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GLOSSARY OF
WEATHER TERMS:
Have you ever wondered what the difference is between
rain, showers and drizzle?
Many people believe that meteorologists use these terms
indiscriminately, assuming that if it "rains" or if there is a "chance of
showers," it's time to get out the umbrella.
In fact, these terms have their own specific meanings.
A better understanding of weather terminology will help
with planning outdoor activities throughout the year.
To that end, a glossary of all the weather terms used in
the current conditions and forecasts has been prepared, so that you can get
the most out of your forecasts and then some.
For complete 'A-Z' glossary of weather
(meteorological) terms, visit our Online Store.
The "Wx-Course" Pro;
Online Product #0017.
A
Glossary courtesy of
The "Wx-Course" Pro;
Online Product #0017.
"Absolute
Humidity"
A mass of water vapor in a given volume of air.
"Acre Foot"
The amount of water needed to cover one acre
under a foot of water.
"Adiabatic
Process"
This is the process by which fixed
relation- ships are maintained during changes in temperature, volume,
and pressure in a body of air, without heat being added or removed. The
change in air temperature caused by pressure
changes as air rises or sinks.
"Advection"
Horizontal movement of any meteorological
property, such as warmth or humidity.
"Advection Fog"
A type of fog caused by the movement of moist
air over a cold surface, and the consequent cooling of that air to below its
dew point. Sea fog is one type of advection fog that involves moist air in
transport over a cold body of water.
"Aeronomy"
The science of the physics and chemistry of upper atmospheres of
planets, including earth.
"A Few Clouds"
Scattered clouds, with clear skies dominant.
Cloud cover between 1/8 and 2/8.
"Ageostrophic Flows"
Air flows that move across isobars or height
contours at an angle toward either high or low pressure.
"Air Mass"
In meteorology, an extensive body of air within
which the conditions of temperature and moisture in a horizontal plane are
essentially uniform.
"Altimeter"
A special type of aneroid barometer used in
airplanes to measure altitude.
"Altitude, Density"
The altitude in the standard atmosphere at
which the air has the same density as the air at the point of question. An
aircraft will have the same performance characteristics as it would have in
a standard atmosphere at this altitude.
"Altitude, Pressure"
The altitude in the standard atmosphere at
which the pressure is the same as at the point if question. Since an
altimeter operates solely on pressure, this is the uncorrected altitude
indicated by an altimeter set at standard sea level pressure of 29.92 inches
or 1013 millibars.
"A Mix of Sun and Clouds"
At times there will be more clouds than sun, and at other times there
will be more sun than clouds. Cloud cover between 3/8 and 7/8.
"Anemometer"
An instrument for measuring wind speed.
"Aneroid"
Not using liquid. An aneroid barometer has a
needle connected to the top of a small box that has had some of its air
taken out. A change in air pressure moves the box lid up and down, and this
moves the needle. The air pressure can be read from the needle’s position on
a scale.
"Aneroid Barometer"
A device to measure air pressure that uses an
aneroid, which is sealed, flexible metal bellows with some air removed that
expands and contracts with air pressure changes.
"Antarctic Circle"
Latitude 66 degrees, 32 minutes south. Area to
the south is the Antarctic.
"Anticyclone"
An area of high pressure. The winds of an
anticyclone blow in a spiral outward from the center.
"ASOS"
Automated Surface Observation System. A suite
of weather instruments that automatically report certain weather conditions,
such as sky cover, temperature, and wind.
"Atmosphere"
The layer of gases, or air, that surrounds the
earth. The atmosphere is made up mostly of the gases nitrogen and oxygen.
"Atmospheric Pressure"
Also called barometric pressure. The pressure exerted by the
atmosphere as a consequence of gravitational attraction exerted upon the
"column" of air lying directly above the point in question.
B
Glossary courtesy of
The "Wx-Course" Pro;
Online Product #0017.
"Back-Door Cold Front"
A cold front that moves from the
northeast, instead of the more usual northwest or north, in the
eastern United States.
"Barograph"
A device for recording air pressure.
"Barometer"
A device used for measuring air pressure.
"Barometric Pressure"
The pressure exerted by the weight of the
column of air above a given point, expressed in pounds per square inch,
inches of mercury, millibars, or kilopascals.
"Beaufort Wind Scale"
Scale used to classify wind speed, devised in
1805 by British Admiral Francis Beaufort to classify winds at sea.
"Biosphere"
The Earth’s living things.
"Blizzard"
Snow falling with winds faster than 35 mph and
visibility of one-quarter mile or less over an extended time period.
"Blizzard Conditions"
A storm with an air temperature below -1 F(-18
C), with snowfall resulting in significant accumulations, and strong winds.
The winds will produce severe wind chill values, blowing snow and whiteouts.
Visibility will be severely reduced and roadways can be slippery, resulting
in very dangerous driving conditions. Stay home.
"Blowing Snow"
Created when high winds lift snow from the surface and
blow it to heights of 6 feet (2 meters) or more. No snow will be falling,
but visibility may be severely reduced and roadways can be slippery.
C
Glossary courtesy of
The "Wx-Course" Pro;
Online Product #0017.
"Calcium Magnesium Acetate"
A chemical compound used used to melt ice.
"Carbon Dioxide"
A colorless, odorless gas given out by animals
when they breath. This gas is taken in by plants, which give out oxygen.
