind, on a constant pressure surface. Variations in height are
usually shown by lines of equal height (contour lines).
constant-height chart
A chart showing variables, such as pressure,
temperature, and wind, at a specific altitude above sea level.
Variation in horizontal pressure is depicted by isobars. The most
common constant-height chart is the surface chart, which is also
called the sea level chart.
contact nucleation (freezing)
The process by which contact with a nucleus such
as an ice crystal causes supercooled liquid droplets to change into
ice.
continental arctic air mass
An air mass characterized by extremely low
temperatures and very dry air.
continental divide
A drainage divide that separates streams flowing
toward opposite sides of a continent, often into different oceans.
continental margin
The submerged area around the edge of a continent
that is geologically part of the continent. The continental margin
usually includes the continental shelf, the continental slope and
the continental rise.
continental polar air mass
An air mass characterized by low temperatures and
dry air. Not as cold as arctic air masses.
continental rise
The gently sloping surface found at the base of
the continental slope. The continental rise marks the transition
between the continental slope and the abyssal plain; it is formed of
sediment deposited from the continental slope.
continental shelf
The submerged margin of the continental land mass.
It extends from the coastline to the shelf break. Water depths over
the continental shelf are usually less than 200 m.
continental slope
Extends from the seaward edge of the continental
shelf (shelf break) down to the continental rise.
continental tropical air mass
An air mass characterized by high temperatures and
low humidity.
contour line
A line that connects points of equal elevation
above a reference level, most often sea level.
contrail (condensation trail)
A cloudlike streamer frequently seen forming
behind aircraft flying in clear, cold, humid air.
convection
Generally, the movement of portions of a fluid due
to density differences arising from heating of the fluid. In
meteorology, the term is used specifically to describe vertical
transport of heat and moisture, especially by updrafts and
downdrafts in an unstable atmosphere. The terms"convection" and
"thunderstorms" often are used interchangeably, although
thunderstorms are only one form of convection. Cbs, towering cumulus
clouds are visible forms of convection. However, convection is not
always made visible by clouds. Convection that occurs without cloud
formation is called dry convection, while the visible convection
processes referred to above are forms of moist convection.
convection current
The movement of a fluid in a closed loop due to
convection.
convective adjustment
A numerical procedure applied in many atmospheric
models to approximate the vertical non-radiative heat transport.
This procedure adjusts the lapse rate whenever necessary so that
some prescribed critical lapse rate is never exceeded.
convective condensation level (ccl)
The level above the surface marking the base of a
cumuliform cloud that is forming due to surface heating and rising
thermals.
convective instability
Instability arising in the atmosphere when a
column of air exhibits warm, moist, nearly saturated air near the
surface and cold, dry air aloft. When the lower part of the layer is
lifted and saturation occurs, it becomes unstable.
convergence
(Meteorology) An atmospheric condition that exists
when the winds cause a horizontal net inflow of air into a specified
region.
(Oceanography) When waters of different
origins come together at a point or along a line (convergence line),
the denser water from one side sinks under the lighter water from
other side. The ocean convergence lines are the polar, subtropical,
tropical, and equatorial. Also see divergence.
convergent
Tending to converge; inclining towards each other
or toward a common point. See convergence.
cooling degree-day
A form of degree-day used in estimating the amount
of energy necessary to reduce the effective temperature of warm air.
A cooling degree-day is a day on which the average temperature is
one degree above a desired base temperature.
core
The central part of the Earth below a depth of
2900 km. It is thought to be composed of iron and nickel and to be
molten on the outside with a central solid inner core.
coriolis effect (coriolis
force)
The deflection of a particle's motion due to the
rotation of the earth. The coriolis effect causes particle
trajectories to be deflected to the right in the Northern Hemisphere
and to the left in the Southern Hemisphere. In synoptic scale
weather systems (hurricanes and large mid-latitude storms), the
coriolis force causes the air to rotate around a low pressure center
in a cyclonic direction. The air flowing around a hurricane spins
counter-clockwise in the northern hemisphere, and clockwise in the
southern hemisphere (as does the earth, itself). In both
hemispheres, this rotation is called cyclonic. If the earth did not
rotate, the air would flow directly in towards the low pressure
center, but on a spinning earth, the coriolis force results in the
are arcing in towards the low pressure center. The coriolis force is
of much too small a magnitude to have any relevance to the direction
of rotation in a sink or toilet.
Corona
(Meteorology) A series of colored rings
concentrically surrounding the disk of the sun or moon. Smaller than
the halo, the corona is caused by the diffraction of light around
small water droplets of uniform size.
country breeze
A light breeze that blows into a city from the
surrounding countryside. It is best observed on clear nights when
the urban heat island is most pronounced.
covalent bond
A relatively strong chemical bond in which an
electron pair is shared by two atoms, simultaneously filling the
outer electron shells of both.
creep
The slow, more or less continuous, downslope
movement of mineral, rock, and soil particles under gravitational
stress.
crepuscular rays
Alternating light and dark bands of light that
appear to fan out from the sun's position, usually at twilight.
Cretaceous
The final period of the Mesozoic era (144-65
million years ago) after the Jurassic and before the Tertiary period
of the Cenozoic era.
crust
The outermost layer or shell of the Earth,
representing less than 1% of the Earth's total volume.
crustal
Having to do with the crust, especially of the
Earth.
crustal deformation
The bending up or down of sedimentary strata due
to pressure originating within the Earth's crust (e.g. mountain
building).
cryosphere
The portion of the climate system consisting of
the world's ice masses and snow deposits, which includes the
continental ice sheets, mountain glaciers, sea ice, surface snow
cover, and lake and river ice. Changes in snow cover on the land
surfaces are by and large seasonal and closely tied to the mechanics
of atmospheric circulation. The glaciers and ice sheets are closely
related to the global hydrologic cycle and to variations of sea
level and change in volume and extent over periods ranging from
hundreds to millions of years.
crystals
A crystal is a homogeneous, solid body of a
chemical element, compound or isomorphous mixture. In a crystal the
atoms are repeated in a regular arrangement and this arrangement may
be outwardly expressed by the plane faces of the crystal.
cumulonimbus
An exceptionally dense and vertically developed
cloud, often with a top in the shape of an anvil. The cloud is
frequently accompanied by heavy showers, lightning, thunder, and
sometimes hail. It is also known as a thunderstorm cloud.
cumulus
A cloud in the form of individual, detached domes
or towers that are usually dense and well defined. It has a flat
base with a bulging upper part that often resembles cauliflower.
Cumulus clouds of fair weather are called cumulus humilis. Those
that exhibit much vertical growth are called cumulus congestus or
towering cumulus.
cumulus stage
The initial stage in the development of an air
mass thunderstorm in which rising, warm, humid air develops into a
cumulus cloud.
curvature effect
In cloud physics, as cloud droplets decrease in
size, they exhibit a greater surface curvature that causes a more
rapid rate of evaporation.
cut-off low
(Meteorology) A cold upper-level low that has
become displaced out of the basic westerly flow and lies to the
south of this flow.
cyclogenesis
(Meteorology) Development or intensification of a
low-pressure center (cyclone).
cyclone
A closed counter-clockwise movement of air-known
as a circulation around a low pressure centre; usually called a low.
Winds blow counterclockwise in the Northern Hemisphere and clockwise
in the Southern Hemisphere. The term is frequently misused to
describe a tornado.
cyclonic circulation
Circulation (or rotation) which is in the same
sense as the Earth's rotation, i.e., counterclockwise (in the
Northern Hemisphere) as would be seen from above. Nearly all storms
and strong or violent tornadoes exhibit cyclonic rotation, but some
smaller vortices, sometimes rotate anticyclonically (clockwise).
Compare with anticyclonic rotation.
daily range of temperature
The difference between the maximum and minimum
temperatures for any given day.
dart leader
The discharge of electrons that proceeds
intermittently toward the ground along the same ionized channel
taken by the initial lightning stroke.
decomposition
The breakdown of matter by bacteria and fungi. It
changes the chemical makeup and physical appearance of materials.
deep water
That part of the ocean below the main thermocline.
deforestation
The removal of forest stands by cutting and
burning to provide land for agricultural purposes, residential or
industrial building sites, roads, etc. or by harvesting the trees
for building materials or fuel. Oxidation of organic matter releases
CO2 to the atmosphere, and regional and global impacts may result.
delta
A body of sediment formed at or near the mouth of
a river where it enters the ocean, or a lake. This is commonly
triangular or fan shaped.
dendrochronology
The dating of past events and variations in the
environment and the climate by studying the annual growth rings of
trees. The approximate age of a temperate forest tree can be
determined by counting the annual growth rings in the lower part of
the trunk. The width of these annual rings is indicative of the
climatic conditions during the period of growth; wide annual rings
signify favorable growing conditions, absence of diseases and pests,
and favorable climatic conditions, while narrow rings indicate
unfavorable growing conditions or climate.
dendroclimatology
The use of tree growth rings as proxy climate
indicators. Tree rings record responses to a wider range of climatic
variables over a larger part of the Earth than any other type of
annually dated proxy record.
density
The ratio of the mass of a substance to the volume
occupied by it. Air density is usually expressed as g/cm3 or kg/m3.
deposition
(Meteorology) A process that occurs in subfreezing
air when water vapor changes directly to ice without becoming a
liquid first.
deposition nuclei
(Meteorology) Tiny particles (ice nuclei) upon
which an ice crystal may grow by the process of deposition.
desert pavement
An arrangement of pebbles and large stones that
remains behind as finer dust and sand particles are blown away by
the wind.
desertification
The progressive destruction or degradation of
vegetative cover especially in arid or semiarid regions bordering
existing deserts. Overgrazing of rangelands, large-scale cutting of
forests and woodlands, drought, and burning of extensive areas all
serve to destroy or degrade the land cover. The climatic impacts of
this destruction include increased albedo leading to decreased
precipitation, which in turn leads to less vegetative cover;
increased atmospheric dust loading could lead to decreased monsoon
rainfall and greater wind erosion and/or atmospheric pollution.
Devonian
A period of the Paleozoic era (408-360 million
years ago). The Devonian follows the Silurian and precedes the
Mississippian era.
dew
As the surface of the earth cools at night, warm
moist air near the ground is chilled and water vapour in the air
condenses into droplets on the grass and other objects. Dew is
particularly heavy on clear nights, when the earth cools rapidly.
When a blanket of cloud insulates the earth, the cooling rate is
slower.
dew cell
An instrument used to determine the dew-point
temperature.
dew point (dew-point temperature)
The temperature to which air must be cooled (at
constant pressure and constant water vapor content) for saturation
to occur.
diatoms
A microscopic, single celled plant having a
silicon impregnated cell wall in two parts. Grows in both fresh and
salt water.
dielectric constant
The dielectric constant of a solvent measures the
ability of a solvent to separate electric charges, and therefore to
dissolve ionic compounds.
diffraction
The bending of light around objects, such as cloud
and fog droplets, producing fringes of light and dark or colored
bands.
dimethylsulphide (DMS)
Volatile sulfur compound excreted by
phytoplankton, which is broken down photochemically, removed by
bacteria, or released into the atmosphere. Its biological function
is unclear.
dinoflagellates
A flagellated, single celled organism, less than
1mm in size, classified as plants or animals since some contain
chlorophyll. Found in two main groups: armored and naked.
diorite
A plutonic (intrusive) igneous rock that is
coarse-grained and composed mainly of plagioclase feldspar, pyroxene
and some amphibole (especially hornblende) and biotite with small
amounts of quartz and K-feldspar.
discharge area
Area where subsurface water, including both ground
water and vadose water is discharged to the land surface, to bodies
of surface water, or to the atmosphere.
dispersion
The separation of white light into its different
component wavelengths.
dissipating stage
The final stage in the development of an air mass
thunderstorm when downdrafts exist throughout the cumulonimbus
cloud.
diurnal
Daily; related to actions which are completed in
the course of a calendar day, and which typically recur every
calendar day (e.g., diurnal temperature rises during the day, and
falls at night).
divergence
(Meteorology) An atmospheric condition that exists
when the winds cause a horizontal net outflow of air from a specific
region. In oceanography, a horizontal flow of water, in different
directions, from a common center or zone; it is often associated
with upwelling. Also see convergence.
divergent
tending to diverge; to move or lie in different
directions from a point; to branch off.
doldrums
The region near the equator that is characterized
by low pressure and light, shifting winds.
dolomite
A common rock-forming mineral, CaMg(CO3)2, that is
white, colorless or tinged yellow, brown, pink or gray. Part of the
magnesium may be replaced by ferrous iron and less frequently by
manganese. Dolomite is found in extensive beds as dolomite rock.
doppler lidar
The use of light beams to determine the velocity
of objects such as dust and falling rain by taking into account the
Doppler shift.
doppler radar
Radar that can measure radial velocity, the
instantaneous component of motion parallel to the radar beam (i.e.,
toward or away from the radar antenna).
doppler shift (effect)
The change in the frequency of waves that occur
when the emitter or the observer is moving toward or away from the
other.
downburst
A severe localized downdraft that can be
experienced beneath a severe thunderstorm. (Compare Microburst and
Macroburst.)
downdraft
A small-scale column of air that rapidly sinks
toward the ground, usually accompanied by precipitation as in a
shower or thunderstorm.
downstream
In the same direction as a stream or other flow,
or toward the direction in which the flow is moving.
downwelling
The process of accumulation and sinking of warm
surface waters along a coastline. A change of air flow of the
atmosphere can result in the sinking or downwelling of warm surface
water. The resulting reduced nutrient supply near the surface
affects the ocean productivity and meteorological conditions of the
coastal regions in the downwelling area.
drainage basin
The total area that contributes water to a single
drainage system.
drizzle
Small water drops between 0.2 and 0.5 mm in
diameter that fall slowly and reduce visibility more than light
rain.
drought
A period of abnormally dry weather sufficiently
long enough to cause serious effects on agriculture and other
activities in the affected area.
dry adiabatic rate
The rate of change of temperature in a rising or
descending unsaturated air parcel. The rate of adiabatic cooling or
warming is about 10oC/1000 m (5.5oF/1000 ft).
dry adiabats
Lines on an adiabatic chart that show the dry
adiabatic rate for rising or descending air. They represent lines of
constant potential temperature.
dry climate
A climate deficient in precipitation where annual
potential evaporation and transpiration exceed precipitation.
dry haze
See Haze.
dry line
A boundary that separates warm, dry air from warm,
moist air. It usually represents a zone of instability along which
thunderstorms form.
dry-bulb temperature
The air temperature measured by the dry-bulb
thermometer of a psychrometer.
dry-summer subtropical climate
A climate characterized by milk, wet winters and
warm to hot, dry summers. Typically located between 30 and 45o
latitude on the western side of continents. Also called
Mediterranean climate.
dust devil (or whirlwind or dust whirl)
A small but rapidly rotating wind made visible by
the dust, sand, and debris it picks up from the surface. It develops
best on clear, dry, hot afternoons.
dust veil index
A quantitative method developed by H. H. Lamb for
comparing the magnitude of volcanic eruptions. The formulae use
observations either of the depletion of the solar beam, temperature
lowering in middle latitudes, or the quantity of solid matter
dispersed as dust. The reference dust veil index is 1000, assigned
to the Krakatoa 1883 eruption, and the index is calculated using all
three methods, where the information is available, for statistical
comparison purposes. Abbreviated D.V.I.
dust whirl
See dust devil.
dynamics
Generally, any forces that produce motion or
affect change. In operational meteorology, dynamics usually refer
specifically to those forces that produce vertical motion in the
atmosphere.
Earth
Our home planet. Earth is the fifth in size of the
9 major planets making up our solar system and is the third (between
Venus and Mars) in order of distance from the sun (about 150 million
km).
easterly wave
A migratory wavelike disturbance in the tropical
easterlies. Easterly waves occasionally intensify into tropical
cyclones.
eccentricity
(Astronomy) The eccentricity is the amount of
deviation of the orbit of a planet from a perfect circle. Planetary
orbits are elliptical and the "eccentricity of the ellipse" is the
ratio of the distance between the center and a focus of an ellipse
to the length of its semi-major axis or e2 = (a2
- b2)/ a2, where e = eccentricity, a =
semi-major axis, and b = semi-minor axis. An ellipse always has an
eccentricity greater than 0 (e = 0 is a circle) and less than 1 (e =
1 is a parabola).
eccentricity (of the Earth's orbit)
The deviation of the earth's orbit from elliptical
to nearly circular.
ecosystem
The interacting system of a biological community
and its nonliving environmental surroundings.
eddy
A small volume of air (or any fluid) that behaves
differently from the larger flow in which it exists In oceanography,
it is a circular movement of water or air that is formed where
currents pass obstructions or between two adjacent currents that are
flowing counter to each other.
eddy viscosity
The internal friction produced by turbulent flow.
Ekman spiral
An idealized description of the way the
wind-driven ocean currents vary with depth. In the atmosphere it
represents the way the winds vary from the surface up through the
friction layer.
