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A
ABSOLUTE HUMIDITYA type ofhumidity that considers the mass
ofwater vapor present per unit volume ofspace. Also considered as
thedensityof the water vapor. It is usually expressed in gramsper
cubic meter.
ABSOLUTE INSTABILITYWhen the lapse rateof a column ofair is
greater than the dryadiabatic lapse rate. Theterm absolute is used
because this applies whether or not the air is dry or
saturated.Related term: instability
ABSOLUTE TEMPERATURE SCALEA temperature scale with afreezing
pointof +273K (Kelvin) and aboiling pointof +373K.Related term:
Kelvin Temperature Scale
ABSOLUTE ZEROConsidered to be the point at which theoretically
no molecular activity exists or thetemperatureat which the volume
of a perfect gas vanishes. The value is 0Kelvin,-273.15Celsius and
-459.67 Fahrenheit.
ABSORPTIONThe process in which incident radiant energy is
retained by a substance. The absorbedradiation is then transformed
into molecular energy.
ABYSSAL PLAIN
The flat, gently sloping or nearly level region of the sea
floor.ADIABATIC PROCESSA thermodynamic change of state in a system
in which there is no transfer ofheat or massacross the boundaries
of the system. In this process, compression will result in
warmingand expansion will result in cooling.
ADVECTIONThe horizontal transfer of any property in the
atmosphere by the movement ofair (wind).Examples include heat and
moistureadvection.
ADVECTION FOGFogthat develops when warm moistair moves over a
colder surface, cooling that air tobelow its dew point.Related
terms: Arctic Sea Smoke and sea fog
ADVISORYStatements that are issued by the National Weather
Service for probable weather situationsof inconvenience that do not
carry the danger ofwarning criteria, but, if not observed,
could lead to hazardous situations. Some examples include snow
advisories stating possibleslick streets, or fog advisories for
patchy fog condition causing temporary restrictions
tovisibility.
AFOSAcronym for Automation ofField Operations and Services. It
is the computer system thatlinksNational Weather Service offices
together forweather data transmission.
AIRThis is considered the mixture of gases that make up the
earth's atmosphere. The principalgases that compose dry air are
Nitrogen (N2) at 78.09%, Oxygen (O2) at 20.946%, Argon(A) at 0.93%,
and Carbon Dioxide (CO2) at 0.033%. One of the most
importantconstituents of air and most important gases in
meteorology is water vapor (H2O).
AIR MASSAn extensive body ofair throughout which the horizontal
temperature and moisturecharacteristics are similar.
AIR MASS THUNDERSTORMA thunderstorm that is produced by
convection within an unstable air massthrough
aninstabilitymechanism. Such thunderstorms normally occur within a
tropicalor warm, moistair mass during the summer afternoon as the
result of afternoon heating and dissipate soonafter sunset. Such
thunderstorms are not generally associated withfronts and are
lesslikely to become severethan other types of thunderstorms.
However, that does notpreclude them from having brief heavy
downpours.
AIR POLLUTIONThe soiling of the atmosphere by contaminants to
the point that may cause injury to health,property, plant, or
animal life, or prevent the use and enjoyment of the outdoors.
AIR QUALITY STANDARDS
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The maximumlevel which will be permitted for a
givenpollutant.Primary standards are tobe sufficiently stringent to
protect the public health. Secondary standards must protect
thepublic welfare, including property and aesthetics.
ALASKAN WINDSThe downslope air flow that blows through the
Alaskan valleys. It is usually given localnames, such as Knik,
Matanuska, Pruga, Stikine, Taku, Take, Turnagain, or Williwaw.
ALBEDO
The ratio of the amount ofradiation reflected from an object's
surface compared to theamount that strikes it. This varies
according to the texture, color, and expanse of theobject's surface
and is reported in percentage. Surfaces with high albedo include
sandandsnow, while low albedo rates include forests and freshly
turned earth.Related term: Dave's Dictionary
ALBERTA CLIPPERA fast moving, snow-producing weather system that
originates in the lee of the CanadianRockies. It moves quickly
across the northern United States, often bring gusty winds andcold
Arctic air.
ALEUTIAN LOWA semi-permanent,subpolar area oflow pressure
located in the Gulf of Alaska near theAleutian Islands. It is a
generating area for storms and migratory lows often reachmaximum
intensity in this area. It is most active during the latefallto
late spring. Duringthe summer, it is weaker, retreating towards the
North Pole and becoming almostnonexistent. During this time, the
North Pacific High pressure system dominates.
Related term: Icelandic LowALTIMETERAn instrument used to
determine thealtitude of an object with respect to a fixed level.
Thetype normally used bymeteorologists measures the altitude with
respect tosea levelpressure.