"Ceilometer"
A cloud height measuring system. It projects
light on the cloud, detects the reflection by a photoelectric cell, and
determines eight by triangulation.
"Celsius Temperature Scale"
Abbreviated C. A temperature scale with zero
degrees as the melting point of pure ice and 100 degrees as the boiling
point of pure water at standard sea level atmospheric pressure.
"Chaos Theory"
The theory that some systems, such as weather,
are ultimately unpredictable because of the effects of small-scale events
that can’t be included in the prediction equations.
"Chinook"
A warm, dry foehn wind blowing down the eastern
slopes the Rocky Mountains over the adjacent plains in the United States and
Canada.
"Chinook Effect"
The temperature response to down slope winds.
Most often used in the context of the Rocky Mountains, Chinook effects can
increase surface temperatures by 20 to 40 degrees Fahrenheit in as little as
15 minutes. The temperature increase is the result of subsiding air, which
heats up and lowers humidities as it flows down the leeward slopes of a
prominent mountain or ridge.
"Chlorofluorocarbons"
Also known as CFC’s. Man-made substance used as
coolant and computer-chip cleaner, which have been shown to destroy
stratospheric ozone when they break down.
"Chromosphere"
A thin layer of gas on the sun’s surface.
"Cirrocumulus"
A cirriform cloud appearing as a thin sheet of
small white puffs resembling flakes or patches of cotton without shadows;
sometimes confused with altocumulus.
"Cirrostratus"
A cirriform cloud appearing as a whitish veil,
usually fibrous, sometimes smooth; often produces halo phenomena; may
totally cover the sky.
"Cirrus"
A cirriform cloud in the form of thin, white
feather-like clouds in patches or narrow bands; have a fibrous and/or silky
sheen; large ice crystals often trail downward a considerable vertical
distance in fibrous, slanted, or irregularly curved wisps called mares’
tails.
"Clear Air Turbulence"
Abbreviated CAT. Turbulence encountered in air
where no clouds are present; more popularly applied to high level turbulence
associated with wind shear.
"Clear"
Skies will be almost totally clear, perhaps
with a few small light clouds visible. Cloud cover will be between 0 and
1/10. Clear is the nighttime term equivalent to "Sunny".
"Clearing"
Most of the sky will be covered in clouds, but
you will see more and more sunshine throughout the forecast period. It will
take only a portion of the period for the sky to become less than half
covered in clouds.
"Clear with Cloudy Periods"
The nighttime equivalent of "Sunny with Cloudy
Periods".
"Climate"
An established, distinct pattern of weather
conditions that exists in a place over a period of many years; Average
weather over a long time period, usually 30 years.
"Climate Model"
Mathematical model containing equations that
describe climatic interactions.
"Cloud"
Moisture in the air that has condensed onto
particles of dust or smoke.
"Cloud Seeding"
The use of silver iodide, dry ice or other
substances to enhance precipitation.
"Clouding Over"
Skies will be mostly clear, but more and more
cloud cover will develop throughout the forecast period. The sky will become
completely overcast before the end of the forecast period.
"Cloudy"
No sunshine visible. Cloud cover will be 5/8 or
more.
"Cloudy with Clear Breaks"
The nighttime equivalent of "Cloudy with Sunny
Breaks".
"Cloudy with Drizzle"
The sky will be overcast and a heavy mist or
very light continuous rain is expected to fall. Your slow pulse wipers will
be too fast in clearing the windshield.
"Cloudy with Flurries"
The cloud cover will be as described for
"Cloudy", and the precipitation light. The flurries will be short-lived and
intermittent, and will not amount to any significant accumulation on the
ground. Visibility will be reduced, but roadways will not generally be
slippery.
"Cloudy with Freezing Drizzle"
Similar to freezing rain, but the size of the
supercooled rain drops will be less than .02 inch diameter (0.5 mm). The
effects will be the same as for "Freezing Rain", but the buildup of ice will
be slower.
"Cloudy with Freezing Rain"
The sky condition will be as described above,
with precipitation consisting of supercooled liquid rain drops that
instantly freeze on contact. Visibility will generally be good, but roadways
will be extremely slippery and dangerous. Freezing rain will create a layer
of ice on bare surfaces and existing snow.
"Cloudy with Isolated Flurries"
The cloud cover will be as described for
"Cloudy", and the precipitation very light. The flurries will be brief and
intermittent, and will not amount to any significant accumulation on the
ground. Visibility will be reduced, but roadways will not generally be
slippery.
"Cloudy with Light Rain"
The sky will be overcast and the precipitation
will be strong enough to get things wet in a short time. The rain will
interfere with outdoor activities. Puddles will form quickly and water may
run down the expressways, causing slippery road conditions. Precipitation
will persist throughout the forecast period.
"Cloudy with Light Snow"
The cloud cover will be as described for
"Cloudy", the precipitation light. The snow will be continuous but will not
usually amount to any significant accumulations for the forecast period.
Visibility will be reduced and roadways can be slippery, depending on the
duration of the snow.
"Cloudy with Periods of Rain"
The sky will be overcast and the precipitation
strong enough to get things wet in a short time, but the rain may also stop
for a short period of time. The rain will interfere with outdoor activities.