El Nino
Spanish for the "Christ Child". Fishermen in Peru
and Ecuador used to use this term to refer to a warming of coastal
waters around Christmas time that occurs most years. It has come to
be used as a term for abnormal warming events that occur, on an
average of two or three times a decade and typically last for a few
seasons. It has important consequences to climate as well as for
ocean states (fishing etc). El Nino is an irregular variation of
ocean current that from January to March flows off the west coast of
South America, carrying warm, low-salinity, nutrient-poor water to
the south. It does not usually extend farther than a few degrees
south of the equator, but occasionally it does penetrate beyond 12
degrees S, displacing the relatively cold Peru Current. The effects
of this phenomenon are generally short-lived, and fishing is only
slightly disrupted. Occasionally (in 1891, 1925, 1941, 1957-58,
1965, 1972-73, 1976, and 1982-83), the effects are major and
prolonged. Under these conditions, sea surface temperatures rise
along the coast of Peru and in the equatorial eastern Pacific Ocean
and may remain high for more than a year, having disastrous effects
on marine life and fishing. Excessive rainfall and flooding occur in
the normally dry coastal area of western tropical South America
during these events. Some oceanographers and meteorologists consider
only the major, prolonged events as El Nino phenomena rather than
the annually occurring weaker and short-lived ones. The name was
originally applied to the latter events because of their occurrence
at Christmas time.
electrical thermometers
Thermometers that use elements that convert energy
from one form to another (transducers). Common electrical
thermometers include the electrical resistance thermometer,
thermocouple, and thermistor.
electromagnetic waves
See Radiant energy.
electronegativity
The power of an atom in a molecule to attract
electrons to itself.
embryo
In cloud physics, a tiny ice crystal that grows in
size and becomes an ice nucleus.
emissions
Materials (gases, particles, vapors, chemical
compounds, etc.) that come out of smokestacks, chimneys, and
tailpipes.
emissivity
The ratio of the radiation emitted by a surface to
that emitted by a black body at the same temperature.
energy
The property of a system that generally enables it
to do work. Some forms of energy are kinetic, radiant, potential,
chemical, electric, and magnetic.
energy balance models
An analytical technique to study the solar
radiation incident on the Earth in which explicit calculations of
atmospheric motions are omitted. In the zero- dimensional models,
only the incoming and outgoing radiation is considered. The outgoing
infrared radiation is a linear function of global mean surface air
temperature, and the reflected solar radiation is dependent on the
surface albedo. The albedo is a step function of the global mean
surface air temperatures, and equilibrium temperatures are computed
for a range of values of the solar constant. The one-dimensional
models have surface air temperature as a function of latitude. At
each latitude, a balance between incoming and outgoing radiation and
horizontal transport of heat is computed. (Abbreviated as EBM.).
energy flow
The movement of energy through the ecosystem.
Energy enters as radiant energy from the sun, is transformed into
kinetic energy, through the movement of the atmosphere and oceans,
or chemical energy through the metabolic activities of life on
earth, and finally exits as heat.
entrainment
The mixing of environmental air into a preexisting
air current or cloud so that the environmental air becomes part of
the current or cloud.
environment
The sum of all external conditions affecting the
life, development, and survival of an organism.
environmental lapse rate
The rate of decrease of air temperature with
elevation. It is most often measured with a radiosonde.
eon
The formal geochronologic unit of highest rank,
next above era. For example, the Phanerozoic Eon encompasses the
Paleozoic, Mesozoic and Cenozoic Eras.
epoch
The formal geochronologic unit lower in rank than
period. For example, the Quaternary Period is divided into the
Holocene Epoch and Pleistocene Epoch.
equilibrium line
The level on a glacier where accumulation equals
ablation and the net balance equals zero.
equilibrium vapor pressure
The necessary vapor pressure around liquid water
that allows the water to remain in equilibrium with its environment.
Also called saturation vapor pressure.
era
The formal geochronologic unit next after eon.
Examples are the Paleozoic Era, the Mesozoic Era and the Cenozoic
Era.
erosion
The general process or group of processes whereby
the material of the Earth's crust are loosened, dissolved, or worn
away, and simultaneously moved from one place to another, by natural
agents.
erratics
A large boulder carried by glacial ice to an area
far removed from its origin.
estuary
Regions of interaction between rivers and
near-shore ocean waters, where tidal action and river flow create a
mixing of fresh and salt water.
euphotic
zone
The layer of a body of water that receives
sufficient sunlight for photosynthesis. The depth of this layer,
which is about 80 m, is determined by the water's extinction
coefficient, the cloudiness, and the sunlight's angle of incidence.
evaporation
The process by which a substance passes from the
liquid state to a gaseous state.
evaporation (mixing) fog
Fog produced when sufficient water vapor is added
to the air by evaporation, and the moist air mixes with relatively
drier air. The two common types are steam fog, which forms when cold
air moves over warm water, and frontal fog, which forms as warm
raindrops evaporate in a cool air mass.
evapotranspiration
Discharge of water from the Earth's surface to the
atmosphere by evaporation from bodies of water, or other surfaces,
and by transpiration from plants.
exosphere
The outermost portion of the atmosphere.
extratropical cyclone
A cyclone storm that most often forms along a
front in middle and high latitudes. Also called a middle latitude
storm, a depression, and a low. It is not a tropical storm or
hurricane.
eye
A region in the center of a hurricane (tropical
storm) where the winds are light and skies are clear to partly
cloudy.
eye wall
A wall of dense thunderstorms that surrounds the
eye of a hurricane.
F scale
See Fujita Scale.
Fahrenheit scale
A temperature scale where 32 is assigned to the
temperature where water freezes and 212 to the temperature where
water boils (at sea level).
fall streaks
Falling ice crystals that evaporate before
reaching the ground.
fall wind
A strong, cold katabatic wind that blows downslope
off snow-covered plateaus.
fata morgana
A complex mirage that is characterized by objects
being distorted in such a way as to appear as castlelike features.
fault
(Geology) A fracture or zone of fractures along
which there has been displacement of the sides relative to one
another parallel to the fracture.
feedback mechanisms
A sequence of interactions in which the final
interaction influences the original one. Also see positive feedback
and negative feedback.
feldspar
A group of abundant rock-forming minerals
consisting of silicates of aluminum and one or more of the metals
potassium (K), sodium (Na) or calcium (Ca). Examples are
K-plagioclase, Ca-plagioclase and Na-plagioclase feldspars.
Feldspars are the most widespread of any mineral group and
constitute 60% of the earth's crust.
fen
A type of wetland that accumulates peat deposits;
they are less acidic than bogs, deriving most of their water from
groundwater rich in calcium and magnesium.
Ferrel cell
The name given to the middle latitude cell in the
3-cell model of the general circulation.
fetch
The distance that the wind travels over open
water, or an area where waves are generated by a wind of constant
direction and speed.
firn
Material that is transitional between snow and
glacier ice. It is formed from snow after passing through one summer
melt season and becomes glacier ice after its permeability to liquid
water falls to zero.
first detection
(Climate) Identification of a precursor signal,
detectable above the noise of natural climatic variability, of a
significant change in a climate parameter and attribution of this
change to an increase in atmospheric carbon dioxide concentration.
The signal may be estimated by numeric modeling of the climate, and
the noise can be estimated using instrumental data. For any modeled
signal that is estimated, the corresponding noise can be estimated
from observational data, and a signal-to-noise ratio can be
calculated to provide a quantitative measure of detectability.
fissure
An open fracture in a rock.
flagellae
A long, whip-like, cell organelle projecting from
the cell surface but enclosed in a cell membrane. Flagellae propel
the cell through a fluid by moving with an undulating motion.
flaring
The burning of waste gases through a flare stack
or other device before releasing them to the air.
flash flood
A flood that rises and falls quite rapidly with
little or no advance warning, usually as the result of intense
rainfall over a relatively small area.
flow law
In glaciology, a constitutive relation for the
analysis of three-dimensional deformation states of ice subjected to
stress.
flurry
Precipitation in the form of snow from a
convective cumulus- type cloud. Flurries are characterized by the
suddenness with which they start and stop, by their rapid changes in
intensity, and usually by rapid changes in the appearance of the
sky.
foehn
See Chinook Wind.
fog
A cloud based at the earth's surface consisting of
tiny water droplets or, under very cold conditions, ice crystals or
ice fog; generally found in calm or low wind conditions. Liquid fog
particles are less than 40 micrometers in diameter and are formed by
condensation of vapor in air. Under foggy conditions, visibility is
reduced to less than one kilometer.
fold
A curve or bend in a structure with a flat
(planar) surface such as rock strata. A fold is usually a result of
deformation, where a layer of rock is subjected to pressure and heat
which bends the rock (rather than breaking it).
forced convection
On a small scale, a form of mechanical stirring
taking place when twisting eddies of air are able to mix hot surface
air with the cooler air above. On a larger scale, it can be induced
by the lifting of warm air along a front (frontal uplift) or along a
topographical barrier (orographic uplift).
forecast
A forecast provides a description of the most
significant weather conditions expected during the current and
following day. The exact content depends upon the intended user,
such as the Public or Marine forecast audiences.
fossil
Any remains, trace or imprint of a plant or animal
that has been preserved in the Earth's crust since some past
geologic or prehistoric time.
fossil fuel
Any hydrocarbon deposit that can be burned for
heat or power, such as petroleum, coal, and natural gas.
fracture
A break in a rock caused by mechanical failure
under stress. Fractures include cracks, joints and faults.
free convection
See convection.
freeze
A condition occurring over a widespread area when
the surface air temperature remains below freezing for sufficient
time to damage certain agricultural crops. A freeze most often
occurs as cold air is advected into a region, causing freezing
conditions to exist in a deep layer of surface air. Also called
advection frost.
freezing nuclei
(Meteorology) Particles that promote the freezing
of supercooled liquid droplets.
freezing point
The temperature at which a substance changes from
a liquid state to a solid state (= melting point).
freezing rain (freezing drizzle)
Rain which freezes on impact to form a coating of
ice upon the ground and on the objects it strikes. This coating is
called a glaze. A freezing rain warning is usually issued when
slippery driving and walking conditions are expected, and/or when
freezing rain may damage trees, power lines, or other structures.
friction layer
The atmospheric layer near the surface usually
extending up to about 1 km (3300 ft) where the wind is influenced by
friction of the earth's surface and objects on it.
front
(Meteorology) A boundary or transition zone
between two air masses of different density, and thus (usually) of
different temperature. A moving front is named according to the
advancing air mass, so a cold front is the leading edge of an
advancing cold air mass, while a warm front is the trailing edge of
a retreatinq cold air mass.
frontal fog
See evaporation fog.
frontal inversion
A temperature inversion encountered upon ascending
through a sloping front, usually a warm front.
frontal thunderstorms
Thunderstorms that form in response to forced
convection (forced lifting) along a front. Most go through a cycle
similar to those of air-mass thunderstorms.
frontal wave
A wave-like deformation along a front in the lower
levels of the atmosphere. Those that develop into storms are termed
unstable waves, while those that do not are called stable waves.
frontogenesis
(Meteorology) The formation, strengthening, or
regeneration of a front.
frontolysis
The weakening or dissipation of a front.
frost (also called hoarfrost)
Water vapour which deposits directly as a solid on
a surface colder than the surrounding air and which has a
temperature below freezing. It is not frozen dew. A "killing frost"
is a frost severe enough to end the growing season.
frost point
The temperature at which the air becomes saturated
with respect to ice when cooled at constant pressure and constant
water vapor content.
frost wedging
The forcing apart of rocks by the expansion of
water as it freezes in fractures and pore spaces.
frostbite
The partial freezing of exposed parts of the body,
causing injury to the skin and sometimes to deeper tissues.
frozen dew
The transformation of liquid dew into tiny beads
of ice when the air temperature drops below freezing.
Fujita scale
A scale developed by T. Theodore Fujita for
classifying tornadoes according to the damage they cause and their
rotational wind speed. A scale of wind damage intensity in which
wind speeds are inferred from an analysis of wind damage:F0 (weak):
40- 72 mph,light damage.F1 (weak): 73-112 mph,moderate damage. F2
(strong):113-157 mph, considerable damage.F3 (strong): 158-206 mph,
severe damage.F4 (violent):207-260 mph, devastating damage.F5
(violent):261-318 mph, (rare) incredible damage.All tornadoes, and
most other severe local windstorms, are assigned a single number
from this scale according to the most intense damage caused by the
storm.
funnel cloud
Simply,a rotating conelike cloud that extends
downward from the base of a thunderstorm. When it reaches the
surface it is called a tornado. It is a cloud extending from the
base of a towering cumulus or Cb, associated with a rotating column
of air that is not in contact with the ground (and hence different
from a tornado). A condensation funnel is a tornado, not a funnel
cloud, if either a) it is in contact with the ground or b) a debris
cloud or dust whirl is visible beneath it.
gabbro
A mafic, coarse-grained, plutonic igneous rock
consisting mainly of Ca-plagioclase feldspar, pyroxene and possibly
olivine, but no quartz. Gabbros form from the slow crystallization
of magma, which is basaltic in composition.
galaxy
A huge assembly of stars (between millions and
hundreds of millions) held together by gravity.
gale
A strong wind. A gale warning is issued for
expected winds of 34 to 47 knots (63-87km/h) over the water.
gas law
The thermodynamic law applied to a perfect gas
that relates the pressure of the gas to its density and absolute
temperature.
general circulation models (GCM)
Hydrodynamic models of the atmosphere on a grid or
spectral resolution that determine the surface pressure and the
vertical distributions of velocity, temperature, density, and water
vapor as functions of time from the mass conservation and
hydrostatic laws, the first law of thermodynamics, Newton's second
law of motion, the equation of state, and the conservation law for
water vapor. Abbreviated as GCM. Atmospheric general circulation
models are abbreviated AGCM, while oceanic general circulation
models are abbreviated OGCM.
general circulation of the atmosphere
Large-scale atmospheric motions over the entire
earth.
geochemical
Relating to the chemistry of rocks. Geochemistry
is the branch of science dealing with the chemistry of the Earth.
geological time scale
An arbitrary chronologic arrangement of geologic
events used as a measure of the relative or absolute duration or age
of any part of geologic time. The time scale is usually presented in
chart form rock-stratigraphic, time-stratigraphic or geologic-time
units, as currently understood.
geologist
A person who is trained in and works in any of the
geological sciences.
geomorphology
The study of present-day landforms, including
their classification, description, nature, origin, development, and
relationships to underlying structures. Also the history of geologic
changes as recorded by these surface features. The term is sometimes
restricted to features produced only by erosion and deposition.
geosphere
The solid mass (lithosphere) of the Earth as
distinct from the atmosphere and hydrosphere or all three of these
layers combined.
geostationary satellite
A satellite that orbits the earth at the same rate
that the earth rotates and thus remains over a fixed place above the
equator.
geostrophic flow
A type of movement where the Coriolis force
balances exactly the horizontal pressure force.
geostrophic wind
A theoretical horizontal wind blowing in a
straight path, parallel to the isobars or contours, at a constant
speed. The geostrophic wind results when the coriolis force exactly
balances the horizontal pressure gradient force.
giant nuclei
(Meteorology) See condensation nuclei.
glacial maximum
The position or time of the greatest advance of a
glacier (e.g., the greatest equatorward advance of Pleistocene
glaciation).
glacial outwash
The sand and gravel washed out by a glacier by
melt water streams and deposited in front of a glacier.
glacial rebound
The isostatic adjustment of previously glaciated
areas after glacial retreat (e.g., the uplift of Scandinavia after
the most recent glaciation).
glaciated cloud
A cloud or portion of a cloud where only ice
crystals exist.
glaciation
The formation, movement and recession of
glaciers and
ice-sheets. Glaciation is a collective term describing all the
geological processes associated with glacial activity.
glacier
A mass of land ice that is formed by the
cumulative recrystallization of material that is transitional
between snow and ice (see Firn). A glacier flows slowly (at present
or in the past) from an accumulation area to an ablation area. Some
well-known glaciers are: the Zermatt, Stechelberg, Grindelwald,
Trient, Les Diablerets, and Rhone in Switzerland; the Nigards,
Gaupne, Fanarak, Lom, and Bover in Norway; the Wright, Taylor, and
Wilson Piedmont glaciers in Antarctica; the Bossons Glacier in
France; the Emmons and Nisqually glaciers on Mt. Ranier, Washington;
Grinnell glacier in Glacier National Park, Montana; the Dinwoody
glacier in the Wind River Mountains and the Teton glacier in Teton
National Park, both in Wyoming; and many glaciers in the Canadian
Rockies.
glacier flow (ice flow)
The slow downward or outward movement of ice in a
glacier caused by gravity.
global scale
The largest scale of atmospheric motion. Also
called the planetary scale.
global warming
An increase in the global mean temperature, which
usually occurs in years or over long periods of time (decades,
centuries, mellenia, etc.).
glory
(Meteorology) Colored rings that appear around the
shadow of an object.
Glossopteris
Late Paleozoic fern-like plants whose seeds were
several millimeters in diameter (too large to have been dispersed
across the ocean by winds). This plant has been found on South
America, South Africa, Australia and India and relatively close to
the South Pole. As a result, Glossopteris has been used as
supporting evidence in the theory of plate tectonics to show that
the continents were once connected.
Gondwana
The Late Paleozoic continent of the Southern
Hemisphere that some geologists believe was derived from the
supercontinent, Pangaea. Gondwana means, "land of the Gonds".
Gondwanaland
A variation of Gondwana introduced by Suess. See
Gondwana.
gradient wind
A theoretical wind that blows parallel to curved
isobars or contours.
granite
A coarse-grained, felsic, plutonic igneous
composed of K-feldspar as well as quartz, plagioclase feldspar, and
in lesser amounts, mica and amphibole.
graupel
Ice particles between 2 and 5 mm in diameter that
form in a cloud often by the process of accretion. Snowflakes that
become rounded pellets due to riming are called graupel or snow
pellets.
green flash
A small green color that occasionally appears on
the upper part of the sun as it rises or sets.
greenhouse effect
A popular term used to describe the roles of water
vapor, carbon dioxide, and other trace gases in keeping the Earth's
surface warmer than it would be otherwise. These "radiatively
active" gases are relatively transparent to incoming shortwave
radiation, but are relatively opaque to outgoing longwave radiation.