ALTIMETER SETTINGThe pressure value to which an
aircraftaltimeter scale is set so that it will indicate
thealtitudeabove mean sea level of an aircraft on the ground at the
location for which thevalue was determined.
ALTITUDEIn meteorology, the measure of a height of an airborne
object in respect to a constantpressure surface or abovemean sea
level.
ALTOCUMULUSComposed of flattened, thick, gray, globular masses,
thismiddle cloud genus is primarilymade ofwater droplets. In the
mid-latitudes, cloud bases are usually found between 8,000
and 18,000 feet. A defining characteristic is that it often
appears as a wavy billowy layer ofcloud, giving it the nickname of
"sheep" or "woolpack" clouds. Sometimes confused withcirrocumulus
clouds, its elements (individual clouds) have a larger mass and
cast a shadowon other elements. It may form several sub-types, such
asaltocumulus castellanusoraltocumulus lenticularis. Virga may also
fall from these clouds.
ALTOCUMULUS CASTELLANUSA middle cloud with vertical development
that forms from altocumulus clouds. It iscomposed primarily ofice
crystals in its higher portions and characterized by its
turrets,protuberances, or crenelated tops. Its formation indicates
instability andturbulence at thealtitudes of occurrence.
ALTOSTRATUSThis middle cloud genus is composed ofwater droplets,
and sometimes ice crystals, In themid-latitudes, cloud bases are
generally found between 15,000 and 20,000 feet. White togray in
color, it can create a fibrous veil or sheet, sometimes obscuring
the sun or moon. Itis a good indicator ofprecipitation, as it often
precedes astormsystem. Virgaoften fallsfrom these clouds.
AMERICAN METEOROLOGICAL SOCIETYAn organization whose membership
promotes the education and professional advancementof the
atmospheric,hydrologic, and oceanographic sciences.For further
information, contact the AMS.Related term: National Weather
Association
ANABATIC WINDA wind that is created by air flowing uphill.Valley
breezes, produced by local daytimeheating, are an example of these
winds. The opposite of a katabatic wind.
ANEMOMETER
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An instrument that measures the speed or force of the
wind.Related term: Dave's Dictionary
ANEROID BAROMETERAn instrument for measuring the atmospheric
pressure. It registers the change in the shapeof an evacuated metal
cell to measure variations on the atmospheric pressure. The
aneroidis a thin-walled metal capsule or cell, usually made of
phosphor bronze or beryllium copper.The scales on the glass cover
measure pressure in bothinchesand millibars.
Related term: mercurial barometerANOMALOUS PROPAGATIONThis
refers to the non-standard propagation of a beam of energy, radio
or radar, undercertain atmospheric conditions, appearing as false
(non-precipitation) echoes. May bereferred to as A.P.
ANTARCTICOf or relating to the area around the geographic South
Pole, from 90 South to theAntarctic Circle at approximately 66
1/2South latitude,including the continent ofAntarctica. Along the
Antarctic Circle, the sun does not set on the day of
thesummersolstice (approximately December 21st) and does not rise
on the day of the winter solstice(approximately June 21st).
ANTARCTIC OCEANAlthough not officially recognized as a separate
ocean body, it is commonly applied to thoseportions of the
Atlantic, Pacific, and Indian Oceans that reach the Antarctic
continent ontheir southern extremes.
ANTICYCLONEA relative pressuremaximum.An area ofpressure that
has diverging winds and arotationopposite to the earth's rotation.
This is clockwise the Northern Hemisphere and counter-clockwise in
the Southern Hemisphere. It is the opposite of an area oflow
pressure, or acyclone.Related term: high pressure
ANVILThe upper portion of a cumulonimbus cloud that becomes flat
and spread-out, sometimesfor hundreds of miles downstream from the
parent cloud. It may look smooth or fibrous,but in shape, it
resembles a blacksmith's anvil. It indicates the mature or decaying
stage ofa thunderstorm.
APHELIONThe point on the earth's orbit that is farthest from the
sun. Although the position is part ofa 21,000 year cycle, currently
it occurs around July, when the earth is about 3 million
milesfarther from the sun than at perihelion. This term can be
applied to any other celestial body
in orbit around the sun. It is the opposite of
perihelion.APOGEE
The point farthest from the earth on the moon's orbit. This term
can be applied to any otherbody orbiting the earth, such as
satellites. It is the opposite ofperigee.
ARCTICOf or relating to the area around the geographic North
Pole, from 90 North to the ArcticCircle at approximately 66 1/2
Northlatitude.
ARCTIC AIR MASSAn air mass that develops around the Arctic, it
is characterized by being cold from surfaceto great heights. The
boundary of this air mass is often defined by the Arctic front, a
semi-permanent, semi-continuous feature. When this air mass moves
from its source region, itmay become more shallow in height as it
spreads southward.