The ground will get wet, puddles will form, but the precipitation will be
expected to cease for short periods of time over the forecast period.
"Cloudy with Rain or Snow Showers"
The type of precipitation expected to fall can
either be rain or snow throughout the forecast period. The precipitation
starts and stops. No significant accumulations are expected. If the
precipitation falls as snow, visibility will be reduced and roadways can
become slippery.
"Cloudy with Scattered Thundershowers"
The sky will be cloudy, with some showers or
light rain possible; thunder and lightning will be associated with the
precipitation. The showers or rain may be heavy for short periods of time.
"Cloudy with Showers"
The sky will be overcast and the strength of
the precipitation will be light. The showers will be intermittent, and will
generally interfere with outdoor activities. The ground will get wet.
"Cloudy with Sunny Breaks"
A least half covered in clouds, but not totally
cloudy. Cloud cover will be between 5/8 to 7/8.
"Cloudy with Thundershowers"
The sky will become cloudy with a few showers
or light rain possible; thunder and lightning will be associated with the
precipitation. The showers or rain may be heavy for extended periods of
time.
"Cloudy with Thunderstorms"
The sky will be cloudy, with showers or light
rain possible; thunder and lightning will be associated with the
precipitation. The showers or rain will be heavy for extended periods of
time.
"Cloudy with Wet Flurries"
The sky condition will be as for "Cloudy", with
intermittent precipitation. The snow will be partially melted before
reaching the ground, and will accumulate as a light dusting of wet sticky
snow if it does not melt instantly on contact. Visibility will be reduced,
but roadways will generally not get slippery.
"Coastal Fog"
Fog will be expected to form near the shore
line, and may cause reduced visibility on roadways near the shore.
"Cold Air Damming"
A situation in the atlantic piedmont and
coastal plain where low-level cold air is shunted southwards, ahead of an
approaching cold front. High pressure to the northeast of the advancing cold
front is necessary to help provide the mechanism for advecting the cold air
to the south. The low-level cold air dams up against the Appalachians, hence
the term. It is an event common to the fall and winter months.
"Cold Front"
A warm-cold air boundary with the cold air
advancing.
"Condense"
To change from a gas or vapor to a liquid. An
example is when steam condenses into water.
"Condensation"
The change of a vapor to a liquid.
"Condensation Nuclei"
Small particles in the air that attract water,
and encourage condensation.
"Conduction"
Transfer of heat within a substance or from one
substance to another by molecular action.
"Constant Pressure Chart"
A chart of a constant pressure surface; may
contain analyses of height, wind, temperature, humidity, and/or other
elements.
"Continental Air Mass"
An air mass that forms over land, making it
generally dry. It may be warm or cold.
"Contour"
In meteorology, (1) a line of equal height on a
constant pressure chart; analogous to contours on a relief map; (2) in radar
meteorology, a line on a radar scope of equal echo intensity.
"Convection"
Transfer of heat by the movement of the heated
material. In meteorology, the up and down air
motions caused by heat.
"Convective Storms"
Storms created by rising warm air;
such as thunderstorms.
"Convergence"
The condition that exists when the distribution
of winds within a given area is such that there is a net horizontal inflow
of air into the area. In convergence at lower levels, the removal of the
resulting excess is accomplished by an upward movement of air; consequently,
areas of low-level convergent winds are regions favorable to the occurrence
of clouds and precipitation.
"Coriolis Effect"
The apparent curving motion of anything, such
as wind caused by the Earth’s rotation, first described in 1835 by French
scientist Gustave-Gaspard Coriolis.
"Corona"
One layer of the sun’s atmosphere. Also a
circle of light around the sun or moon caused by clouds.
"Cutoff Low"
An area of an upper atmosphere low pressure
that is cut off from the general west-to-east wind flow.
"Cryosphere"
The Earth’s ice.
"Cumulonimbus"
A cumuliform cloud type; it is heavy and dense,
with considerable vertical extent in the form of massive towers; often with
tops in the shape of an anvil or massive plume; under the base of
cumulonimbus, which often is very dark, there frequently exists virga,
precipitation and low ragged clouds (scud), either merged with it or not;
frequently accompanied by lightning, thunder, and sometimes hail;
occasionally produces a tornado or a waterspout; the ultimate manifestation
of the growth of a cumulus cloud, occasionally extending well into the
atmosphere.
"Cumulonimbus Mamma"
A cumulonimbus cloud having hanging
protuberances, like pouches, festoons, or udders, on the under side of the
cloud; usually indicative of severe turbulence.
"Cumulus"
A cloud in the form of individual detached
domes or towers which are usually dense and well defined; develops
vertically in the form of rising mounds of which the bulging upper part
often resembles a cauliflower; the sunlit parts of these clouds are mostly
brilliant white; their bases are relatively dark and nearly horizontal.
"Cyclogenesis"
Any development or strengthening of cyclonic
(counterclockwise, in the northern hemisphere) circulation in the
atmosphere. It is applied to the development of cyclonic circulation where
it previously did not exist (commonly, the initial appearance of a low or
trough).
"Cyclone"
An area of low-atmospheric-pressure with winds
blowing around it, counterclockwise in the Northern Hemisphere, clockwise in
the Southern Hemisphere.
D
Glossary courtesy of
The "Wx-Course" Pro;
Online Product #0017.