The latter radiation, which would otherwise escape to space, is
trapped by these gases within the lower levels of the atmosphere.
The subsequent reradiation of some of the energy back to the surface
maintains surface temperatures higher than they would be if the
gases were absent. There is concern that increasing concentrations
of greenhouse gases, including carbon dioxide, methane, and manmade
chlorofluorocarbons, may enhance the greenhouse effect and cause
global warming. See Atmospheric greenhouse effect.
greenhouse gases
Those gases, such as water vapor, carbon dioxide,
tropospheric ozone, nitrous oxide, and methane, that are transparent
to solar radiation but opaque to longwave radiation. Their action is
similar to that of glass in a greenhouse. Also see greenhouse effect
and trace gas.
ground cover
Plants grown to keep soil from eroding.
ground fog
See radiation fog.
grounding line
The boundary between the area where an ice shelf
or a glacier is floating on water and where it is in contact with
the shore or underlying earth (grounded).
groundwater
The supply of fresh water found beneath the
surface of the Earth (usually in aquifers) that often supplies wells
and springs.
growing degree-day
A form of the degree-day used as a guide for crop
planting and for estimating crop maturity dates.
growth water-use efficiency
A measure at the individual plant level of how
well plants use available water in growth. The units of dry matter
synthesized are divided by the units of water lost.
Gulf Stream
A warm, swift, narrow ocean current flowing along
the east coast of the United States.
Gulf Stream meander
A transient winding bend in the Gulf Stream. These
bends intensify as the Gulf Stream merges into the North Atlantic
and can break up into detached eddies at about 40 degrees S.
gust
A sudden, brief increase in wind speed, generally
less than 20 seconds.
gust front
A boundary that separates a cold downdraft of a
thunderstorm from warm, humid surface air. On the surface its
passage resembles that of a cold front.
guyot
A type of seamount with a flat, platform top.
gyres
Major circular flow patterns in the oceans. The
wind- driven eastward- and westward-flowing equatorial currents are
blocked by the continents and rotate slowly in a clockwise direction
in the North Atlantic and Pacific Oceans and in a counter- clockwise
direction in the South Atlantic, South Pacific, and Indian Oceans.
haboob
A dust or sandstorm that forms as cold downdrafts
from a thunderstorm turbulently lift dust and sand into the air.
Hadley cell
A direct thermally-driven and zonally symmetric
large- scale atmospheric circulation first proposed by George Hadley
in 1735 as an explanation for the trade winds. It carries momentum,
sensible heat, and potential heat from the tropics to the
mid-latitudes (30 degrees). The poleward transport aloft is
complemented by subsidence in the subtropical high pressure ridge
and a surface return flow. The variability of this cell and the
Walker cell is hypothesized to be a major factor in short-term
climatic change.
hail
Precipitation in the form of lumps of ice
associated with thunderstorms. Hail size usually ranges from that of
a small pea to the size of cherries, but has been observed as large
as oranges. Hail occurs most frequently during the summer when
thunderstorm activity is at a peak. Extensive damage in the
agricultural industry is caused each year by hailstorms.
hailstones
Transparent or partially opaque particles of ice
that range in size from that of a pea to that of golf balls.
hailstreak
The accumulation of hail at the earth's surface
along a relatively long (10 km), narrow (2 km) band.
halite
An evaporite mineral composed of sodium chloride
(NaCl). Halite has perfect cubic cleavage and is often found in
cubes with curved faces although it also occurs in granular and
compacted form.
halo
In ages past, the huge rings or haloes around the
sun or the moon were thought to portend everything from storms to
great personal disasters. We now know that they are the optical
result of the refraction of light from the sun or moon by ice
crystals in the very high cloud (9,000 meters) called cirrus or
cirrostratus. On occasion, only two bright spots on either side of
the sun can be seen. These are known as sun dogs and are caused when
the ice crystals occur in a certain uniform arrangement.
halocline
In the oceans, a well-defined vertical gradient of
salinity.
haze
Fine dry or wet dust or salt particles dispersed
through a portion of the atmosphere. Individually these are not
visible but cumulatively they will diminish visibility. Dry haze
particles are very small, on the order of 0.1 mm. Wet haze particles
are larger. Haze is distinquished from fog by its bluish or
yellowish tinge.v
heat
The amount of energy in a bunch of molecules. The
amount of heat is equal to the number of molecules multiplied by the
temperature of the molecules.
heat capacity
The quantity of energy required to increase the
temperature of a system or substance one degree of temperature. In
the metric system it is expressed in Joules per degree Celscius.
heat flux (thermal flux)
The amount of heat that is transferred across a
surface of unit area in a unit of time.
heat index (hi)
An index that combines air temperature and
relative humidity to determine an apparent temperature -- how hot it
actually feels.
heat island effect
A dome of elevated temperatures over an urban area
caused by the heat absorbed by structures and pavement.
heat lightning
Distant lightning that illuminates the sky but is
too far away for its thunder to be heard.
heating degree-day
A form of the degree-day used as an index for fuel
consumption.
heatstroke
A physical condition induced by a person's
overexposure to high air temperatures, especially when accompanied
by high humidity.
heiligenschein
A faint white ring surrounding the shadow of an
observer's head on a dew-covered lawn.
heterosphere
The region of the atmosphere above about 85 km
where the composition of the air varies with height.
high inversion fog
A fog that lifts above the surface but does not
completely dissipate because of a strong inversion (usually
subsidence) that exists above the fog layer).
high pressure
In the Northern Hemisphere, an area of high
atmospheric pressure with a closed, clockwise movement of air
(circulation). Also known as an anticylone.
Holocene
The most recent epoch of the Quaternary period,
covering approximately the last 10,000 years.
homosphere
The region of the atmosphere below about 85 km
where the composition of the air remains fairly constant.
hook-shape echo
The shape of an echo on a radar screen that
indicates the possible presence of a tornado.
horse latitudes
The belt of latitude at about 30 to 35 where winds
are predominantly light and weather is hot and dry.
hotspot
The expression at the Earth's surface of a mantle
plume, or column of hot, buoyant rock rising in the mantle beneath a
lithospheric plate.
humid continental climate
A climate characterized by severe winters and mild
to warm summers with adequate annual precipitation. Typically
located over large continental areas in the Northern Hemisphere
between about 40 and 70 latitude.
humidex
A so called "comfort index", the humidex reading
provides a value based on a combination of temperature and humidity
which represents what the temperature would be expected to be if the
air were dry. If you consider the effect of evaporative cooling....
then in dry air at any given temperature you would not feel so warm
as in moist air at the same temperature simply because your body
could not cool down as quickly due to the reduced evaporative
cooling.... Extremes of temperature and high humidity can cause
health concerns in some people as a result.
humidity
Generally, a measure of the water vapor content of
the air. Popularly, it is used synonymously with relative humidity
See relative humidity.
humiture
An index that relates air temperature and relative
humidity to how hot it feels.
humus
The generally dark layer of soil which contains
mainly decomposed organic matter.
hurricane
(also known as Typhoon, Tropical Cyclones,
Willy-Willies) Tropical storms with wind speeds of 64 knots (117
km/h) up to 240 knots (414 km/h) that can be thousands of square
kilometers in size. Such systems usually have a lifespan of several
days. In the North Atlantic, the hurricane season is from May to
November, but the majority of storms occur in August, September and
October. A hurricane warnng is issued if winds are expected to
exceed 64 knots (117 km/h).
hurricane warning
A warning given when it is likely that a hurricane
will strike an area within 24 hours.
hurricane watch
A hurricane watch indicates that a hurricane poses
a threat to an area (often within several days) and residents of the
watch area should be prepared.
hydration sphere
A collection of water molecules that surround an
ion.
hydration sphere
A collection of water molecules that surround an
ion.
hydrogen bond
A weak, non-covalent electrostatic attraction
between molecules where the positively charged hydrogen atom of one
molecule is attracted to the negatively charged oxygen atom in
another molecule.
hydrologic budget
A quantitative accounting of all water volumes and
their changes with time for a basin or area.
hydrologic cycle
The cycle of evaporation and condensation that
controls the distribution of the earth's water as it evaporates from
bodies of water, condenses, precipitates, and returns to those
bodies of water.
It is a major factor in determining climate through its influence on
surface vegetation, the clouds, snow and ice, and soil moisture. The
hydrologic cycle is responsible for 25 to 30 percent of the
mid-latitudes' heat transport from the equatorial to polar regions.
hydrology
The science dealing with the properties,
distribution, and circulation of water.
hydrophobic
The ability to resist the condensation of water
vapor. Usually used to describe "water-repelling" condensation
nuclei.
hydrosphere
The aqueous envelope of the Earth, including the
oceans, freshwater lakes, rivers, saline lakes and inland seas, soil
moisture and vadose water, groundwater, and atmospheric vapor.
hydrostatic equation
In the vector equation of motion, the form assumed
by the vertical component when all Coriolis, earth-curvature,
frictional, and vertical-acceleration terms are considered
negligible compared with those involving the vertical pressure force
and the force of gravity. The error in applying the hydrostatic
equation to the atmosphere for cyclonic-scale motions is less than
0.01%. In extreme situations, the strong vertical accelerations in
thunderstorms and mountain waves can be 1% of gravity.
hydrostatic equilibrium
The state of the atmosphere when there is a
balance between the vertical pressure gradient force and the
downward pull of gravity.
hydrostatic pressure
The pressure resulting from the weight of
overlying water. Hydrostatic pressure increases by 1 atmosphere for
every 10 metres depth.
hydrothermal vent
Fissures in the ocean’s crust emitting seawater
whose temperature is 50-700°C. Such vents often have a unique
associated fauna sustained by chemosynthetic bacteria.
hygrometer
An instrument designed to measure the air's water
vapor content. The sensing part of the instrument can be hair (hair
hygrometer), a plate coated with carbon (electrical hygrometer), or
an infrared sensor (infrared hygrometer).
hygroscopic
The ability to accelerate the condensation of
water vapor. Usually used to describe "water seeking" condensation
nuclei.
hypothermia
The deterioration in one's mental and physical
condition brought on by a rapid lowering of human body temperature.
hypoxia
A condition experienced by humans when the brain
does not receive sufficient oxygen.
hypsithermal period
The period about 4000 to 8000 years ago when the
Earth was apparently several degrees warmer than it is now. More
rainfall occurred in most of the subtropical desert regions and less
in the central midwest United States and Scandinavia. It is also
called the altithermal period and can serve as a past climate analog
for predicting the regional pattern of climate change should the
mean Earth surface temperature increase from an increase in
atmospheric carbon dioxide concentration.
ice age
A glacial epoch or time of extensive glacial
activity. Also, as the ice age, which refers to the latest
glacial epoch, the Pleistocene Epoch.
ice and snow albedo
The reflectivity of ice and snow-covered surfaces.
The albedo of freshly fallen snow may be as much as 90%, while older
snow may have values of 75% or less. The larger the areal extent of
snow and ice cover, the higher the albedo value. The surface albedo
will also increase as a function of the depth of snow cover up to 13
cm and be unaffected by increased snow cover after reaching that
depth.
ice/snow-albedo-temperature feedback
Interactions that can be described as a
theoretical concept of a feedback mechanism in which the interacting
elements are the areal extent of polar ice and snow cover, the
albedo of the polar region (dependent on areal extent of ice and
snow), absorption of solar radiation (dependent on the albedo),
temperature (dependent on the absorption of solar radiation) and the
area of ice and snow cover (dependent on temperature). Less snowfall
would mean more absorption of solar radiation, therefore a surface
warming would occur. Climate modeling studies indicate an
amplification effect (i.e., positive feedback) of the ice and
snow-albedo feedback on increased surface air temperatures caused by
increases in the atmospheric concentration of carbon dioxide.
ice cover
During the present time, the extent, especially
the thickness, of glacier ice on a land surface. Also the same as
ice concentration, which is the ratio of an area of sea ice to the
total area of sea surface within some large geographic area.
ice crystal process
A process that produces precipitation. The process
involves tiny ice crystals in a supercooled cloud growing larger at
the expense of the surrounding liquid droplets. Also called the
Bergeron-Findeisen process.
ice flow
See glacier flow.
ice fog
A type of fog composed of tiny suspended ice
particles that forms at very low temperatures.
ice front
The floating vertical cliff that forms the seaward
face or edge of a glacier or an ice shelf that enters water. It can
vary from 2 to 50 m in height.
ice nuclei
Particles that act as nuclei for the formation of
ice crystals in the atmosphere.
ice pellets
See sleet.
ice sheet (continental glacier)
A glacier of considerable thickness and more than
50,000 sq km in area. It forms a continuous cover of ice and snow
over a land surface. An ice sheet is not confined by the underlying
topography but spreads outward in all directions. During the
Pleistocene Epoch, ice sheets covered large parts of North America
and northern Europe but they are now confined to polar regions
(e.g., Greenland and Antarctica).
ice shelf
A sheet of very thick ice with a level or gently
undulating surface. It is attached to the land on one side, but most
of it is floating. On the seaward side, it is bounded by a steep
cliff (ice front) 2 to 50 m or more above sea level. Ice shelves
have formed along polar coasts (e.g., Antarctica and Greenland);
they are very wide with some extending several hundreds of
kilometers toward the sea from the coastline. They increase in size
from annual snow accumulation and seaward extension of land
glaciers. They decrease in size from warming, melting, and calving.
Icelandic low
The subpolar low pressure area that is centered
near Iceland on charts that show mean sea level pressure.
igneous
A rock or mineral that solidified from molten or
partly molten material, that is, from magma. Igneous rocks
constitute one of the three main classes into which rocks are
divided, the others being sedimentary and metamorphic.
Indian summer
An unseasonably warm spell with clear skies near
the middle of autumn. Usually follows a substantial period of cool
weather.
inferior mirage
See mirage.
infiltration (soil)
Movement of water from the ground surface into the
soil.
infrared radiation
Electromagnetic radiation lying in the wavelength
interval from 0.7 micrometers to 1000 micrometers. Its lower limit
is bounded by visible radiation, and its upper limit by microwave
radiation. Most of the energy emitted by the Earth and its
atmosphere is at infrared wavelength. Infrared radiation is
generated almost entirely by large-scale intra-molecular processes.
The tri-atomic gases, such as water vapor, carbon dioxide, and
ozone, absorb infrared radiation and play important roles in the
propagation of infra- red radiation in the atmosphere. Abbreviated
IR; also called "longwave radiation".
infrared radiometer
An instrument designed to measure the intensity of
infrared radiation emitted by an object. Also called infrared
sensor.
insolation v
The solar radiation incident on a unit
horizontal surface at the top of the atmosphere. It is sometimes
referred to as solar irradiance. The latitudinal variation of
insolation supplies the energy for the general circulation of the
atmosphere. Insolation depends on the angle of incidence of the
solar beam and on the solar constant.
instability
(Meteorology) The tendency for air parcels to
accelerate when they are displaced from their original position;
especially, the tendency to accelerate upward after being lifted.
Instability is a prerequisite for thunderstorms to develop -- the
greater the instability, the greater the potential for severe
thunderstorms. See lifted index.
instantaneous transpiration efficiency
A measure at the physiologic level of how
well plants use available water in photosynthesis. It is the
assimilation rate divided by the transpiration rate; that is, the
moles of CO2 taken up are divided by the moles of water
lost through transpiration in a unit of time.
instrument shelter
A boxlike wooden structure designed to
protect weather instruments from direct sunshine and precipitation.
interglacial period
A time interval of relatively mild climate
during the Ice Age when continental ice sheets were absent or
limited in extent to Greenland and the Antarctic.
intertropical convergence zone (ITCZ)
The boundary zone separating the northeast
trade winds of the Northern Hemisphere from the southeast trade
winds of the Southern Hemisphere.
inversion
(Meteorology) Generally, a departure from the
usual increase or decrease in an atmospheric property with altitude.
Specifically it almost always refers to a temperature inversion,
i.e., an increase in temperature with height, or to the layer within
which such an increase occurs.
ion
An electrostatically charged atom or
molecule.
ionosphere
An electrified region of the upper atmosphere
where fairly large concentrations of ions and free electrons exist.
iridescence
(Meteorology) Brilliant spots or borders of
colors, most often red and green, observed in clouds up to about 30°
from the sun.
iridium
An element of the platinum group . A cubic
mineral consisting of more than 80% iridium and includes osmium,
palladium, or related elements, which make up the remainder.
irradiance
The total radiant flux received on a unit
area of a given real or imaginary surface. Also called the radiant
flux density. Units are W m-2 (= J m-2 s-1).
isallobar
A line of equal change in atmospheric
pressure during a specified time interval.
isobar
A line connecting points of equal pressure.
isobaric surface
A surface along which the atmospheric
pressure is everywhere equal. (See Constant pressure chart).
isochron
A line on a map connecting points at which a
characteristic time or interval has the same value.
isodrosotherm
A line connecting points of equal dew point
temperature.
isohyet
A line connecting points of equal
precipitation amounts.
isopleth
General term for a line connecting points of
equal value of some quantity. Isobars, isotherms, isopynics, etc.
all are examples of isopleths.
isopynic
A line on a chart that connects all points of
equal or constant density.
isostatic adjustment
The process whereby lateral transport at the
Earth's surface from erosion or deposition is compensated for by
movements in a subcrustal layer to maintain equilibrium among units
of varying masses and densities.
isotach
A line connecting points of equal wind speed.
isotherm
A line connecting points of equal
temperature.
isotope
One of two or more atoms that have the same
atomic number (i.e., the same number of protons in their nuclei) but
have different mass numbers.
jet maximum
See jet streak.
jet streak
A region of high wind speed that moves through the
axis of a jet stream. Also called jet maximum.
jet stream
Relatively strong winds concentrated in a narrow
stream in the atmosphere, normally referring to horizontal,
high-altitude winds. The position and orientation of jet streams
vary from day to day. General weather patterns (hot/cold, wet/dry)
are related closely to the position, strength and orientation of the
jet stream (or jet streams). A jet stream at low levels is known as
a low-level jet.