ARCTIC JETThejet stream that is situated high in the
stratosphere in and around the Arctic or AntarcticCircles. It marks
the boundary ofpolarand arctic air masses.
ARCTIC SEA SMOKEA type ofadvection fogthat forms primarily over
water whencold air passes across warmerwaters.Related term: steam
fog
ARGON (A)A colorless, odorless inert gas that is the third most
abundant constituent of dry air,comprising 0.93% of the total.
ARIDA term used for an extremely dry climate. The degree to
which a climate lacks effective,life-promotingmoisture. It is
considered the opposite ofhumid when speaking of climates.
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ASOSAcronym for Automated Surface Observing System. This system
is a collection ofautomated weather instruments that collect data.
It performs surface basedobservationsfrom places that do not have a
human observer, or that do not have an observer 24 hoursa day.
ASTRONOMICAL TWILIGHTThe time after nautical twilight has
commenced and when the sky is dark enough, away
from the sun's location, to allow astronomical work to proceed.
It ends when the center ofthe sun is 18 below the horizon.Related
term: twilight
ATMOSPHEREThe gaseous or airportion of the physical environment
that encircles a planet. In the caseof the earth, it is held more
or less near the surface by the earth'sgravitational attraction.The
divisions of the atmosphere include the troposphere, the
stratosphere, the mesosphere,the ionosphere, and the exosphere.
ATMOSPHERIC PRESSUREThe pressure exerted by the atmosphere at a
given point. Its measurement can beexpressed in several ways. One
is in millibars. Another is in inches or millimeters ofmercury
(Hg).Related term: barometric pressure
AURORAIt is created by the radiant energy emission from the sun
and its interaction with the
earth's upper atmosphere over the middle and high latitudes. It
is seen as a bright displayof constantly changing light near the
magneticpoles of each hemisphere. In the NorthernHemisphere, it is
known as the aurora borealis or Northern Lights, and in the
SouthernHemisphere, this phenomena is called the aurora
australis.
AUTUMNThe seasonof the year which occurs as the sun approaches
the wintersolstice, andcharacterized by decreasingtemperaturesin
the mid-latitudes. Customarily, this refers tothe months of
September, October, and November in the North Hemisphere and
themonths of March, April, and May in the Southern Hemisphere.
Astronomically, this is theperiod between the autumnal equinox and
the winter solstice.
AVHRRAcronym for Advanced Very High Resolution Radiometer. It is
the main sensor on the U.S.polar orbitingsatellites.
AVIATION WEATHER CENTERAs one of the National Centers for
Environmental Prediction, it is the national center for
weather information that is used daily by the Federal Aviation
Administration, commercialairlines, and private pilots. It is
entering a new phase of service, growing to accept
globalforecasting responsibilities.For further information, contact
the AWC, located in Kansas City, Missouri.
AWIPSAcronym for Advanced Weather Interactive Processing System.
It is the computerizedsystem that processes NEXRAD and ASOSdata
received atNational Weather ServiceForecast Offices.
AZORES HIGHA semi-permanent,subtropical area ofhigh pressure in
the North Atlantic Ocean thatmigrates east and west with varying
central pressure. Depending on the season, it hasdifferent names.
In the Northern Hemispheric winterand early spring, when the
IcelandicLow dominates the North Atlantic, it is primarily centered
near the Azores Islands. When itis displaced westward, during the
summerand fall, the center is located in the westernNorth Atlantic,
near Bermuda, and is known as theBermuda High.Related term: North
Pacific High
B
BACKINGA counterclockwise shift in the wind directionin the
Northern Hemisphere at a certainlocation. In the Southern
Hemisphere, it is clockwise. This can either happen in
thehorizontal or the vertical (with height). For example, the wind
shifts from the northeast tothe north to the northwest. It is the
opposite ofveering.
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eather.com/glossary/s.html#semihttp://www.weather.com/glossary/s.html#subtrophttp://www.weather.com/glossary/h.html#highhttp://www.weather.com/glossary/c.html#centpresshttp://www.weather.com/glossary/w.html#winterhttp://www.weather.com/glossary/s.html#springhttp://www.weather.com/glossary/i.html#icelowhttp://www.weather.com/glossary/i.html#icelowhttp://www.weather.com/glossary/s.html#summerhttp://www.weather.com/glossary/a.html#autumnhttp://www.weather.com/glossary/b.html#bermudahttp://www.weather.com/glossary/n.html#npachihttp://www.weather.com/glossary/w.html#winddirhttp://www.weather.com/glossary/v.html#veer
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BACKSCATTERA radar echo that isreflected, or scattered, at 180
degrees to the direction of the incidentwave. Also the scattering
ofradiant energy into space before it reaches the earth's
surface.