"Depression"
An area of low atmospheric pressure. It is
another name for a cyclone.
"Derechoes"
Wind storms created by thunderstorms during
which winds blow in straight lines.
"Dew"
Water droplets formed by condensation of water
vapor; Moisture condensation that collects on unprotected objects outdoors
when the air has cooled below its saturation point.
"Dew Point"
The temperature to which air must be cooled at
constant pressure and humidity for condensation (dew) to occur, expressed in
degrees Fahrenheit or Celsius.
"Diamond Dust"
Tiny ice crystals that float in the air
creating pillars of light.
"Diurnal"
Daily, recurrent variations in meteorological
variables. Some diurnal variations at the Earth’s surface includes (1)
Temperature maximums near or after local noon, and temperatures minimums
just before sunrise; (2) relative humidity and fog minimums in late
afternoon, and maximum near sunrise; (3) Increase of winds during day, and
decrease of winds at night.
"Divergence"
The condition that exists when the distribution
of winds within a given area is such that there is a net horizontal flow of
air outward from the region. In divergence at lower levels, the resulting
deficit is compensated for by subsidence of air from aloft; consequently the
air is heated and the relative humidity lowered making divergence a warming
and drying process. Low-level divergent regions are areas unfavorable to the
occurrence of clouds and precipitation. The opposite of convergence.
"Doppler Radar"
Radar that measures speed and direction of a
moving object such as wind.
"Downburst"
Wind blasting down from a thunderstorm or
shower. A strong downdraft which induces an outburst of damaging winds on or
near the ground. The sizes of downbursts vary from one-half mile or less to
more than ten miles.
"Drifting Snow"
Created when winds lift snow from the surface
and blow it to heights less than 6 feet(2 meters). No additional snow will
be falling and visibility will not generally be reduced, but roadways can
still get slippery.
"Drizzle"
Heavy mist or light, fine, continuous rain that
will fall slowly, reducing visibility somewhat more than light rain. Water
droplets will be less than .02 inch diameter (0.2mm and 0.5mm) in size.
Drizzle can often be confused with light rain, however drizzle's water
droplets will be smaller in size than those of rain.
"Drought"
Abnormal dryness for a particular region.
"Dry Adiabatic Lapse Rate"
Rate at which rising air cools or sinking air
warms when no water phase changes are occurring; in both cases 5.4 degrees
Fahrenheit per 1,000 feet.
"Dry Line"
A boundary between ‘warm, dry air’ and ‘warm,
humid air’ along which thunderstorms form, often found on the southern
plains.
E
Glossary courtesy of
The "Wx-Course" Pro;
Online Product #0017.
"Echo"
In radar, (1) the energy reflected or scattered
by a target; (2) the radar scope presentation of the return from a target.
"Eddy"
A local irregularity of wind in a larger scale
wind flow. Small scale eddies produces turbulent conditions.
"Electromagnetic Radiation"
Energy that moves in the form of disturbances
in electrical and magnetic fields. Light and radio waves are examples.
"El Nino"
Linked ocean and atmospheric events, which have
world-wide effects, characterized by warming of water in the tropical
Pacific from around the International Date Line to the coast of Peru.
"Evaporation"
The change from a liquid or a solid to a vapor
or gas.
"Equinox"
Times when the sun crosses the equator. The
spring or vernal equinox occurs around March 21. The autumn's equinox
is around September 21.
"Extratropical Cyclone"
A large scale weather system that forms outside
the tropics with a low-pressure center.
F
Glossary courtesy of
The "Wx-Course" Pro;
Online Product #0017.
"Flash Flood"
Flooding with a rapid water rise.
"Flurries"
Intermittently falling snow, generally
short-lived. Flurries are the solid equivalent of "Rain Showers".
"Foehn"
A warm, dry down slope wind; the warmness and
dryness being due to adiabatic compression upon descent; characteristic of
mountainous regions. A warm, dry wind that blows down out of the mountain
slopes in winter and early spring.
"Fog"
A cloud with its base at the Earth's surface.
Visibility is reduced to less than 5/8 statue mile (1 km). Clouds that form
at ground level.
"Foggy"
Fog is expected to form throughout the
countryside, causing reduced visibility over large areas. The fog will
generally be slow in breaking up, and may eventually result in a mixture of
sun and clouds as the fog lifts and breaks up. Foggy conditions may be
forecast after rain or showers overnight and into the early morning hours.
"Fog Patches"
Some fog is expected to form, but will generally be restricted to low
lying areas. It may cause reduced visibility if encountered. Fog patches
generally break up through the first part of the morning if they form
overnight. They may be forecast after rain or showers at any time of the
day, but generally fog occurs late at night or in the early morning hours.
"Freezing"
The phase change of water from liquid to solid.
"Freezing Drizzle"
Very fine supercooled liquid rain drops with a
diameter of less than .02 inches in diameter (0.5mm).
"Freezing Level"
A level in the atmosphere at which the
temperature is 0 degrees Celsius or 32 degrees Fahrenheit.
"Freezing Nuclei"
Small particles in the air that encourage the
formation of ice.
"Freezing Rain"
Supercooled liquid rain drops that freeze on
contact with the ground. Freezing rain will form a layer of ice on any
surface instantly.
"Front"
Boundary between air masses of different
densities, and usually different temperatures.