Jurassic
The second period of the Mesozoic era (213-144
million years ago), after the Triassic and before the Cretaceous.
katabatic (fall) wind
Any wind blowing downslope. It is usually cold.
Kelvin
A unit of temperature. A Kelvin is denoted by K
and 1 K equals 1° Celcius. Zero Kelvin is absolute zero, or -273.15°
Celcius.
Kelvin scale
A temperature scale with zero degrees equal to the
theoretical temperature at which all molecular motion ceases. Also
called the absolute scale. The units are sometimes called "degrees
Kelvin"; however, the correct SI terminology is "kelvins,"
abbreviated K.
kilopascal (kPa)
The internationally recognized unit for measuring
atmospheric pressure (1 Pa = 1 N m-2; 1 kPa = 1000 N m-2).
Normal atmospheric pressure is 1 atmosphere (atm.) = 1.013 x 105
Pa = 101.3 kPa.
kinetic energy
The energy within a body that is a result of its
motion.
Kirchhoff's law
A law that states: good absorbers of a given
wavelength of radiation are also good emitters of that wavelength.
knot
(Nautical) A unit of speed equal to 1 nautical
mile per hour. 1 knot equals 1.15 mi/hr.
Köppen classification system
A system for classifying climates developed by W.
Köppen that is based mainly on annual and monthly averages of
temperature and precipitation.
Krakatoa (Krakatau)
Active volcano 2667 ft (813 m) high in West
Indonesia. It forms an island between Java and Sumatra. Its eruption
in 1883, which was one of the most violent in modern times,
scattered debris and darkened skies over vast areas. Additional
eruptions occurred in late 1927 and in the l960s.
La Niña
A condition where the central and eastern tropical
Pacific Ocean turns cooler than normal.
lake breeze
A wind blowing onshore from the surface of a lake.
lake-effect snows
Localized snowstorms that form on the downwind
side of a lake. Such storms are common in late fall and early winter
near the Great Lakes as cold, dry air picks up moisture and warmth
from the unfrozen bodies of water.
laminar flow
A non-turbulent flow in which the fluid moves
smoothly in parallel layers or sheets.
land breeze
A coastal breeze that blows from land to sea,
usually at night.
lapse rate
The rapidity with which temperature decreases with
altitude. The normal lapse rate is defined to be 3.6 degrees F per
1000 feet change in altitude. The dry adiabatic lapse rate is about
5.5 degrees F per 1000 feet, and the wet adiabatic lapse rate varies
between 2 and 5 degrees F per 1000 feet.
large nuclei
See condensation nuclei
latent
Present or existing but not manifest, visible or
active. See latent heat.
latent heat
Latent heat is the heat taken up (or released)
when a substance melts (or solidifies). The uptake or release of
latent heat does not change the temperature of the substance, it
serves to change the state of the substance.
latent heat of evaporation
The amount of heat (energy) required to change a
substance from its liquid state to its gaseous state at its boiling
point.
latent heat of fusion
The amount of heat (energy) required to change a
substance from its solid state to its liquid state at its melting
point.
Laurasia
The protocontinenet of the Northern Hemisphere
from which the present continents have been derived by separation
and continental displacement. Some geologists believe that Laurasia
(and Gondwana) was derived from the supercontinent, Pangaea.
Le Chatelier's principle
When an external force is applied to an
equilibrium system, the system adjusts to minimize the effect of the
force.
leachate
A solution obtained by leaching. Leaching occurs
as water percolates through the soil. Soluble ions and substances in
the soil dissolve in the water forming a leachate solution.
leeside low
Storm systems (extratropical cyclones) that form
on the downwind (lee) side of a mountain chain. In the United States
leeside lows frequently form on the eastern side of the Rockies and
Sierra Nevada.
leeward
Situated away from the wind; downwind -- opposite
of windward.
lenticular cloud
A cloud in the shape of a lens.
Lepidodendron
A genus of early tree-like plants (lycophytes)
that grew over 30m in height and 2 m in diameter. Lycophytes were
the dominant plants in the Devonian and Carboniferous forests. Their
remains became partly decomposed to form coal, gas and crude oil.
lidar
An instrument that uses a laser to generate
intense pulses that are reflected from atmospheric particles of dust
and smoke. Lidars have been used to determine the amount of
particles in the atmosphere as well as particle movement that has
been converted into wind speed. Lidar means light detection and
ranging.
life zone
A climatically-defined class that can be
associated with regions of soil and biota with a high uniformity in
species composition and environmental adaptation. See Holdridge life
zone.
lifted index
A common measure of atmospheric instability. Its
value is obtained by computing the temperature that air near the
ground would have if it were lifted to some higher level (around
18,000 feet, usually) and comparing that temperature to the actual
temperature at that level. Negative values indicate instability the
more negative, the more unstable the air is, and the stronger the
updrafts are likely to be with any developing thunderstorms. However
there are no "magic numbers" or threshold LI values below which
severe weather becomes imminent.
lifting condensation level (LCL)
The level at which a parcel of air when lifted dry
adiabatically would become saturated.
lightning
Generally, any and all of the various forms of
visible electrical discharge produced by thunderstorms.
lithosphere
The component of the Earth's surface comprising
the rock, soil, and sediments. The lithosphere is the outer, rigid
shell of the Earth, situated above the asthenosphere and containing
the crust, the uppermost part of the mantle, the continents, and the
plates.
litter
Undecomposed plant residues on the soil surface.
Little Ice Age
A cold period that lasted from about A.D. 1550 to
about A.D. 1850 in Europe, North America, and Asia. This period was
marked by rapid expansion of mountain glaciers, especially in the
Alps, Norway, Ireland, and Alaska. There were three maxima,
beginning about 1650, about 1770, and 1850, each separated by slight
warming intervals.
local winds
Winds that tend to blow over a relatively small
area; often due to regional effects, such as mountain barriers,
large bodies of water, local pressure differences, and other
influences.
loess
A buff-colored, wind-blown deposit of fine silt,
which is frequently exposed in bluffs with steep faces. The
thickness can range from 6 to 30 m. The loess of the USA and Europe
is thought to be the fine materials first transported and deposited
by the waters of melting ice sheets during the glacial period. It
was later blown considerable distances with, in some cases,
deposition in lakes. The origin of Asiatic loess, however, is
apparently wind-blown dust from central Asian deserts.
longwave radiation
The radiation emitted in the spectral wavelength
greater than 4 micrometers corresponding to the radiation emitted
from the Earth and atmosphere. It is sometimes referred to as
terrestrial radiation or infrared radiation, although somewhat
imprecisely.
longwaves in the westerlies
A wave in the major belt of westerlies
characterized by a long length (thousands of kilometers) and
significant amplitude. Also called Rossby waves.
low
(Meteorology) See low pressure area and
extratropical cyclone.
low pressure area (or a "low")
An area of low atmospheric pressure that has a
closed counter-clockwise circulation in the Northern Hemisphere.
Also known as a cyclone.
low-level jet
A region of relatively strong winds in the lower
part of the atmosphere.
low-level jet streams
Jet streams that typically form near the earth's
surface below an altitude of about 2 km and usually attain speeds of
less than 60 knots.
Lystrosaurus
A mammal-like reptile that was strictly a land
dweller. Fossils are found in abundance in South Africa, South
America, Asia and Antarctica. Lystrosaurus is cited as evidence for
the theory of plate tectonics because this creature could not have
swam thousands of kilometers across the Atlantic and Antarctic
oceans. Therefore, this leads to postulate that the continents were
once connected.
macroburst
A strong downdraft (downburst) greater than 4 km
wide that can occur beneath thunderstorms. A downburst less than 4
km across is called a microburst.
macroclimate
The general climate of a large area, such as a
country.
macroscale
The normal meteorological synoptic scale for
obtaining weather information. It can cover an area ranging from the
size of a continent to the entire globe.
mafic
An igneous rock composed chiefly (>50%) of one or
more ferromagnesian, dark-colored minerals (derived from magnesium
and ferric).
magma
A mobile silica melt, which can contain suspended
crystals and dissolved gases. Igneous rocks are considered to have
formed from magma through solidification and related processes.
magnetic
Having the properties of a magnet. Magnetics is
the science of magnetism. Magnetic force is the physical force
experienced by a magnetic substance when placed in a magnetic field
or between magnetized bodies and electric currents.
magnetic storm
A worldwide disturbance of the earth's magnetic
field causes by solar disturbances.
magnetosphere
The region around the earth in which the earth's
magnetic field plays a dominant part in controlling the physical
processes that take place.
mammatus clouds
Clouds that look like pouches hanging from the
underside of a cloud.
mantle
The zone of the Earth between the base of the
crust and the core.
marine climate
A climate controlled largely by the ocean. The
ocean's influence keep winters relatively mild and summers cool.
marine wind warnings
Small craft warning issued if winds are forecast
to be in the range of 20 to 33 knots inclusive. Gale warning issued
if winds are forecast to be in the range of 34 to 47 knots
inclusive. Storm warning issued if the winds are forecast to be in
the range of 48 to 63 knots inclusive. Hurricane force wind warning
issued for winds of 64 knots or greater.
maritime air
Moist air whose characteristics were developed
over an extensive body of water.
maritime polar air mass
An air mass characterized by high temperatures and
high humidity.
marsh
A type of wetland that does not accumulate
appreciable peat deposits and is dominated by herbacious vegetation.
Marshes may be fresh- or saltwater, tidal or nontidal.
mass
A unified body of matter with no specific shape or
a grouping of individual parts or elements that compose a unified
body of unspecified size or quantity.
Physics. A property of matter equal to the measure of an object's
resistance to changes in either the speed or direction of its
motion. The mass of an object is not dependent on gravity and
therefore is different from but proportional to its weight.
mass balance
The application of the principle of the
conservation of matter. For example, the mass of a glacier is not
destroyed or created; the mass of a glacier and all its constitutive
components remains the same despite alterations in their physical
states. The mass balance of a glacier is calculated with the
input/output relationships of ice, firn, and snow, usually measured
in water equivalent. Output includes all ablative processes of
surface melting, basal melting, evaporation, wind deflation,
calving, and internal melting. Input includes direct precipitation,
avalanching, and the growth of superimposed ice.
mature thunderstorm
The second stage in the three-stage cycle of an
air-mass thunderstorm. This stage is characterized by heavy showers,
lightning, thunder, and violent vertical motions inside cumulonimbus
clouds.
Mauna Loa
An intermittently active volcano 13,680 ft (4,170
m) high in Hawaii. Last eruption was in 1984. Also see Mauna Loa
record.
Mauna Loa record
The record of measurements of atmospheric carbon
dioxide concentrations taken at the Mauna Loa Observatory, Mauna
Loa, Hawaii, since March 1958. The Mauna Loa record is the longest
reliable daily record of atmospheric carbon dioxide measurements in
the world.
Maunder minimum
The period from 1654 to 1714 when it was believed
that there were no sunspots. It is now thought that there were some
sunspots during that time but less than the numbers counted after
1800.
maximum thermometer
A thermometer with a small constriction just above
the bulb. It is designed to measure the maximum air temperature.
maximum/minimum thermometer
A thermometer that marks the lowest temperature
(minimum) or highest temperature (maximum) since the previous
reading (usually 1 day).
mean annual temperature
The average temperature at any given location for
the entire year.
mean daily temperature
The average of the highest and lowest temperature
for a 24-hour period.
mean sea level
The average height of the sea surface, based upon
hourly observation of the tide height on the open coast or in
adjacent waters that have free access to the sea. In the United
States, it is defined as the average height of the sea surface for
all stages of the tide over a nineteen year period. Mean sea level,
commonly abbreviated as MSL and referred to simply as sea level,
serves as the reference surface for all altitudes in upper
atmospheric studies.
mechanical turbulence
Turbulent eddy motions caused by obstructions,
such as trees, buildings, mountains and so on.
Mediterranean climate
See Dry-summer subtropical climate.
melting point
The temperature at which a substance changes from
a solid state to a liquid state.
meridional flow
Large-scale atmospheric flow in which the
north-south component (i.e., longitudinal, or along a meridian) is
pronounced. The accompanying zonal (east-west) component often is
weaker than normal. Compare with zonal flow.
mesic environment
A habitat with a moderate amount of water.
mesoclimate
The climate of an area ranging in size from a few
acres to several square kilometers.
mesocyclone
A vertical column of cyclonically rotating air
within a severe thunderstorm.
mesohigh
A relatively small area of high atmospheric
pressure that forms beneath a thunderstorm.
mesopause
The top of the mesosphere. The boundary between
the mesosphere and the thermosphere, usually near 85 km.
mesoscale
The scale of meteorological phenomena that range
in size from a few km to about 100 km. It includes local winds,
thunderstorms, and tornadoes. "Medium size". Size scale referring to
weather systems smaller than a few hundred kilometers across. Squall
lines are an example of mesoscale weather systems.
mesoscale convective complex (MCC)
A large organized convective weather system
comprised of a number of individual thunderstorms. The size of an
MCC can be 1000 times larger than an individual air-mass
thunderstorm.
mesoscale eddies (mode eddies)
In the ocean, dense and irregularly-oval high- and
low- pressure centers about 400 km in diameter. The intensities of
currents in these centers are about 10 times greater than the local
means.
mesosphere
The atmospheric layer between the stratosphere and
the thermosphere. Located at an average elevation between 50 and 80
km above the earth's surface.
Mesozoic
An era of geologic time, from the end of the
Paleozoic to the beginning of the Cenozoic (about 248-65 million
years ago). Termed the era of middle life.
metamorphic
Any rock formed from pre-existing rocks within the
Earth's crust by changes in temperature and pressure and by chemical
action of fluids.
meteorologist
A person who studies meteorology. There are many
different paths within the field of meteorology. For example, one
could be a research meteorologist, radar meteorologist,
climatologist, or operational meteorologist.
meteorology
Meteorology is the study of the physics,
chemistry, and dynamics of the atmosphere and the direct effects of
the atmosphere upon the Earth's surface, the oceans, and life in
general.
mica
A group of silicate minerals exhibiting perfect
cleavage in one direction. Micas split into thin, tough, somewhat
elastic plates with a splendent pearly luster. They are prominent
rock-forming constituents of igneous and metamorphic rocks, and
commonly occur as flakes, scales or shreds.
microburst
A strong localized downdraft (downburst) less than
4 km wide that occurs beneath thunderstorms. A strong downburst
greater than 4 km across is called a microburst.
microclimate
The climate structure of the air space near the
surface of the earth.
micrometer
A unit of length equal to one-millionth of a meter
(= 1 m m).
microscale
The smallest scale of atmospheric motions.
middle latitude cyclones
See extratropical cyclone.
middle latitudes
The region of the world typically described as
being between 30° and 50° latitude.
Milankovitch theory
An astronomical theory formulated by the Yugoslav
mathematician Milutin Milankovitch that associates climate change
with fluctuations in the seasonal and geographic distribution of
insolation determined by periodic variations of the Earth's
eccentricity and obliquity and the longitude of the perihelion.
millibar (mb)
A unit for expressing atmospheric pressure. Sea
level pressure is normally close to 1013 mb.
minimum thermometer
A thermometer designed to measure the minimum air
temperature during a desired time period.
mirage
A refraction phenomenon that makes an object
appear to be displaced from its true position. When an object
appears higher than it actually is, it is called a superior mirage.
When an object appears lower than it actually is, it is an inferior
mirage.
Mississippian
A period of the Paleozoic era (320-286 million
years ago) after the Devonian and before the Pennsylvanian.
mist
Consists of microscopic water droplets suspended
in the air which produce a thin greyish veil over the landscape. It
reduces visibility to a lesser extent than fog. Mist consists of
liquid particles 40 to 500 microns in diameter that are formed by
condensation of vapor in air.
mixed cloud
A cloud containing both water drops and ice
crystals.
mixed layer
The surface layer of ocean above the thermocline
that is mixed by wind action.
mixing depth
The unstable atmospheric layer that extends from
the surface up to the base of an inversion. Within this layer the
air is well stirred.
mixing ratio
The ratio of the mass of water vapor in a given
volume of air to the mass of dry air.
modeling
An investigative technique that uses a
mathematical or physical representation of a system or theory that
accounts for all or some of its known properties. Models are often
used to test the effects of changes of system components on the
overall performance of the system.
moist adiabatic rate
The rate of change of temperature in a rising or
descending saturated air parcel. The rate of cooling or warming
varies but a common value of 6oC/1000 m (3.3oF/1000 ft) is used.
moist adiabats
Lines on an adiabatic chart that show the moist
adiabatic rate for rising and descending air.
molecular viscosity
The small-scale internal fluid friction that is
due to the random motion of the molecules within a smooth-flowing
fluid, such as air.
molecule
A collection of atoms held together by chemical
forces.
monsoon
The word "monsoon" appears to have originated from
the Arabic word "mausim" which means season. It is most often
applied to the seasonal reversals of the wind direction along the
shores of the Indian Ocean, especially in the Arabian Sea, that blow
from the southwest during one half of the year and from the
northeast during the other. As monsoons have come to be better
understood, the definition has been broadened to include almost all
of the phenomena associated with the annual weather cycle within the
tropical and subtropical continents of Asia, Australia and Africa
and the adjacent seas and oceans. It is within these regions that
the most vigorous and dramatic cycles of weather events on the earth
takes place.
monsoon depressions
Weak low-pressure areas that tend to form in
response to divergence in an upper-level jet stream. The circulation
around the low strengthens the monsoon wind system and enhances
precipitation during the summer.
monsoon wind system
A wind system that reverses direction between
winter and summer. Usually the wind blows from land to sea in winter
and from sea to land in summer.
mountain
A general term for any landmass that stands above
its surroundings. It is usually very high, with steep sides and
often with considerable bare-rock surfaces. Mountains are formed
either by the injection of igneous intrusions or from the collision
of tectonic plates.
mountain and valley breeze
A local wind system of a mountain valley that
blows downhill (mountain breeze) at night and uphill (valley breeze)
during the day.
mud flows
The rapid flow of fine-grained unconsolidated
material such as soil, sediments and fragments of bedrock mixed with
as much as 60% water.
multicell storms
Thunderstorms in a line, each of which may be in a
different stage of development.
nacreous clouds
Clouds of unknown composition that have a soft,
pearly luster and that form at altitudes about 25 to 30 km above the
earth's surface. They are also called mother-of-pearl clouds.
nebula
A cloud of interstellar gas and dust.
negative feedback
An interaction that reduces or dampens the
response of the system in which it is incorporated. See Feedback
mechanism
net primary production
The part of the gross primary production that
remains stored in the producer organism (primarily green plants)
after deducting the amount used during the process of respiration.
neutral stability (neutrally stable air)
An atmospheric condition that exists in dry air
when the environmental lapse rate equals the try adiabatic rate. In
saturated air the environmental lapse rate equals the moist
adiabatic rate.
nimbostratus
A dark, gray cloud characterized by more or less
continuously falling precipitation. It is not accompanied by
lightning, thunder, or hail.
noctilucent clouds
Wavy, thin, bluish-white clouds that are best seen
at twilight in polar latitudes. They form at altitudes about 80 to
90 km above the surface.
nocturnal
Related to nighttime, or occurring at night.
nocturnal inversion
See radiation inversion.
normal
The long-term average value of a meteorological
element for a certain area. For example, "temperatures are normal
for this time of year" Usually averaged over 30 years.
northeaster
A name given to a strong, steady wind from the
northeast that is accompanied by rain and inclement weather. It
often develops when a storm system moves northeastward along the
coast of North America.
northern lights
See aurora borealis.
nowcast
A short-term weather forecast, generally out to
six hours or less.
nowcasting
Short-term weather forecasts varying from minutes
up to a few hours.
nuclear fusion
The combination of the nuclei of light atoms to
form heavier nuclei, with the release of energy.
nuclear winter
The dark, cold, and gloomy conditions that
presumably would be brought on by nuclear war.
nucleation
Any process in which the phase change of a
substance to a more condensed state (such as condensation,
deposition, and freezing) is initiated about a particle (nucleus) or
at a certain locus.
numerical weather prediction (nwp)
Forecasting the weather based upon the solutions
of mathematical equations by high-speed computers.
nutation
The motion of the true axis of rotation of the
Earth about its mean position. The principal component of this
motion has a period of about 18.6 years.
nutrient
Any substance assimilated by living things that
promotes growth.
obliquity (of the Earth's axis)
The tilt of the earth's axis. It represents the
angle from the perpendicular to the plane of the earth's orbit.
occluded front (occlusion)
A complex frontal system that ideally forms when a
cold front overtakes a warm front. When the air behind the front is
colder than the air ahead of it, the front is called a cold
occlusion. When the air behind the front is milder than the air
ahead of it, it is called a warm occlusion.
ocean basin
A low part of the lithosphere lying between the
continental masses.
ocean mixing
Processes that involve rates of advection,
upwelling/ downwelling, and eddy diffusion and that determine how
rapidly excess atmospheric carbon dioxide can be taken up by the
oceans.
oceanic front
A boundary that separates masses of water with
different temperatures and densities.
offshore wind
A breeze that blows from the land out over the
water. Opposite of an onshore wind.
olivine
A name applied to a group of olive-green,
grayish-green, or brown silicate minerals containing magnesium and
iron but no aluminum. The group forms an isomorphous system
(Mg,Fe,Mn,Ca)2SiO4, usually forms as granular aggregates (in igneous
rocks) and generally has poor cleavage.
omega high
A ridge in the middle or upper troposphere that
has the shape of a Greek letter omega (?).
onshore wind
A breeze that blows from the water onto the land.
Opposite of an offshore wind.
opacity
The degree of obscuration of light; for example, a
glass window has almost 0% opacity, whereas a concrete wall has 100%
opacity.
optical thickness (optical depth)
In calculating the transfer of radiant energy, the
mass of an absorbing or emitting material lying in a vertical column
of unit cross-sectional area and extending between two specified
levels. Also, the degree to which a cloud prevents light from
passing through it; the optical thickness then depends on the
physical constitution (crystals, drops, and/or droplets), the form,
the concentration, and the vertical extent of the cloud.
orchard heaters
Oil heaters placed in orchards that generate heat
and promote convective circulations to protect fruit trees from
damaging low temperatures. Also called smudge pots.
Ordovician
The second earliest period of the Paleozoic era
(after the Cambrian and before the Silurian) 458-438 million years
ago.
organic matter
Matter, such as humus*, which consists of organic
carbon rich deposits formed by the decay of once living material,
which has been buried.
orographic
Related to, or caused by, physical geography (such
as mountains or sloping terrain).
orographic lift (or orographic uplift)
Lifting of air caused by its passage up and over
mountains or other sloping terrain. Clouds that form in this lifting
process are called orographic clouds.
outflow winds
Winds that blow down fjords and inlets from the
land to the sea. When cold arctic air flows from the interior of BC
onto the coast, the windspeeds through mainland inlets can reach
over 100 km/h.
outgassing
The release of gases dissolved in hot, molten
rock.
overrunning
A condition that occurs when air moves up and over
another layer of air.
ozone (O3)
A pungent-smelling, slightly bluish gas (O3)
which is a close chemical cousin to molecular oxygen (O2).
About 90% of the earth's ozone is located in the stratosphere far
above the surface of the globe, in a frigid region of the atmosphere
known as the stratosphere. Here in this outer region it protects the
earth and its inhabitants from the harmful effects of ultraviolet
radiation from the sun. In the troposphere, it is a chemical oxidant
and major component of photochemical smog.
P/E index (precipitation-evaporation index)
An index that gives the long-range effectiveness
of precipitation in promoting plant growth.
P/E ratio (precipitation-evaporation ratio)
An expression devised for the purpose of
classifying climates; based on monthly totals of precipitation and
evaporation.
Pacific high
See subtropical high.
paleomagnetism
The study of ancient magnetic fields, which are
preserved in the magnetic properties of rocks. ie. studying the
changes in position of the magnetic poles, as well as reversals of
poles in the geological past.
paleosol
An ancient soil or soil horizon that formed on the
surface during the geologic past.
Paleozoic
An era of geologic time, from the end of the
Precambrian to the beginning of the Mesozoic (about 590-248 million
years ago). Termed as the era of ancient life.
paludification
The expansion of a bog caused by the gradual
rising of the water table as accumulation of peat impedes water
drainage.
palynology
The science of reconstructing the past flora and
past climate from pollen data obtained from lake and bog sediments.
The fossil pollen record is a function of the regional flora and
vegetation at a given time and location.
Pangaea
A proposed supercontinent that existed 300-200
million years ago that included most of the continental crust on
Earth. Named by Alfred Wegener, Greek for "all lands", is proposed
to have broken up into Laurasia and Gondwana and ultimately into
present day continents.
parallax
The apparent change in direction or position of an
object as seen from two different points. In astronomy, parallax
refers to the change in angle of a celestial body caused by viewing
it from the surface rather than the center of the Earth (diurnal or
geocentric parallax), or from the Earth at different locations on
the Earth's orbit around the sun (annual or heliocentric parallax).
parcel of air
An imaginary small body of air a few meters wide
that is used to explain the behavior of air.
parhelia
See sundog.
particulate matter
Very small pieces of solid or liquid matter, such
as particles of soot, dust, aerosols, fumes, or mists.
past climate analogs
The reconstructing of past climates at a given
locality from modern climatic conditions in a different elevation or
latitudinal zone to infer past climatic conditions.
pCO
The partial pressure of CO2 in the
atmosphere and the ocean. In the atmosphere, the partial pressure of
CO2 is defined as the pressure the CO2 would
exert if all other gases were removed. The sum of the partial
pressure of all the atmospheric gases will equal the atmospheric
pressure. The partial pressure of CO2 in the atmosphere
is determined by the atmospheric CO2 concentration and
atmospheric temperature. In the ocean, the pCO2 is
determined by the amount of dissolved CO2 and H2CO3.
It varies with alkalinity, latitude, depth, and temperature.
Biological processes in the ocean also exert an influence on the pCO2
in the ocean.
pelagic
Pelagic, meaning ‘open sea’, refers to the water
column from the sea surface to the greatest depths.
Pennsylvanian
A period of the Paleozoic era (320-286 million
years ago) after the Mississippian and before the Permian.
percolation
The slow laminar movement of water through small
openings within a porous material. In a hydrological context,
percolation refers to the movement of water downward and radially
through the subsurface soil layers, usually continuing downward to
the groundwater.
peridotite
A coarse-grained, igneous, ultramafic rock
composed chiefly of olivine with or without other mafic minerals
such as pyroxenes, amphiboles, or micas, and containing little or no
feldspar.
period
The formal geochronologic unit lower in rank than
era and higher than epoch. For example, the Devonian Period and the
Ordovician Period.
permafrost
A layer of soil beneath the earth's surface that
remains frozen throughout the year.
permanent thermocline
The depth horizon from the base of the mixed layer
(several-200m) to the bottom waters (around 1000 m) where the
temperature decreases rapidly.
Permian
The last period of the Paleozoic Era (286-248
million years ago).
periodic table of elements
A tabular arrangement of the elements according to
their atomic numbers so that elements with similar properties are in
the same column. Arranged from left to right and top to bottom
increasing according to their atomic number or mass.
The different rows of the table elements are called periods, hence
periodic table. The period number of an element identifies the
highest energy level an electron in that element occupies. The
number of elements in a period increases as one moves down the
periodic table.
persistence forecast
A forecast that the future weather condition will
be the same as the present condition.
phaneritic
The texture of an igneous rock is phaneritic when
the individual components are distinguishable with the unaided eye.
Phanerozoic
That part of geologic time in which the evidence
of life is abundant. This eon ranges from 590 million years ago to
the present.
phenology
The study of periodic biological phenomena with
relation to climate, particularly seasonal changes. These phenomena
can be used to interpret local seasons and the climatic zones.
photochemical smog
Air pollution caused by chemical reactions among
various substances and pollutants in the atmosphere. See smog.
photodissociation
The splitting of a molecule by a photon.
photoelectric
Of or relating to the electrical effects of light,
including the emission of electrons, the generation of a voltage, or
a change in resistance.
photon
A discrete quantity of energy that can be thought
of as a packet of electromagnetic radiation travelling at the speed
of light.
photosphere
The visible surface of the sun from which most of
its energy is emitted.
photosynthesis
The photochemical and biochemical processes
whereby plants and algae transform radiant energy (sunlight) into
chemical energy (organic compounds). Photosynthesis involves the
consumption of inorganic compounds such as water, carbon dioxide,
nitrate and phosphate and the release of oxygen. On land,
photosynthesis is dependent upon favorable temperature and moisture
conditions as well as the atmospheric carbon dioxide concentration.
phytoplankton
That portion of the plankton community comprised
of tiny plants (algae). Diatoms and dinoflagellates are two common
types of phytoplankton.
pileus cloud
A smooth cloud in the form of a cap. Occurs above,
or is attached to, the top of a cumuliform cloud.
pillow lava
A general term for lavas displaying a pillow
structure arising when lava is extruded underwater.
planet
A large body in the solar system that revolves
around the sun. In our solar system, one of nine celestial bodies
that orbit Sol.
planetary albedo
The fraction (approximately 30%) of incident solar
radiation that is reflected by the earth-atmosphere system and
returned to space, mostly by backscatter from clouds in the
atmosphere.
planetary boundary layer
The transition region between the turbulent
surface layer and the normally nonturbulent free atmosphere. This
region is about 1 km in thickness and is characterized by a
well-developed mixing generated by frictional drag as the air masses
move over the Earth's surface. This layer contains approximately 10%
of the mass of the atmosphere. Also called the atmospheric boundary
layer or frictional layer.
planetary scale
The largest scale of atmospheric motion. Sometimes
called the global scale.
planetesimals
Small chunks of mass that orbit the sun.
plankton
Passively floating or weakly motile aquatic plants
(phytoplankton) and animals (zooplankton).
plate boundaries
Zones of seismic and tectonic activity along the
edges of the lithospheric plates, presumed to indicate relative
motion between plates.
plate tectonics
The theory and study of the formation and division
of the lithosphere into plates as well as the movement, interactions
and destruction associated with them. The rigid plates interact with
one another at their boundaries.
Pleistocene
The earlier of the two epochs of the Quaternary
period starting 2 to 3 million years before the present and ending
about 10,000 years ago. It was a time of glacial activity.
plume
In general, a plume is something whose shape
resembles a feather. The term is usually used to describe a moving
column of something such as smoke or snow. In oceanography, a plume
refers to feather-like flow of one water mass into another, such as
where a river water laden with sediment enters the sea. In geology,
a plume refers to a mass of volcanic rock rising into the crust from
the mantle, which is thought to be the cause of hot spots.
polar easterlies
A shallow body of easterly winds located at high
latitudes poleward of the subpolar low.
polar front
A semipermanent, semicontinuous front that
separates tropical air masses from polar air masses.
polar front jet stream
The jet stream that is associated with the polar
front in middle and high latitudes. It is usually located at
altitudes between 9 and 12 km.
polar front theory
A theory developed by a group of Scandinavian
meteorologists that explains the formation, development, and overall
life history of cyclonic storms that form along the polar front.
polar ice cap climate
A climate characterized by extreme cold, as every
month has an average temperature below freezing.
polar orbiting satellite
A satellite whose orbit closely parallels the
earth's meridian lines and thus crosses the polar regions on each
orbit.
polar tundra climate
A climate characterized by extremely cold winters
and cool summers, as the average temperature of the warmest month
climbs above freezing but remains below 10° C (50° F).
pollutants
Any gaseous, chemical, or organic matter that
contaminates the atmosphere, soil, or water.
popcorn convection
Clouds, showers and thundershowers that form on a
scattered basis with little or no apparent organization, usually
during the afternoon in response to diurnal heating.
positive feedback
An interaction that amplifies the response of the
system in which it is incorporated.
positive vorticity advection (PVA)
A region of positive vorticity usually several
hundred kilometers wide on an upper-level chart that moves with the
general wind flow. It aids in weather prediction by showing where
regions of rising air, clouds, and storms are likely to form.
potential density
The potential density is the density a parcel of
seawater would have if it were at the surface. The potential density
takes into account both the change in volume and the change in
temperature that accompany any change in depth.
potential energy
The energy that a body possesses by virtue of its
position with respect to other bodies in the field of gravity.
potential evapotranspiration (PE)
That amount of moisture that, if it were
available, would be removed from a given land area by evaporation
and transpiration.
potential temperature
The potential temperature is the temperature a
parcel of water would have if it was at the surface. The potential
temperature takes into account the temperature change that occurs
whenever the pressure of the water changes. The potential
temperature is always less than the in situ temperature.
Precambrian
All geologic time before the beginning of the
Paleozoic. It is equivalent to about 90% of geologic time (…4600-590
million years ago).
precession (of the Earth's axis of rotation)
The wobble of the earth's axis of rotation that
traces out the path of a cone over a period of about 23,000 years.
precipitation
Any or all forms of liquid or solid water
particles that fall from the atmosphere and reach the Earth's
surface. It includes drizzle, rain, snow, snow pellets, snow grains,
ice crystals, ice pellets, and hail. The ratio of precipitation to
evaporation is the most important factor in the distribution of
vegetation zones. Precipitation is also defined as a measure of the
quantity, expressed in centimeters or milliliters of liquid water
depth, of the water substance that has fallen at a given location in
a specified amount of time. Deposits of dew, fog, and frost are
excluded.
pressure gradient
The rate of decrease of pressure per unit of
horizontal distance. On the same chart, when the isobars are close
together, the pressure gradient is steep. When the isobars are far
apart, the pressure gradient is weak.
pressure gradient force
The force due to differences in pressure within
the atmosphere that causes air to move and, hence, the wind to blow.