"Frontal Zone"
A front or zone with a marked increase of
density gradient; used to denote that fronts are not truly a "surface" of
discontinuity but rather a "zone" of rapid transition of meteorological
elements.
"Frost"
Water vapor that has turned to ice on an
object.
"Fujita Scale"
Wind damage scale created by Theodore Fujita.
"Funnel Cloud"
A rotating column of air extending from a
cloud, but not reaching the ground.
G
Glossary courtesy of
The "Wx-Course" Pro;
Online Product #0017.
"Glaze"
A coat of smooth ice created when supercooled
drops of water spread out before freezing.
"Glory"
Colored rings around an object’s shadow.
"GOES Satellite"
Geostationary Operational Environmental
Satellite. It is a U.S. weather satellite. These satellites orbit at
altitudes of approximately 23,300 statute miles. At this height, the
satellites orbit with the earth’s rotation, and therefore give continuous,
overlapping coverage of the cloud cover, surface temperatures, and vertical
distributions of the atmospheric temperatures and humidities below. There
are two such satellites - GOES East, centered over the equator at 75 degrees
west longitude, and GOES West, centered at 135 degrees west. "GOES East"
covers the Atlantic Ocean; "GOES West" covers the Pacific. Both cover large
portions of eastern and western North and South America. GOES satellites
provide visible, infrared, and water vapor imagery.
"Gradient"
In meteorology, a horizontal decrease in value
per unit distance of a parameter in the direction of maximum decrease; most
commonly used with pressure, temperature, and moisture.
"Gradual Clearing"
Most of the sky will be covered in clouds, but
you will see more and more sunshine throughout the forecast period. It may
take the entire period for the sky to become less than half covered in
clouds.
"Graupel"
Form of ice created when supercooled water
droplets coat a falling ice crystal.
"Greenhouse Effect"
Warming of a planet caused by the absorption
and re-emission of infrared energy by molecules in the atmosphere.
"Ground Fog"
A layer of fog, often less than 200 feet high,
that forms when the ground cools.
"Gulf Stream"
A warm, well-defined, swift, and relatively
narrow ocean current that flows from the Gulf of Mexico across the Atlantic
to the European Coast. It helps warm Western Europe.
"Gust Front"
Wind flowing out from a thunderstorm.
H
Glossary courtesy of
The "Wx-Course" Pro;
Online Product #0017.
"Hail"
Frozen rain droplets created by the updrafts of
convective thunderstorms. Hail (balls of ice) is generally associated with
severe thunderstorms.
"Halo"
Any of the rings or arcs of light around the
sun or moon caused by ice crystal clouds.
"Hazy"
The sky will appear to be covered in a very
light fog when viewed off in the distance. Haze is created when fine dust
particles disperse through a portion of the atmosphere. It is generally not
noticeable when observed from directly above or below. Typically, haze does
not impair visibility to any great extent.
"Heat Lightning"
Glowing flash in clouds. No thunder is heard
because heat lightning is too far away.
"Heavy Rain"
The sky will be overcast and the precipitation
continuous, strong enough to get things wet very quickly. There may be minor
flooding of storm drains and roadways if the precipitation continues for
very long. Heavy rain causes reduced visibility while driving.
"High"
A short name for an area of high pressure. It
is also called an anticyclone.
"Horse Latitudes"
Latitudes where sailing ships often encountered
extended periods of calm winds. In this region, ship’s captains would order
horses thrown overboard to preserve food and lighten the ships’ loads. The
many horse carcasses seen floating on the still sea there earned the region
its name.
"Humidity"
The amount of moisture in the air in the form
of suspended (and often visible) water vapor.
"Humiditex"
A calculation combining air temperature and
relative humidity, given in degrees Fahrenheit or Celsius. It represents the
heating effect felt due to a lack of body moisture evaporation, and gives
the average person a "feeling" for how hot and stuffy the air is. Humiditex
is given only in the summer months.
"Hurricane"
A violent storm that is made up by a tropical
cyclone. A tropical cyclone: with winds of 74 mph or more.
"Hydrogen"
A gas that is the lightest chemical element. It
can float balloons but it burns very easily.
"Hydrosphere"
The Earth’s water.
"Hygrometer"
An instrument that measures the humidity in the
air.
I
Glossary courtesy of
The "Wx-Course" Pro;
Online Product #0017.
"Ice Age"
A geological time period during which sheets of
ice cover extensive parts of the earth. A time in the Earth’s past when
large areas of the world’s land were covered by thick ice. There were
several ice ages, but only the last one is known as the Ice Age. This Ice
Age took place during the Pleistocene epoch, from 2.5 million years to about
10,000 years ago.
"Ice Caps"
Thick sheets of ice covering large areas of
land, especially in Antarctica and Greenland. Sheets like these covered much
larger areas during ice ages.
"Ice Crystals"
Very fine grains of ice falling slowly to the
ground. Ice crystals are formed under very cold and dry conditions when the
crystals are unable to stick together and form snow. Ice crystals can reduce
visibility.
"Ice Fog"
A type of fog created when water vapor condenses at very cold air
temperatures. Visibility is reduced.
"Ice Pellets"
Transparent pellets of continuously falling
ice. They may be (1) hard grains that rebound on a striking a hard surface,
or (2) pellets of snow encased in ice. Ice pellets start as falling snow
that melts and then refreezes completely before reaching the ground. They
bounce and make a rattling sound on contact with the ground. They are
falling drops of frozen water, also called sleet.