It is directly proportional to the pressure gradient.
pressure tendency
The rate of change of atmospheric pressure within
a specified period of time, most often three hours. Same as
barometric tendency.
prevailing westerlies
The dominant westerly winds that blow in middle
latitudes on the poleward side of the subtropical high-pressure
areas. Also called westerlies.
prevailing wind
The wind direction most frequently observed during
a given period.
primary productivity
See gross primary production and net primary
production.
primitive equations
The Eulerian equations of fluid motion in which
the primary dependent variables are the velocity components of the
fluid. In meteorology, they can be specialized to apply directly to
the cylonic-scale motions.
probability forecast
A forecast of the probability of occurrence of one
or more of a mutually exclusive set of weather conditions.
probability of precipitation
Probability forecasts are subjective estimates of
the chances of encountering measurable precipitation at some time
during the forecast period. Measurable means at least 0.2 mm of rain
or the water equivalent of snow. For example, a 40% probability of
rain today means there are 4 chances in 10 that it will rain.
prognostic chart (prog)
A chart showing expected or forecasted conditions,
such as pressure patterns, frontal positions, contour height
patterns, and so on.
prominence
See solar flare.
proterozoic
The later part of Precambrian time (2500-590
million years ago).
proxy climate indicators
Dateable evidence of a biological or geological
phenomenon whose condition, at least in part, is attributable to
climatic conditions at the time of its formation. Proxy data are any
material that provides an indirect measure of climate and include
documentary evidence of crop yields, harvest dates, glacier
movements, tree rings, varves, glaciers and snow lines, insect
remains, pollen remains, marine microfauna, isotope measurements:
18O, in ice sheets, 18O, 2H, and
13C in tree rings; CaCO3 in sediments; and
speleothems. There are three main problems in using proxy data: (1)
dating, (2) lag and response time, and (3) meteorological
interpretation. Tree rings, pollen deposits from varved lakes, and
ice cores are the most promising proxy data sources for
reconstructing the climate of the last five millennia because the
dating are precise on an annual basis while other proxy data sources
may only be precise to +/- 100 years.
psychrometer
An instrument used to measure the water vapor
content of the air. It consists of two thermometers (dry bulb and
wet bulb). After whirling the instrument, the dew point and relative
humidity can be obtained with the aid of tables.
pycnocline
A region in the ocean where the water density
increases rapidly with depth. A pycnocline acts as a strong barrier
to vertical mixing.
pyrgeometer
An instrument that measures radiation from the
earth's surface into space.
pyroclastic
Pertaining to clastic rock (broken fragments of
existing rock) material formed by volcanic explosions.
pyroxenes
A group of rock-forming silicate minerals composed
of single chains of silicon-oxygen tetrahedra (in contrast to
amphibole which is made of double chains).
quartz
An important rock-forming silicate mineral
composed of silicon-oxygen tetrahedra joined in a three-dimensional
network. Quartz forms the major proportion of most sands and is
widely distributed in igneous, metamorphic and sedimentary rocks.
Quaternary period
The latest period of geologic time, covering the
most- recent 2,000,000 years of the Earth's history. It is divided
into two epochs: the Pleistocene (2 million years ago to
approximately 10,000 years ago) and the Holocene (the period from
approximately 10,000 years ago to the present). The Quaternary
period is the artificial division of time separating pre-human and
human periods. It contains five ice ages and four interglacial ages,
and temperature indicators seem to show sharp and abrupt changes by
several degrees.
radar
An electronic instrument used to detect objects
such as falling precipitation by their ability to reflect and
scatter microwaves back to the receiver. It is an acronym that
stands for "RAdio Detecting And Ranging".
radial velocity
Component of motion toward or away from a given
location. As "seen" by Doppler radar, it is the component of motion
parallel to the radar beam. (The component of motion perpendicular
to the beam cannot be seen by the radar. Therefore, strong winds
blowing strictly from left to right or from right to left, relative
to the radar, cannot be detected.).
radiant energy (radiation)
Energy propagated in the form of electromagnetic
waves. These waves do not need molecules to propagate them, and in a
vacuum they travel at nearly 300,000 km per sec.
radiant flux density
The total flow of radiation received on a unit
area of a given real or imaginary surface. Also called the
irradiance.
radiation balance
The difference between the absorbed solar
radiation and the net infrared radiation. Experimental data show
that radiation from the earth's natural surfaces is rather close to
the radiation from a black body at the corresponding temperature;
the ratio of the observed values of radiation to black body
radiation is generally 0.90-1.0.
radiation fog
Fog produced over land when radiational cooling
reduces the air temperature to or below its dew point. It is also
known as ground fog and valley fog.
radiation inversion
An increase in temperature with height due to
radiational cooling of the earth's surface. Also called a nocturnal
inversion.
radiational cooling
The process by which the earth's surface and
adjacent air cool by emitting infrared radiation.
radiative equilibrium temperature
The temperature achieved when an object, behaving
as a black body, is absorbing and emitting radiation at equal rates.
radiative-convective models
Thermodynamic models that determine the
equilibrium temperature distribution for an atmospheric column and
the underlying surface, subject to prescribed solar radiation at the
top of the atmosphere and prescribed atmospheric composition and
surface albedo. Submodels for the transfer of solar and terrestrial
radiation, the heat exchange between the earth's surface and
atmosphere, the vertical redistribution of heat within the
atmosphere, the atmospheric water vapor content and clouds are
included in these one-dimensional models. Abbreviated as RCM.
radiatively active gases
Gases that absorb incoming solar radiation or
outgoing infrared radiation thus affecting the vertical temperature
profile of the atmosphere. Most frequently being cited as being
radiatively active gases are water vapor, CO2, methane, nitrous
oxide, chlorofluorocarbons, and ozone.
radioisotope dating
Precise method for calculating the age of geologic
materials by measuring the relative percentages of parent and
daughter isotopes of a given radiometric element.
radiometer
See Infrared radiometer.
radiosonde
An instrument lifted into the air via balloon that
measures various atmospheric parameters such as temperature,
pressure wind and humidity, up to a height of approximately 30,000
meters (100,000 feet). The height of each pressure level of the
observation is computed from data received via radio signals.
rain
Precipitation in the form of liquid water drops
that have diameters greater than that of drizzle.
rain gauge
An instrument designed to measure the amount of
rain that falls during a given time interval.
rain shadow
The region on the leeside of a mountain where the
precipitation is noticeably less than one on the windward side.
rainbow
Rainbows occur when sunlight is refracted and then
reflected by raindrops. The raindrops act like a prism, breaking the
light into the colors of a rainbow, with red on the outer, and blue
on the inner edge. On occasion the light can be reflected from both
the front and back of the raindrops and two rainbows are visible,
with the color bands in the second opposite to those in the primary
rainbow. Rainbows can be seen when the sun is shining and the air
contains water spray or raindrops. This condition occurs frequently
during or immediately following showers. Rainbows are always
observed in the portion of the sky opposite the sun. The sun, the
observer's eye, and the centre of the rainbow arc always fall on a
straight line.
rawinsonde observation
A radiosonde observation that includes wind data.
recharge
The process by which water is added to a reservoir
or zone of saturation, often by runoff or percolation from the soil
surface.
recharge area
A geographic area where water is absorbed and
moves downward toward the water table. The area that acts as a
catchment area.
reflection
The process whereby a surface turns back a portion
of the radiation that strikes it.
reflectivity
The ratio of the energy carried by a wave that is
reflected from a surface to the energy of a wave incident on the
surface.
refraction
The change in direction of a wave as it passes
from one medium to another of different density, which changes its
velocity.
refractive index
The ratio of the velocity of light in two
substances of different density.
regolith
The term used for the layer of unconsolidated
weathered material, which includes mineral grains, rock fragments
and other superficial deposits which lie above the solid bedrock.
relative humidity
A ratio, expressed in percent, of the amount of
atmospheric moisture present relative to the amount that would be
present if the air were saturated. Since the latter amount is
dependent on temperature, relative humidity is a function of both
moisture content and temperature. As such, relative humidity by
itself does not directly indicate the actual amount of atmospheric
moisture present. See dew point.
relative sea level
The height of the boundary between sea and air as
measured in relationship to a fixed reference point on land.
reservoir
Any natural or artificial holding area used to
store, regulate, or control a substance.
residence time
The size of any specific reservoir or pool of mass
(e.g., carbon) divided by the total flux of mass into or out of that
pool.
respiration
A biochemical process by which living organisms
take up oxygen from the environment and consume organic matter,
releasing both carbon dioxide and heat.
return stroke
The luminous lightning stroke that propagates
upward from the earth to the base of a cloud.
Revelle factor
The ratio of the instantaneous fractional change
in the partial pressure of CO2 (pCO2) exerted by seawater to the
fractional change in total CO2 dissolved in the ocean waters. The
buffer factor relates the partial pressure of CO2 in the ocean to
the total ocean CO2 concentration at constant temperature,
alkalinity and salinity. The Revelle factor is a useful parameter
for examining the distribution of CO2 between the atmosphere and the
ocean, and measures in part the amount of CO2 that can be dissolved
in the mixed surface layer.
rhyolite
A fine-grained volcanic rock composed of quartz,
K-feldspar and plagioclase. It is the extrusive equivalent of
granite.
ridge
(meteorology) An elongated area of relatively high
atmospheric pressure; the opposite of trough.
rift valley
A valley that has formed along a rift where
tensional forces have pulled the crust apart.
rift zone
A system of crustal fractures arising from
tensional forces.
rifting
The splitting apart of continental masses. A rift
is a long, narrow continental trough that is bounded by normal
faults. It marks a zone along which the entire thickness of the
lithosphere has ruptured under extension. The great continent
Panagea was split apart by rifting.
rime ice
A white, granular deposit of ice formed by the
freezing of water drops when they come in contact with an object.
riming
See accretion.
ring of fire
The "ring of fire" refers to the regions of the
Earth's surface where the most volcanic and earthquake activity
occurs. This zone marks both divergent plate boundaries (where the
tectonic plates are moving apart), convergent plate boundaries
(where the plates are coming together and one plate being subducted
under the other) and transform fault boundaries (where two plates
are sliding past each other). These regions encircle the globe
forming a "ring".
rock
An aggregate of one or more minerals that forms an
appreciable part of the lithosphere.
rocketsonde
A rocket-borne instrument for measurement and
transmission of upper-air meteorological data in the lower 76,000
meters (250,000 feet) of the atmosphere, especially that portion
inaccessible to radiosonde techniques.
roll cloud
A dense, roll-shaped cloud attached to the lower
front part of the main cloud. It often forms with thunderstorms
along the leading edge of a gust front. Also called an arcus cloud.
Rossby waves
See Longwaves in the westerlies.
rotor cloud
A turbulent cumuliform type of cloud that forms on
the leeward side of large mountain ranges. The air in the cloud
rotates about an axis parallel to the range.
rotors
Turbulent eddies that form downwind of a mountain
chain, creasing hazardous flying conditions.
runoff
That part of precipitation, snowmelt, or
irrigation water that flows from the land to streams or other
surface waters without infiltrating the ground.
Saffir-Simpson scale
A scale relating a hurricane's central pressure
and winds to the possible damage it is capable of inflicting.
salinity
The total weight of inorganic salts dissolved in
seawater. Usually expressed on a weight basis, the average salinity
of ocean water is 34.5 grams per kilogram (= 34.5 parts per thousand
(ppt)).
salt water intrusion
The invasion of fresh, surface, or groundwater by
salt water.
saltation
The bouncing movement of sand and small particles
along the surface due to the wind.
sand
Sedimentary material composed of fragments ranging
in diameter from 0.0625 mm to 2 mm. Sand particles are larger than
silt particles but smaller than pebbles. Much sand is composed of
quartz grains because quartz is abundant and resists chemical and
mechanical disintegration, however other materials such as shell and
rock fragments can also form sand.
sandstone
A sedimentary rock composed mostly of sand-sized
particles, usually cemented by calcite, silica, or iron oxide.
sand dunes
A warm, dry wind that blows into southern
California from the east off the elevated desert plateau. Its warmth
is derived from compressional heating.
satellite photo
A photograph of the earth taken by satellites.
saturated zone
Also called the phreatic zone. Soil or rock zone
below the level of the water table in which pores are completely
filled with groundwater.
saturation (of air)
An atmospheric condition whereby the water vapor
is the maximum possible at the existing temperature and pressure.
saturation vapor pressure
The maximum amount of water vapor necessary to
keep moist air in equilibrium with a surface of pure water or ice.
It represents the maximum amount of water vapor that the air can
hold at any given temperature and pressure. (See Equilibrium vapor
pressure.)
savanna
A tropical or subtropical region of grassland and
drought-resistant vegetation. Typically found in tropical
wet-and-dry climates.
scales of motion
The hierarchy of atmospheric circulations from
tiny gusts to giant storms.
scattering
The process by which small particles in the
atmosphere deflect radiation from its path into different
directions.
scintillation
The apparent twinkling of a star due to its light
passing through regions of differing air densities in the
atmosphere.
sea breeze
Sea breezes occur during the daytime in warm sunny
weather when the air over a land is heated more rapidly than that
over an adjacent water surface. As a result, the warmer air rises
and relatively cool air from the sea flows onshore to replace it. At
night, the air over the land cools faster than that over the nearby
ocean and causes the air circulation to be in the opposite direction
a land breeze. Land breezes are usually weaker than sea breezes and
have a less noticeable effect upon the temperature.
sea breeze convergence zone
A region where sea breezes, having started in
different regions, flow together and converge.
sea level pressure
The atmospheric pressure at mean sea level.
sea surface temperature (SST)
The temperature of the layer of seawater
(approximately 0.5 m deep) nearest the atmosphere.
sea surface temperature anomalies
Temperature of emitted energy from the sea
surface. SST anomaly = (SST minus SST mean).
seafloor spreading
The lateral movement of the seafloor away from the
oceanic ridge as new lithosphere is created along the crest of the
ridge by igneous activity.
seamount
An isolated, conical mound of volcanic origin
rising more than 1000 m above the ocean floor.
seasonal thermocline
A region of rapidly changing temperature at the
ocean surface that forms in temperature latitudes during the summer
months
seasonal variation
The change in a set of meteorological parameters
averaged over three months. Seasonal variation is the largest
climatic variation, and temperature is the most frequently observed
meteorological parameter. Often, monthly averaged data are grouped
into seasons, according to the prescribed definition.
secchi disk (secchi disc)
Secchi Disc (Disk Transparency) is a measure of
the clarity of the water, and an accurate method for estimating lake
water quality. A black and white disk - secchi disc (or disk) - is
lowered into the water until it just disappears from sight -- this
depth measurement is recorded. The deeper the measurement, the
clearer the water. Secchi disk measurements give a general
indication of problems with algae, zooplankton, water color and
silt.
secular carbon dioxide trend
The fairly uniform and accelerating increase of
carbon dioxide concentration in the atmosphere, as illustrated by
the Mauna Loa record. The secular trend reflects the increase in
global atmospheric carbon dioxide concentrations produced by
combustion of fossil fuels, kilning of limestone, and possibly a net
biospheric release of carbon dioxide resulting from deforestation.
sediment
Solid fragments of material (such as gravel, sand
and mud) originating from the weathering of rocks or as skeletal
remains of marine organisms.
sedimentary rock
Rock formed by the accumulation and consolidation
of sediment.
seiches
Standing waves that oscillate back and forth over
an open body of water.
selective absorbers
Substances such as water vapor, carbon dioxide,
clouds, and snow that absorb radiation only at particular
wavelengths.
semi-arid climate
A dry climate where potential evaporation and
transpiration exceed precipitation. Not as dry as the arid climate.
Typical vegetation is short grass.
semipermanent highs and lows
Areas of high pressure (anticyclones) and low
pressure (extratropical cyclones) that tend to persist at a
particular latitude belt throughout the year. In the Northern
Hemisphere, typically they shift slightly northward in summer and
slightly southward in winter.
sensible heat
Heat (energy) input or output that results in a
change in temperature of a substance.
sensible heat loss
In meteorology, sensible heat loss is used to
describe the excess radiative energy that has passed from the
Earth's surface to the atmosphere through advection, conduction, and
convection processes.
sensible temperature
The sensation of temperature that the human body
feels in contrast to the actual temperature of the environment as
measured with a thermometer.
severe thunderstorm
A thunderstorm which produces tornadoes, large
hail, or winds of 50 knots (80 kph) or more. Structural wind damage
may imply the occurrence of a severe thunderstorm.
sferics
Radio waves produced by lightning. A contraction
of atmospherics.
shale
A fine-grained clastic sedimentary rock formed by
the consolidation of clay and mud.
shear
See wind shear.
sheet lightning
A fairly bright lightning flash from distant
thunderstorms that illuminates a portion of the cloud.
shelf break
An abrupt change in slope, marking the boundary
between the continental shelf and the continental slope.
shelterbelt
A belt of trees or shrubs arranged as a protection
against strong winds.
shortwave (in the atmosphere)
A small wave that moves around longwaves in the
same direction as the air flow in the middle and upper troposphere.
Shortwaves are also called shortwave troughs.
shortwave radiation
A term most often used to describe the radiant
energy emitted from the sun, in the visible and near ultraviolet
wavelengths.
shower
Intermittent precipitation from a cumuliform
cloud, usually of short duration but often heavy.
Siberian high
A strong, shallow area of high pressure that forms
over Siberia in winter.
signal-to-noise ratio
A quantitative measure of the statistical
detectability of a signal, expressed as a ratio of the magnitude of
the signal relative to the variability. For first detection of a CO2-induced
climate change, the model signal is the mean change or anomaly in
some climatic variable, usually surface air temperature, attributed
by a numerical model to increased concentrations of carbon dioxide.
Observed noise is the standard deviation or natural variability
computed from observations of that variable and adjusted for sample
size, autocorrelation, and time averaging.
silica
silicon dioxide (SiO2)
silicate
A mineral containing silicon and oxygen (SiO4),
where 4 oxygen atoms surround 1 silicon atom.
siliceous
Said of a rock containing abundant silica (silicon
dioxide, SiO2).
siltstone
A fine-grained clastic sedimentary rock composed
mostly of silt-size particles.
Silurian
A period of the Paleozoic era (438-408 million
years ago). The Silurian follows the Ordovician and precedes the
Devonian.
sinuous
Having many curves or turns, winding.
sleet
A type of precipitation consisting of transparent
pellets of ice 5 mm or less in diameter. Same as ice pellets.
slumps
A slump is a type of mass movement in which
material moves along a curved surface of rupture.
smog
Originally smog meant a mixture of smoke and fog.
Today, smog means air that has restricted visibility due to
pollution, or pollution formed in the presence of sunlight -
photochemical smog.
smog front (also smoke front)
The leading edge of a sea breeze that is
contaminated with smoke or pollutants.
smoke
Particles suspended in air after incomplete
combustion of materials.
smudge pots
See orchard heaters.
snow
A solid form of precipitation composed of ice
crystals in complex hexagonal form.
snow flurries
Light showers of snow that fall intermittently.
snow grains
Precipitation in the form of very small, opaque
grains of ice. The solid equivalent of drizzle.
snow pellets
White, opaque, approximately round ice particles
between 2 and 5 mm in diameter that form in a cloud either from the
sticking together of ice crystals or from the process of accretion.
snow rollers
A cylindrical spiral of snow shaped somewhat like
a child's muff and produced by the wind.
snow squall (shower)
An intermittent heavy shower of snow that greatly
reduces visibility.
snowflake
An aggregate of ice crystals that falls from a
cloud.
solar constant
The rate at which solar energy is received on a
surface at the outer edge of the atmosphere perpendicular to the
sun's rays when the earth is at a mean distance from the sun. The
value of the solar constant is about two calories per square
centimeter per minute or about 1375 W m-2 in the SI
system of measurement.
solar cycle
The periodic change in sunspot numbers. It is the
interval between successive minima and is about 11.1 years.
solar flare
A rapid eruption from the sun's surface that emits
high energy radiation and energized charged particles.
solar wind
An outflow of charged particles from the sun that
escapes the sun's outer atmosphere at high speed.
solute effect
The dissolving of hygroscopic particles, such as
salt, in pure water, thus reducing the relative humidity required
for the onset of condensation.
sonic boom
A loud explosive-like sound caused by a shock wave
emanating from an aircraft (or any object) traveling at or above the
speed of sound.
sounding
(meteorology) A plot of the vertical profile of
temperature and dew point (and often winds) above a fixed location.
Soundings are used extensively in weather forecasting, e.g., to
determine instability, locate temperature etc.
source regions
Regions where air masses originate and acquire
their properties of temperature and moisture.
Southern Oscillation
A large-scale atmospheric and hydrospheric
fluctuation centered in the equatorial Pacific Ocean. It exhibits a
nearly annual pressure anomaly, alternatively high over the Indian
Ocean and high over the South Pacific. Its period is slightly
variable, averaging 2.33 years. The variation in pressure is
accompanied by variations in wind strengths, ocean currents,
sea-surface temperatures, and precipitation in the surrounding
areas. El Nino occurrences are associated with the phenomenon.
Southern Oscillation Index (SOI)
An indicator based on the pressure gradient
between the quasi-stationary low pressure region and the center of a
subtropical high pressure cell. A positive index corresponds to an
anomalously high pressure difference between the two centers of
action.
specific heat
The ratio of the heat absorbed (or released) by
the unit mass of the system to the corresponding temperature rise
(or fall).
specific humidity
The ratio of the mass of water vapor in a given
parcel to the total mass of air in the parcel.
spontaneous (homogeneous) nucleation
(freezing)
The freezing of pure water without the benefit of
any nuclei.
spreading zone
The area around the mid-ocean ridge where the
newly formed crust is moving away from the center axis of the ridge
at rates of 1-10 centimeters per year.
squall line
A solid or nearly solid line or band of active
thunderstorms.
St. Elmo's fire
A bright electric discharge that is projected from
objects (usually pointed) when they are in a strong electric field,
such as during a thunderstorm.
stable air
See absolutely stable air.
standard atmosphere
A hypothetical vertical distribution of
atmospheric temperature, pressure, and density in which the air is
assumed to obey the gas law and the hydrostatic equation. The lapse
rate of temperature in the troposphere is taken at 6.5oC/1000 m or
3.6oF/1000 ft.
standard atmospheric pressure
A pressure of 1013.25 millibars (mb), 29.92 inches
of mercury (Hg), 760 millimeters (mm) of mercury, 14.7 pounds per
square inch (lb in-2), 101,325 pascals (Pa).
station pressure
The actual air pressure computed at the observing
station.
stationary front
A front that is nearly stationary with winds
blowing almost parallel and from opposite directions on each side of
the front.
statistical-dynamical models
Computer programs that calculate simplified
climate models based on versions of the conservation equations that
have been averaged over longitude, with the effects of the synoptic
eddies parameterized statistically in the meridional plane.
steady-state forecast
A weather prediction based on the past movement of
surface weather systems. It assumes that the systems will move in
the same direction and at approximately the same speed as they have
been moving. Also called trend forecasting.
steam fog
See evaporation (mixing) fog.
steering winds (or steering currents)
A prevailing synoptic scale flow which governs the
movement of smaller features embedded within it.
steppe
An area of grass-covered, treeless plains that has
a semi-arid climate.
stepped leader
An initial discharge of electrons that proceeds
intermittently toward the ground in a series of steps in a
cloud-to-ground lightning stroke.
steric height
The mean dynamic depth (or height) of the ocean
for the month minus the annual mean dynamic depth for the same
isobaric reference level.
storm surge
An abnormal rise of the sea along a shore;
primarily due to the winds of a storm, especially a hurricane.
strata
(Geology) A tabular or sheet-like layer of
sedimentary rock that is visually different from other layers above
and below.
stratification
Separating into layers or strata.
stratiform
Having extensive horizontal development, as
opposed to the more vertical development characteristic of
convection. Stratiform clouds cover large areas but show relatively
little vertical development. Stratiform precipitation, in general,
is relatively continuous and uniform in intensity (i.e., steady rain
versus rain showers).
stratocumulus
Low-level clouds, existing in a relatively flat
layer but having individual elements. Elements often are arranged in
rows, bands, or waves. Stratocumulus often reveals the depth of the
moist air at low levels, while the speed of the cloud elements can
reveal the strength of the low-level jet.
stratosphere
The region of the upper atmosphere extending from
the tropopause (8 to 15 km altitude) to about 50 km. The thermal
structure is determined by its radiation balance and is generally
very stable with low humidity.
stratospheric polar night jet
A jet stream that forms near the top of the
stratosphere over polar latitudes during the winter months.
stratus
A low, generally gray cloud layer with a fairly
uniform base. Stratus may appear in the form of ragged patches, but
otherwise does not exhibit individual cloud elements as do cumulus
and stratocumulus clouds. Fog usually is a surface-based form of
stratus.
streamline
A line that shows the wind flow pattern.
striations
One of multiple scratched lines, usually parallel,
inscribed on a rock surface by a glacier.
strike
The bearing (compass direction) of a horizontal
line on a bedding plane, a fault plane, or some other planar
structural feature.
strike slip
A fault in which the movement has occurred
parallel to the strike of the fault.
subduction
The subsidence of the leading edge of a
lithospheric plate down into the mantle.
subduction zone
An elongate zone in which one lithospheric plate
descends below another. A subduction zone is typically marked by an
oceanic trench, lines of volcanoes and crustal deformation
associated with mountain building.
sublimation
The process whereby ice changes directly into
water vapor without melting.
submarine canyon
A V-shaped trench or valley with steep sides cut
into a continental shelf or continental slope.
subpolar climate
A climate observed in the Northern Hemisphere that
borders the polar climate. It is characterized by severely cold
winters and short, cool summers. Also known as taiga climate and
boreal climate.
subpolar low
A belt of low pressure located between 50° and 70°
latitude. In the Northern Hemisphere, this "belt" consist of the
Aleutian low in the North Pacific and the Icelandic low in the North
Atlantic. In the Southern Hemisphere, it exists around the periphery
of the Antarctic continent.
subsidence
(meteorology) Sinking (downward) motion in the
atmosphere, usually over a broad area ~ it often implies clearing
skies.
subsidence inversion
A temperature inversion produced by compressional
warming -- the adiabatic warming of a layer of sinking air.
subtropical front
A zone of temperature transition in the supper
troposphere over subtropical latitudes, where warm air carried
poleward by the Hadley cell meets the cooler air of the middle
latitudes.
subtropical high
A semipermanent high in the subtropical
high-pressure belt centered near 30° latitude. The Bermuda high is
located over the Atlantic Ocean off the east coast of North America.
The Pacific high is located off the west coast of North America.
subtropical jet stream
The jet stream typically found between 20° and 30°
latitude at altitudes between 12 and 14 km.
suction vortices
Small, rapidly rotating whirls perhaps 10 m in
diameter that are found within large tornadoes.
Suess effect
The relative change in the 14C/C or
13C/C ratio of any carbon pool or reservoir caused by the
addition of fossil- fuel CO2 to the atmosphere. Fossil
fuels are devoid of 14C because of the radioactive decay
of 14C to 14N during long underground storage
and are depleted in 13C because of isotopic fractionation
eons ago during photosynthesis by the plants that were the
precursors of the fossil fuels. Carbon dioxide produced by the
combustion of fossil fuels is thus virtually free of 14C
and depleted in 13C. The term Suess effect originally
referred to the dilution of the 14C/C ratio in
atmospheric CO2 by the admixture of fossil-fuel produced
CO2, but the definition has been extended to both the
14C and 13C ratios in any pool or reservoir of
the carbon cycle resulting from human disturbances.
summer solstice
Approximately June 22 in the Northern Hemisphere
when the sun is highest in the sky and directly overhead at latitude
23½°N, the Tropic of Cancer.
sun pillar
A vertical streak of light extending above (or
below) the sun. It is produced by the reflection of sunlight off ice
crystals.
sundog
A colored luminous spot produced by refraction of
light through ice crystals that appears on either side of the sun.
Also called parhelia.
sunspot
A relatively dark, sharply defined region on the
solar disk, marked by an umbra approximately 2000K cooler than the
effective photospheric temperature, surrounded by a less dark but
also sharply bounded penumbra. The average spot diameter is about
3700 km, but can range up to 245,000 km. Most sunspots are found in
groups of two or more, but they can occur singly. Sunspots are
cyclic, with a period of approximately 11 years. The quantitative
description of sunspot activity is called the Wolf sunspot number,
denoted R. The Wolf sunspot number is also referred to as Wolfer
sunspot number, Zurich relative sunspot number, or relative sunspot
number.
supercell storm
An enormous severe thunderstorm whose updrafts and
downdrafts are nearly in balance, allowing it to maintain itself for
several hours. It can produce large hail and tornadoes.
supercooled cloud (or cloud droplets)
A cloud composed of liquid droplets at
temperatures below 0° C (32° F). When the cloud is on the ground it
is called supercooled fog or cold fog.
superior mirage
See mirage.
supernova
A tremendous explosion of a massive star.
supersaturated air
A condition that occurs in the atmosphere when the
relative humidity is greater than 100 percent.
surface air temperature
The temperature of the air near the surface of the
Earth, usually determined by a thermometer in an instrument shelter
about 2 m above the ground. The true daily mean, obtained from a
thermograph, is approximated by the mean of 24 hourly readings and
may differ by 1.0 degrees C from the average based on minimum and
maximum readings. The global average surface air temperature is 15
degrees C.
surface albedo
The fraction of solar radiation incident on the
Earth's surface that is reflected by it. Reflectivity varies with
ground cover, and during the winter months it varies greatly with
the amount of snow cover (depth and areal extent). Roughness of
terrain, moisture content, solar angle, and angular and spectral
distribution of ground- level irradiations are other factors
affecting surface albedo.
surface current
The horizontal movement of seawater at the sea
surface.
surface inversion
See radiation inversion.
surface tension
A condition that exists on the free surface of
water or other liquid by reason of intermolecular forces
unsymmmetrically disposed around the surface molecules, tending to
make the surface layer behave like an elastic membrane.
surface water
All water naturally open to the atmosphere.
swamp
A type of wetland that is dominated by woody
vegetation and does not accumulate appreciable peat deposits; it may
be fresh- or saltwater, and tidal or non-tidal.
symbiotic
Literally "living together". Symbiosis is the
living together in intimate association of two dissimilar organisms.
Reef-forming corals are a good example of symbiosis in which
dinoflagellate algae (called zooxanthellae) reside within the
tissues of the host animal (a colonial anthozoan).
syncline
A fold in which the limbs dip toward the axis
(concave upward). After erosion, the youngest beds are exposed in
the central core of the field. Contrast with anticline.
synoptic scale
The typical weather map scale that shows features
such as high- and low-pressure areas and fronts over a distance
spanning a continent. Also called the cyclonic scale.
synoptic scale (or large scale)
Size scale referring generally to weather systems
with horizontal dimensions of several hundred miles or more. Most
high and low pressure areas seen on weather maps are synoptic-scale
systems. Compare with mesoscale.
taiga (boreal forest)
The open northern part of the coniferous forest.
Taiga also refers to subpolar climate.
tangent arc
An arc of light tangent to a halo. IT forms by
refraction of light through ice crystals.
Tcu
An abbreviation sometimes used to denote a
towering cumulus cloud (cumulus congestus).
plate tectonics
A theory that explains the global distribution of
geological phenomena such as seismicity, volcanism, continental
drift, and mountain building in terms of the formation, destruction,
movement, and interaction of the earth's lithospheric plates. The
dynamics of plate movement.
A broad segment of the lithosphere (including the rigid upper
mantle, plus the oceanic and continental crust) that floats on the
underlying asthenosphere and moves independently of the other
plates.
temperature
A measure of the internal energy in a molecule. In
general, the degree of hotness or coldness measured against some
definite scale by means of a thermometer. Common scales are Celcius,
Fahrenheit and Kelvin.
tephra
Any rock material produced by a volcano.
terminal velocity
The constant speed obtained by a falling object
when the upward drag on the object balances the downward force of
gravity.
terrestrial radiation
The total infrared radiation emitted by the Earth
and its atmosphere in the temperature range of approximately
200-300K. Because the Earth is nearly a perfect radiator, the
radiation from its surface varies as the fourth power of the
surface's absolute temperature. Terrestrial radiation provides a
major part of the potential energy changes necessary to drive the
atmospheric wind system and is responsible for maintaining the
surface air temperature within limits for livability.
Tertiary
The first period of Cenozoic era (65-2 million
years ago) after the Cretaceous of the Mesozoic era and before the
Quaternary. The Tertiary is divided into five epochs: The Paleocene,
Eocene, Oligocene, Miocene and Pliocene.
Texas norther
A strong, cold wind from between the northeast and
northwest associated with a cold outbreak of polar air that brings a
sudden drop in temperature. Sometimes called a blue norther.
theodolite
An instrument used to track the movements of a
pilot balloon.
thermal
A small, rising parcel of warm air produced when
the earth's surface is heated unevenly.
thermal belts
Horizontal zones of vegetation found along
hillsides that are primarily the result of vertical temperature
variations.
thermal circulations
Air flow resulting primarily from the heating and
cooling of air.
thermal lows and thermal highs
Areas of low and high pressure that are shallow in
vertical extent and are produced primarily by surface temperatures.
thermal tides
Atmospheric pressure variations due to the uneven
heating of the atmosphere by the sun.
thermal turbulence
Turbulent vertical motions that result from
surface heating and the subsequent rising and sinking of air.
thermistor
An electrical resistance device used in the
measurement of temperature.
thermocline
A transition layer of water in the ocean, with a
steeper vertical temperature gradient than that found in the layers
of ocean above and below. The permanent thermocline separates the
warm mixed surface layer of the ocean from the cold deep ocean
water, and is found between 100- and 1000-m depths. The thermocline
first appears at the 55-60 degree N and S latitudes, where it forms
a horizontal separation between temperate and polar waters. The
thermocline reaches its maximum depth at mid-latitudes and is
shallowest at the equator and at its northern and southern limits.