"Ice Pellet Showers"
Ice pellet showers are the same as ice pellets,
but the precipitation is intermittent rather than continuous.
"Increasing Cloudiness"
Skies will be mostly clear, but more and more
cloud cover will develop throughout the forecast period. It will not become
completely overcast before the end of the forecast period.
"Insolation"
The total solar radiation received at the
Earth’s surface; also, the rate of delivery of direct solar radiation per
unit of horizontal surface area.
"Instability"
A general term to indicate various states of
the atmosphere in which spontaneous convection will occur when prescribed
criteria are met; indicative of turbulence.
"Intertropical Convergence Zone"
The area near the equator, called "The
Doldrums" by sailors, where the trade winds converge.
"Inversion"
Stable air condition in which air near the
ground is cooler than air at a higher altitude. An increase in temperature
with height. A reversal of the normal decrease with height in the
troposphere; may also be applied to other meteorological properties.
"Isobar"
A line of equal or constant barometric
pressure. Lines connecting points of equal atmospheric pressure on a weather
map.
"Isotherm"
Lines connecting points of equal or constant
air temperatures on a weather map.
J
Glossary courtesy of
The "Wx-Course" Pro;
Online Product #0017.
"Jetstreams"
A narrow band of upper atmosphere wind with
speeds greater than 57 miles per hour.
K
Glossary courtesy of
The "Wx-Course" Pro;
Online Product #0017.
"Katabatic Wind"
Any wind blowing down an incline. The same as a
"gravity wind," a katabatic wind that is warm is called a "FOEHN". If it’s
cold, it’s called a "fall wind," or a "BORA".
"Kilopascal"
A metric unit of air pressure. Millibars
divided by 10.
L
Glossary courtesy of
The "Wx-Course" Pro;
Online Product #0017.
"Lake Effect Snow"
Cold, dry air blows over a lake (20 or more degrees warmer), the
water evaporates into the colder, dry air, which rises to form turbulent
clouds. Clouds then form in bands, dump heavy snow inland on the opposite
side with heavy icing aloft. For example, the prevailing wind pushes
moisture-laden air over colder terrain.
"La Nina"
El Nino’s unruly twin sister. Scientists call
La Nina "the girl". La nina arises when trade winds return to the eastern
pacific, bring to the surface of the sea deeper, colder water that replaces
the abnormally warm waters of her brother El Nino.
"Latent Heat"
Energy stored when water vapor evaporates into
vapor or ice melts into liquid. It’s released as heat when vapor condenses
or water freezes.
"Lapse Rate"
The rate of decrease of an atmospheric variable
with height; commonly refers to decrease of temperature with height.
"Lee Wave"
Any stationary wave disturbance caused by a
barrier in a fluid flow. In the atmosphere when sufficient moisture is
present, this wave will be evidenced by lenticular clouds to the lee of
mountain barriers; also called mountain wave or standing wave.
"Lenticular Cloud"
A species of cloud whose elements have the form
of more or less isolated, generally smooth lenses or almonds. These clouds
appear most often in formations of orographic origin, the result of lee
waves, in which case they remain nearly stationary with respect to the
terrain (standing cloud), but they also occur in regions without marked
orography.
"Lightning"
A visible discharge of electricity produced by
a thunderstorm.
"Little Ice Age"
Period from the mid-16th century to the
mid-19th century during which average global temperatures were lower than
during previous and subsequent periods.
"Long Wave"
A south-to-north wave that appears in the
normal west-to-east flow of upper atmosphere winds.
"Low"
A short name for an area of low pressure, or a
cyclone.
M
Glossary courtesy of
The "Wx-Course" Pro;
Online Product #0017.
"Magnetic Field"
The area of force that surrounds a magnet. A
magnetic field surrounds the Earth, but it is
weakest at the poles.
"Mainly Clear"
The nighttime equivalent of "Mainly Sunny".
"Mainly Clear with a Chance of Showers"
The sky is expected to be less than half
covered with clouds, but there still may be brief periods of precipitation,
generally occurring in the early evening.
"Mainly Cloudy"
Mostly covered in clouds, possibly with brief
periods of sunshine. Cloud cover will be 5/8 to 7/8.
"Mainly Cloudy with a Few Showers"
Cloud cover as for "Mainly Cloudy", with light
precipitation of short duration. The showers will be short-lived, but may
interfere with outdoor activity. The ground will get damp.
"Mainly Cloudy with Isolated Showers"
Cloud cover as for "Mainly Cloudy", with very
light precipitation of short duration. The showers will be very short-lived
and generally will not interfere with any outdoor activity. You may get a
bit damp, but the shower will end quickly, and you will dry off almost
immediately.
"Mainly Cloudy with Scattered Showers"
Cloud cover as for "Mainly Cloudy", with light
precipitation of short duration. The showers will come and go, and generally
interfere with outdoor activities. The ground will get wet.
"Mainly Sunny"
Clear, with a small area of clouds visible.
Cloud cover will be between 0/8 to 2/8.
"Mainly Sunny with a Chance of Showers"
The sky is expected to be less than half
covered with clouds, but there still may be brief periods of precipitation,
generally occurring in the late afternoon.