The thermocline is stably stratified, and transfer of water and
carbon dioxide across this zone occurs very slowly. Thus, the
thermocline acts as a barrier to the downward mixing of carbon
dioxide.
thermodynamics
In general, the relationships between heat and
other properties (such as temperature, pressure, density, etc.) In
forecast discussions, thermodynamics usually refers to the
distribution of temperature and moisture (both vertical and
horizontal) as related to the diagnosis of atmospheric instability.
thermograph
An instrument that measures and records air
temperature.
thermohaline
Refers to the combined effects of temperature and
salinity that contribute to density variations in the oceans.
thermometer
An instrument for measuring temperature.
thermosphere
The atmospheric layer above the mesosphere (above
about 85 km) where the temperature increases rapidly with height.
thunder
The sound due to rapidly expanding gases along the
channel of a lightning discharge.
thunderstorm
Thundershower. A local storm, produced by a
cumulonimbus cloud, and accompanied by thunder and lightning.
tidal marsh
Low, flat marshlands traversed by channels and
tidal hollows and subject to tidal innundation; normally, the only
vegetation present are salt-tolerant bushes and grasses.
topography
The general shape and form of the land surface.
tornado
A tornado appears as a violent funnel-shaped wind
vortex in the lower atmosphere with upward spiralling winds of high
speeds spawned by severe thunderstorms. The tornado usually appears
from a bulge in the base of a cumulonimbus cloud. It has a typical
width of tens to hundreds of meters and a lifespan of minutes to
hours. In area, it is one of the least extensive of all storms, but
in violence, it is the world's most severe. More tornadoes occur in
the United States than in any other country.
tornado outbreak
A series of tornadoes that forms within a
particular region -- a region that may include several states. Often
associated with widespread damage and destruction.
tornado vortex signature (TVS)
An image of a tornado on the Doppler radar screen
that shows up as a small region of rapidly changing wind speeds
include a mesocyclone.
tornado warning
A warning issued when a tornado has actually been
observed either visually or on a radar screen. It is also issued
when the formation of tornadoes is imminent.
tornado watch
A forecast issued to alert the public that
tornadoes may develop within a specified area.
towering cumulus
A large cumulus cloud with great vertical
development, usually with a cauliflower-like appearance, but lacking
the characteristic anvil shaped top of a Cb. (Often shortened to
"towering cu," and abbreviated TCU.)
trace (of precipitation)
An amount of precipitation less than 0.01 inch
(0.025 cm).
trade wind inversion
A temperature inversion frequently found in the
subtropics over the eastern portions of the tropical oceans.
trade winds
The winds that occupy most of the tropics and blow
from the subtropical highs to the equatorial low.
transform fault
A special type of strike-slip fault forming the
boundary between two moving lithospheric plates, usually along an
offset segment of the oceanic ridge.
transient tracers
Chemical elements (often radioactive) or compounds
that have finite lifetimes. Atmospheric testing of nuclear weapons
from the mid-1950s to the early 1960s released large quantities of
radionuclides to the atmosphere. Atmosphere-ocean exchange processes
have transferred some of these elements to the oceans. Studying the
behavior and distribution of these specific isotopes and other
chemical tracers in the ocean will provide information on: residence
times of the water and its dissolved components in gyres, basins,
etc.; the mode and rate of formation and the subsequent spreading
rates of specific water types, such as the polar water of the
Norwegian and Greenland Seas; deep-ocean circulation and ocean-
mixing processes, such as advection and upwelling; and the flux of
anthropogenic carbon dioxide into the ocean through its correlation
with several different transient tracers.
transpiration
The process in plants by which water is taken up
by the roots and released as water vapor by the leaves. The term can
also be applied to the quantity of water thus dissipated.
tree rings
Annual growth increments of trees that indicate,
among other factors, the climatic conditions that enhance or limit
growth. Tree ring widths and indexes have been used to search for
solar-terrestrial relationships and climatic cycles and to
reconstruct past climates. See also dendroclimatology and
dendrochronology.
trench
A narrow, elongate depression of the deep ocean
floor oriented parallel to the trend of a continent or an island
arc. Ocean trenches are usually about 2 km deeper than the
surrounding ocean floor, and may be thousands of kilometers long.
triassic
The first period of the Mesozoic era (248-213
million years ago) after the Permian of the Paleozoic era and before
the Jurassic.
triple junction
A point where three lithospheric plates meet.
trophic level
A segment of the food chain in which all organisms
obtain food and energy in, basically, the same manner (e.g.,
photosynthesis, herbivory, or carnivory) and in which all organisms
are the same number of links from the photosynthetic segment.
tropical depression
A mass of thunderstorms and clouds generally with
a cyclonic wind circulation of between 20 and 34 knots.
tropical disturbance
An organized mass of thunderstorms with a slight
cyclonic wind circulation of less than 20 knots.
tropical easterly jet
A jet stream that forms on the equatorward side of
the subtropical highs near 15 km.
tropical monsoon climate
A tropical climate with a brief dry period of
perhaps one or two months.
tropical rain forest
A type of forest consisting mainly of lofty trees
and a dense undergrowth near the ground.
tropical storm
Organized thunderstorms with a cyclonic wind
circulation between 35 and 64 knots.
tropical wet climate
A tropical climate with sufficient rainfall to
produce a dense tropical rain forest.
tropical wet-and-dry climate
A tropical climate poleward of the tropical wet
climate where a distinct dry season occurs, often lasting for two
months or more.
tropopause
The upper boundary of the troposphere, usually
characterized by an abrupt change in temperature with height from
positive (decreasing temperature with height) to neutral or negative
(temperature constant or increasing with height).
tropopause jets
Jet streams found near the tropopause, such as the
polar front and subtropical jet streams.
troposphere
The layer of the atmosphere from the earth's
surface up to the tropopause, characterized by decreasing
temperature with height. It's the layer of the atmosphere where most
of the weather occurs.
trough
(meteorology) An elongated area of relatively low
atmospheric pressure, usually extending from the centre of a low
pressure region. The opposite of ridge.
tufa
A chemical sedimentary rock composed of calcium
carbonate, formed by evaporation as a thin, soft, spongy
encrustation around the mouth of a calcareous spring or seep (not to
be confused with tuff).
tuff
A fine-grained, pyroclastic rock composed of
volcanic ash and dust particles compacted and cemented together (not
to be confused with tufa).
tundra
A type of ecosystem dominated by lichens, mosses,
grasses, and woody plants. It is found at high latitudes (arctic
tundra) and high altitudes (alpine tundra). Arctic tundra is
underlain by permafrost and usually very wet.
turbidity current
A current in air, water or other fluid caused by
differences in the amount of suspended matter (such as mud, silt or
volcanic dust). Marine turbidity currents, laden with suspended
sediment, move rapidly down continental slopes and spread out over
the abyssal floor.
turboclair
A fog-clearing technique that uses hot gases from
jet engines to heat the air and evaporate warm fog.
turbulence
(meteorology) The vertical motion of the air, at
times violent, which can cause the up-and-down movement of a plane,
etc.
turbulent mixing
The physical mixing of water where the pattern of
flow is turbulent, that is, the path of motion is very irregular
with eddies and swirls.
turnover rate
The fraction of the total amount of mass (e.g.,
carbon) in a given pool or reservoir that is released from or that
enters the pool in a given length of time. The turnover rate of
carbon is often expressed as GtC/year.
twilight
The time at the beginning of the day immediately
before sunrise and at the end of the day after sunset when the sky
remains illuminated.
typhoon
A hurricane that forms in the western Pacific
Ocean.
ultraviolet radiation
Electromagnetic radiation with wavelengths longer
than X-rays but shorter than visible light.
unstable air
See Absolutely unstable air.
updraft
A small-scale current of rising air. If the air is
sufficiently moist, then the moisture condenses to become a cumulus
cloud or an individual tower of a towering cumulus or Cb.
upslope flow
Air that flows toward higher terrain, and hence is
forced to rise. The added lift often results in widespread low
cloudiness and stratiform precipitation if the air is stable, or an
increased chance of thunderstorm development if the air is unstable.
upslope fog
Fog formed as moist, stable air flows upward over
a topographic barrier.
upslope precipitation
Precipitation that forms due to moist, stable air
gradually rising along an elevated plain. Upslope precipitation is
common over the western Great Plains, especially east of the Rocky
Mountains.
upstream
Toward the source of the flow, or located in the
area from which the flow is coming.
upwelling
The vertical motion of water in the ocean by which
subsurface water of lower temperature and greater density moves
toward the surface of the ocean. Upwelling occurs most commonly
among the western coastlines of continents, but may occur anywhere
in the ocean. Upwelling results when winds blowing nearly parallel
to a continental coastline transport the light surface water away
from the coast. Subsurface water of greater density and lower
temperature replaces the surface water, and exerts a considerable
influence on the weather of coastal regions. Carbon dioxide is
transferred to the atmosphere in regions of upwelling. This is
especially important in the Pacific equatorial regions, where 1-2
GtC/year may be released to the atmosphere. Upwelling also results
in increased ocean productivity by transporting nutrient-rich waters
to the surface layer of the ocean.
urban heat island
The increased air temperatures in urban areas as
contrasted to the cooler surrounding rural areas.
UV
See ultraviolet.
valley breeze
See mountain breeze.
valley fog
See radiation fog.
vapor
The gaseous phase of substances that are liquid or
solid at atmospheric pressure (e.g., steam).
vapor pressure
The pressure exerted by the water vapor molecules
in a given volume of air.
varve
A layer of sediment deposited in lakes during one
year. Each layer consists of two parts, which are deposited at
different seasons and which differ in color and texture; thus, the
layers can be counted and measured. In a complete series, the number
of layers gives the date on which the ground was vacated by the
retreating ice.
veering winds
Winds which shift in a clockwise direction with
time at a given location (e.g., from southerly to westerly), or
which change direction in a clockwise sense with height (e.g.,
southeasterly at the surface turning to southwesterly aloft).
ventifact
A rock that has been cut, shaped or faceted by
wind-driven particles.
vernal equinox
The equinox at which the sun approaches the
Northern Hemisphere and passes directly over the equator. Occurs
around March 20.
virga
Streaks or wisps of precipitation falling from a
cloud but evaporating before reaching the ground.
viscosity
The resistance of fluid flow. See molecular
viscosity and eddy viscosity.
visibility
The greatest distance an observer can see and
identify prominent objects.
visible radiation (light)
Radiation with a wavelength between 0.4 and 0.7
micrometers.
volume
The amount of space occupied by a
three-dimensional object or region of space, expressed in cubic
units. Amount; quantity: as in a low volume of water. Also an
amplitude or loudness of a sound. A control, as on a radio, for
adjusting amplitude or loudness.
vort max
(Slang; short for vorticity maximum), a center, or
maximum, in the vorticity field of an airmass.
vorticity
A measure of the spin of a fluid, usually small
air parcels. Absolute vorticity is the combined vorticity due to the
earth's rotation and the vorticity due to the air's circulation
relative to the earth. Relative vorticity is due to the curving of
the air flow and wind shear.
Walker cell
A zonal circulation of the atmosphere confined to
equatorial regions and driven principally by the oceanic temperature
gradient. In the Pacific, air flows westward from the colder,
eastern area to the warm, western ocean, where it acquires warmth
and moisture and subsequently rises. A return flow aloft and
subsidence over the eastern ocean complete the cell
wall cloud
An area of rotating clouds that extends beneath a
severe thunderstorm and from which a funnel cloud may appear. Also
called a collar cloud.
warm advection
Transport of warm air into an area by winds.
warm clouds
Clouds that form at temperatures above freezing.
warm front
A front that moves in such a way that warm air
replaces cold air.
warm occlusion
See occluded front.
warm sector
The region of warm air within a wave cyclone that
lies between a retreating warm front and an advancing cold front.
warm-core low
A low-pressure area that is warmer at its center
than at its periphery. Tropical cyclones exhibit this temperature
pattern.
water equivalent
The depth of water that would result from the
melting of a snow sample. Typically about 10 inches of snow will
melt to 1 inch of water, producing a water equivalent of 10 to 1.
water table
The upper surface of groundwater or the level
below an unconfined aquifer that is permenantly saturated with
water.
water vapor
Water present in the atmosphere in gaseous form;
the source of all forms of condensation and precipitation. Water
vapor, clouds, and carbon dioxide are the main atmospheric
components in the exchange of terrestrial radiation in the
troposphere serving as a regulator of planetary temperatures via the
greenhouse effect. Approximately 50 percent of the atmosphere's
moisture lies within about 1.84 km of the earth's surface, and only
a minute fraction of the total occurs above the tropopause.
water vapor feedback
A process in which an increase in the amount of
water vapor increases the atmosphere's absorption of longwave
radiation, thereby contributing to a warming of the atmosphere.
Warming, in turn, may result in increased evaporation and an
increase in the initial water vapor anomaly. This feedback, along
with carbon dioxide, is responsible for the greenhouse effect and
operates virtually continuously in the atmosphere.
watershed
A region drained by or contributing water to a
stream, lake or other body of water. A term used in Great Britain
for a drainage divide.
waterspout
In general, a tornado occurring over water.
Specifically, it normally refers to a small, relatively weak
rotating column of air over water beneath a Cb or towering cumulus
cloud.
wave
(Meteorology) The intersection of warm and cold
fronts.
wave crest
The highest part of a wave.
wave cyclone
An extratropical cyclone that forms and moves
along a front. The circulation of winds about the cyclone tends to
produce a wavelike deformation on the front.
wavelength
The horizontal distance between similar points on
two successive waves, measured perpendicular to the crest.
wave period
The interval of time required for a wave crest to
travel a distance equal to one wavelength; the interval of time
required for two successive wave crests to pass a fixed point.
waves
To move freely back and forth or up and down in
the air and with the wind. Waves are generated on the surface of
water (ocean/lakes) by wind. The wind transfers energy to the water,
through friction between the air molecules and the water molecules.
Waves of water do not move horizontally, they only move up and down
(a wave does not represent a flow of water).
wavetrain
A series of waves.
wave trough
The lowest part of a wave, between two crests.
weather
State of the atmosphere with respect to heat or
cold, wetness or dryness, calm or storm, clearness or cloudiness.
Also, weather is the meteorological day-to-day variations of the
atmosphere and their effects on life and human activity. It includes
temperature, pressure, humidity, clouds, wind, precipitation and
fog.
weather balloon
Large balloons filled with helium or hydrogen and
carry radiosondes (weather instruments) aloft to measure temperature
pressure and humidity as the balloon rises through the air. The
whole contraption is attached to a small parachute so that when the
balloon inevitably breaks, the radiosonde doesn't hurtle back to
earth dangerously quickly.
weather elements
The elements of air temperature, air pressure,
humidity, clouds, precipitation, visibility, and wind that determine
the present state of the atmosphere, the weather.
weather types
Certain weather patterns categorized into similar
groups. Used as an aid in weather prediction.
weight
A unit measure of gravitational force: a table of
weights and measures or a measure of the heaviness of an object. The
force with which a body is attracted to Earth or another celestial
body, equal to the product of the object's mass and the acceleration
of gravity.
wet haze
See haze.
wet-bulb depression
The difference in degrees between the air
temperature (dry-bulb temperature) and the wet-bulb temperature.
wet-bulb temperature
The lowest temperature that can be obtained by
evaporating water into the air.
wetlands
An area that is regularly saturated by surface
water or groundwater and subsequently is characterized by a
prevalence of vegetation that is adapted for life in saturated-soil
conditions.
whirlwinds
See Dust devils.
wind
Air in motion relative to the earth's surface.
wind chill
The combined cooling effect of wind and
temperature is called wind chill. The wind chill factor is a measure
of this cooling effect. The larger the wind chill factor, the faster
the rate of cooling. Note, however, that an object will not be
cooled below the actual air temperature, it will just get there
faster. The rate at which a body will cool depending on temperature,
wind and humidity differences. Expressed as units of energy lost
over a given area (e.g. watts per square metre). In still air one's
body releases heat that stays close to the body so in cold air it
takes a little time for the body to cool down. With wind, the warm
air around one's body is blown away and one cools down more quickly.
At cold temperature and high wind the threat of problems due to
exposure to cold become more severe.
wind direction
The direction from which the wind is blowing.
wind machines
Fans placed in orchards for the purpose of mixing
cold surface air with warmer air above.
wind profiler
A Doppler radar capable of measuring the turbulent
eddies that move with the wind. Because of this, it is able to
provide a vertical picture of wind speed and wind direction.
wind rose
A diagram that shows the percent of time that the
wind blows from different directions at a given location over a
given time.
wind shear
(meteorology) Variation in wind speed and/or
direction) over a short distance. Shear usually refers to vertical
wind shear, i.e., the change in wind with height, but the term also
is used in Doppler radar to describe changes in radial velocity over
short horizontal distances.
wind sock
A tapered fabric shaped like a cone that indicates
wind direction by pointing away from the wind. Also called a wind
cone.
wind vane
An instrument used to indicate wind direction.
wind waves
Water waves that form due to the flow of air over
the water's surface.
wind-chill factor
The cooling effect of any combination of
temperature and wind, expressed as the loss of body heat. Also
called wind-chill index.
windward
Upwind, or the direction from which the wind is
blowing; the opposite of leeward.
windward side
The side of an object facing into the wind.
winter chilling
The amount of time the air temperature during the
winter must remain below a certain value so that fruit and nut trees
will grow properly during the spring and summer.
winter solstice
Approximately December 22 in the Northern
Hemisphere when the sun is lowest in the sky and directly overhead
at latitude 23½° S, the Tropic of Capricorn.
xerophytes
Drought-resistant vegetation.
zonal flow
Large-scale atmospheric flow in which the
east-west component (i.e., latitudinal) is dominant. The
accompanying meridional (north-south) component often is weaker than
normal. Compare with meridonal flow.
zonal wind flow
A wind that has a predominate west-to-east
component.
zonally-averaged models
Statistical-dynamical or energy-balance models in
which only the latitudinally averaged quantities are determined and
the effects of the longitudinally varying transports are determined
parametrically. Abbreviated as ZAM.
zooplankton
That portion of the plankton community comprised
of tiny aquatic animals eaten by fish.