"Mainly Sunny with a Chance of T-storms"
The sky is expected to be less than half
covered with clouds, but there still may be brief periods of showery
precipitation with thunder and lightning, generally occurring at the end of
the afternoon.
"Maritime Air Mass"
An air mass that forms over an ocean, making it
humid. It may be warm or cold.
"Meridional Flow"
A north to south to north flow of high altitude
winds.
"Mesocyclone"
A rotating, upward-moving column of air in a
thunderstorm that can spawn tornadoes.
"Mesoscale"
In meteorology, weather systems and events up
to about 250 miles across.
"Meteor"
Any natural phenomena in the atmosphere.
"Meteorological Bomb"
An extratropical cyclone in which the center
pressure drops an average of one millibar an hour for 24 hours. Usually
refers to storms off the U.S. east coast.
"Microburst"
A downburst less than 2.5 miles in diameter. A
locally, extremely high-intensity column of descending air.
"Mid Latitudes"
Region of the Earth outside the polar and
tropical regions, between latitudes 23.5 degrees and 66.5 degrees.
"Millibar"
Abbreviated mb. The metric unit of air pressure
measurement. An internationally used unit of pressure equal to 1,000 dynes
per square centimeter. It is convenient for reporting atmospheric pressure.
"Mercury"
A silver-colored metal that is a liquid at
normal temperatures. It is often used in thermometers and barometers.
"Mixed Precipitation"
Mixed precipitation is a combination of liquid
and solid precipitation, and is generally continuous. The precipitation
falling must either consist of rain, snow and ice pellets, or rain and ice
pellets. Some freezing rain can be mixed in as well. The concentration of
precipitation types falling can change throughout the forecast period and is
not quantified. Visibility will be reduced and roadways can be extremely
slippery. A small accumulation of snow and/or ice pellets can be expected,
depending on the proportion of solid to liquid precipitation.
"Moist Adiabatic Lapse Rate"
The rate at which rising air cools or air warms
when water is changing phases in the air. The rate varies.
"Monsoon"
A wind that changes direction according to the
season. Persistent, widespread, seasonable winds from the ocean bring rain,
while winter winds from the land are dry. Also, the rains that it brings to
parts of the world in summer.
"Mountain Wave"
Up and down air motions created as wind flows
over mountains. The standing wave or lee wave to the lee of a mountain
barrier.
"Mountain Winds"
Winds that blow either up or down mountains,
caused by different rates of heating and cooling of mountaintops and
valleys.
"Multi Cell Storms"
Thunderstorms consisting of clusters of
single-cell thunderstorms.
N
Glossary courtesy of
The "Wx-Course" Pro;
Online Product #0017.
"National Climatic Data Center"
Also known as NCDC. The National Oceanic and
Atmospheric Administration office in Ashville, North Carolina, keeps climate
records.
"National Huricane Center"
Also known as NHC. National Weather Service
office in Coral Gables, Florida, tracks and forecasts hurricanes and other
weather in the Atlantic, Gulf of Mexico, Caribbean Sea, and parts of the
Pacific.
"National Meteorological Center"
Also known as NMC. National Weather Service
center in Camp Springs, Maryland, prepares worldwide computer forecasts.
Hurricane and Severe Storms centers are part of NMC.
"National Severe Storms Forecast Center"
National Weather Service center on Kansas City,
Missouri, issues watches for severe thunderstorms and tornadoes across the
nation.
"National Severe Storms Laboratory"
Also known as NSSL. national Oceanic and
Atmospheric Administration Laboratory in Norman, Oklahoma, studies severe
thunderstorms.
"National Weather Service"
Federal agency observes and forecasts weather.
Formerly the U.S. Weather Bureau. It’s part of the National Oceanic and
Atmospheric Administration, which is part of the Department of Commerce.
"Neap Tides"
Tides at the moon’s first and third quarters,
characterized by a small rise and fall.
"NexRad"
Next generation Weather Radar System. Installed
in the 1990’s by the National Weather Service, the Defense Department, and
the Federal Aviation Administration.
"Nimbostraus"
A principal cloud type, grey colored, often
dark, the appearance of which is rendered diffuse by more or less
continuously falling rain or snow, which in most cases reaches the ground.
It is thick enough throughout to blot out the sun.
"Numerical Forecasting or Prediction"
Use of computers to solve mathematical
equations and produce weather forecasts.
O
Glossary courtesy of
The "Wx-Course" Pro;
Online Product #0017.
"Observatory"
A place or building where people study the
stars and planets. Observatories of today use cameras linked with
high-powered tracking telescopes.
"Obscuration"
Denotes sky hidden by surface-based obscuring
phenomena and vertical visibility restricted overhead.
"Occluded Front"
Also commonly called Occlusion, or Frontal
Occlusion. A composite of two fronts as a cold front overtakes a warm front
or quasi-stationary front.
"One-Hundred-(100)-Year-Floods"
Water levels that, on average, should occur
once a century. This is the same as a water level with a 100 to 1 chance of
occurring in any single year.
"Orographic"
Of, pertaining to, or caused by mountains as in
orographic clouds, orographic lift, or orographic precipitation.
"Overcast"
Totally covered with clouds, with clear skies
not visible. Cloud cover will be 8/8.
"Ozone"
Form of oxygen with molecules that consist of
three oxygen atoms compared to two atoms for ordinary oxygen molecules.
"Ozone Hole"
The destruction of about 40 percent of the
ozone in the stratosphere over Antarctica each spring.
P
Glossary courtesy of
The "Wx-Course" Pro;
Online Product #0017.
"Partial Clearing"
Most of the sky will be covered in clouds, but
you will see more and more sunshine throughout the forecast period. It will
take longer than the forecast period for the sky to become less than half
covered in clouds.
"Partly Cloudy"
Clouds dominant, with little clear sky showing.
Cloud cover will be between 3/8 and 4/8.
"Perigean Spring Tides"
Unusually high tides caused by the Earth and
moon being at their nearest approach to each other, at the same time the
Earth, sun and moon are in a nearly straight line.
"Phase Changes"
Changes of a substance among solid, liquid and
gas forms.
"POES Satellite"
Polar Orbiting Operational Environmental
Satellite. These satellites orbit along north-south tracks at nominal
altitudes of approximately 530 statute miles. This is a comparatively low
orbiting altitude, but it allows greater resolution of the weather features
below, and better remote sensing of atmospheric soundings. POES satellite
measure in the visible, infrared, water vapor, and various radiometric
spectra, one of which is used to make ozone measurements. POES satellites
also contain SARSAT (Search And Rescue Satellite Aided Tracking) equipment,
which allows them to locate and relay emergency locater transmitter (ELT)
and other emergency broadcasts from pilots and mariners in distress. Two
POES satellites make 14 orbits each per day. This provides nearly complete
coverage of the Earth below.
"Polar Front"
The semi permanent, semi continuous front
separating air masses of tropical and polar origins.
"Polar Regions"
Regions of the Earth north of 66.5 degrees
north latitude around the North Pole, and south of 66.5 degrees south
latitude around the South Pole.
"Polar Vortex"
Strong, winter, upper-atmosphere winds around
the polar regions.
"Precipitation"
Any form of water in the form of droplets or
ice that falls to the Earth, whether as rain, snow, sleet, or hail.
"Precipitation Fog"
Fog that forms when precipitation falls into
cold air.
"Pre-Frontal Squall Lines"
Lines of thunderstorms ahead of an advancing
cold front.
"Pressure Gradient Force"
Force acting on air caused by air pressure
differences.
Q
Glossary courtesy of
The "Wx-Course" Pro;
Online Product #0017.
"Quasi-Stationary Front"
Also known as and commonly called Stationary
Front. A front which is stationary or nearly so;
conventionally, a front which is moving at a speed of less than 5 knots is
generally considered to be quasi-stationary.
R
Glossary courtesy of
The "Wx-Course" Pro;
Online Product #0017.
"Radar"
A device that bounces radio waves off an object
in order to measure the distance to it and to keep track of its movements.
"Radiation"
The flow of particles and rays, such as light
and radio waves. It also means the energy released from an atom.
"Radiation Fog"
Fog
characteristically resulting when traditional cooling of the Earth’s surface
lowers the air temperature near the ground to or below its initial dew point
on calm, clear nights.
"Rain"
Precipitation that reaches the Earth as large
droplets. It may begin as smaller droplets high in the sky, but as the
droplets fall, they intermingle and combine to become larger droplets. We
classify rain by the amount that falls and by the results of the free-fall
mixing action, describing rain as light, moderate, or heavy.
"Rain Forests"
Forests growing in tropical areas that have
heavy rainfall.
"Rainbow"
Arc or circle of colored light caused by the
refraction and reflection of light by water droplets.
"Rain Gauge"
An instrument that collects precipitation,
enabling weather observers to measure the amount.
"Rain and Snow"
A mix of both rain and snow is expected for the
forecast period. The precipitation will be continuous. The concentration of
rain and snow can change throughout the forecast period and is not
quantified. Small accumulations of snow can sometimes occur depending on the
proportion of snow to rain. Visibility will be reduced and roadways can be
very slippery.
"Rain and Snow Mixed"
A mixture of snow and rain. The proportion of
snow to rain is not quantified.
"Rain Changing to Snow"
At the beginning of the forecast period, the
precipitation will start as rain, then completely change over to snow by the
end of the forecast period. There may be a brief period of freezing rain
and/or ice pellets during the transition. The precipitation will be
continuous throughout the forecast period. Visibility will be reduced and
roadways will begin to ice over as the temperature drops and the
precipitation changes to snow, resulting in very slippery roads and
dangerous driving conditions.
"Rain or Snow"
The type of precipitation expected to fall will
be either rain or snow throughout the forecast period. The precipitation
will be continuous. If the precipitation is snow, accumulations can be
significant, visibility will be reduced and roadways can be slippery.
"Rain Showers"
Intermittently falling rain, generally
short-lived.
"RDF"
Rain Drop Frequency, a term used in radar
weather summaries.
"Relative Humidity"
The ratio between the amount of water vapor in
the air compared to the maximum amount of water vapor the air can hold, at a
given temperature and pressure. This is expressed as a percentage.
S - Z
The complete 'A-Z' glossary of
weather (meteorological) terms, is available online.
The "Wx-Course"
Pro; Online Product #0017.
Independent Self-Study for the professional meteorologist, flight dispatcher
or airline pilot.
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