BALL LIGHTNINGA relatively rare form oflightning consisting of a
luminous ball, often reddish in color, whichmoves rapidly along
solid objects or remains floating in mid-air.Related term: globe
lightning
BAROCLINITYThe state of stratification in a fluid in which
surfaces ofconstant pressure intersect surfacesof constant density.
Also known as baroclinicity. An example is the
tighttemperaturegradient along the East Coast of the United States
during the winter that gives rise tointense cyclogenesis.
BAROGRAPHAn instrument that continuously records a barometer's
reading ofatmospheric pressure.Related term: aneroid barometer
BAROMETERAn instrument used to measure atmospheric pressure. Two
examples are the aneroidbarometer and the mercurial barometer.
BAROMETRIC PRESSUREThe pressure exerted by the atmosphere at a
given point. Its measurement can beexpressed in several ways. One
is in millibars. Another is in inches or millimeters ofmercury
(Hg).
Related term: atmospheric pressureBAROTROPYThe state of a fluid
in which surfaces of constant density ortemperature are coincident
withsurfaces ofconstant pressure. It is considered zero
baroclinity.
BARRIER WINDSRefers to the westerly flow ofair along the
northern slope of the Brooks Range in northernAlaska that precedes
the arrival of colder air from the north.
BATHYTHERMOGRAPHA device used to obtain a record oftemperature
against depth (pressure) in theocean. Maybe referred to as a
B.T.
BEAUFORT WIND SCALEA system of estimating and reporting wind
speeds. It is based on the Beaufort Force orNumber, which is
composed of the wind speed, a descriptive term, and the visible
effectsupon land objects and/or sea surfaces. The scale was devised
by Sir Francis Beaufort(1777-1857), hydrographer to the British
Royal Navy.
BELLOT WINDSRefers to thewinds in the Canadian Arctic that blow
through the narrow Bellot Straitbetween Somerset Island and the
Boothia Peninsula, connecting the Gulf of Boothia andFranklin
Strait.
BERMUDA HIGHA semi-permanent,subtropical area ofhigh pressure in
the North Atlantic Ocean thatmigrates east and west with varying
central pressure. Depending on the season, it hasdifferent names.
When it is displaced westward, during the Northern Hemispheric
summerand fall, the center is located in the western North
Atlantic, near Bermuda. In the winterand early spring, it is
primarily centered near the Azores Islands.Related term: Azores
High
BERNOULLI'S THEOREMA statement of the conservation of energy for
a steady, nonviscous, incompressible levelflow. It is an inverse
relationship in whichpressures are least where velocities are
greatest.Theorized by Daniel Bernoulli (1700-1782), a Swiss
mathematician and physicist.
BIOSPHEREThe transition zone between the earth and the
atmosphere within which most terrestrial lifeforms are found. It is
considered the outer portion of the geosphereand the inner or
lowerportion of the atmosphere.
BLACK BLIZZARDA local term for a violent duststormon the
south-central Great Plains that darkens the skyand casts a pall
over the land.Related term: black roller
BLACK ICE
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Thin, new ice on fresh or salt water that appears dark in color
because of its transparency.Also refers to thin, transparent ice on
road surfaces.
BLIZZARDA severe weather condition characterized by
lowtemperatures,winds 35 mph or greater,and sufficient falling
and/orblowing snowin the air to frequently reduce visibility to
1/4mile or less for a duration of at least 3 hours. A severe
blizzard is characterized bytemperatures near or below 10F, winds
exceeding 45 mph, and visibility reduced by snow
to near zero.BLOCKING HIGHThe development of a warm ridge or
cutoffhighaloft at high latitudes which becomesassociated with a
cold highat the surface, causing a split in thewesterlywinds. Such
a highwill move very slowly, tending to move westward during
intensification and eastward duringdissipation. It prevents the
movement of migratory cyclonesacross its latitudes.Related terms:
cut-off highand Omega block
BLOWING DUSTDust that is raised by thewind to heights of six
feet or greater. It is reported as "BLDU" inanobservation and on
theMETAR.
BLOWING SANDSand that is raised by thewind to heights of six
feet or greater. It is reported as "BLSA" inanobservation and on
theMETAR.
BLOWING SNOWSnowthat is raised by the wind to heights of six
feet or greater. It is reported as "BLSN" in
anobservation and on theMETAR.BLOWING SPRAYSalt spray that is
raised by the wind to heights of six feet or greater. It is
reported as"BLPY" in an observation and on the METAR.
BLUE NORTHERRefers to a swift-moving cold frontalpassage in the
southern Great Plains, marked by adark, blue-black sky with strong
wintery winds from the northwest or north andtemperatures that may
drop 20F to 30F in a few minutes.Related term: Texas Norther
BOILING POINTThe temperature at which a liquid changes to a
vaporous state. The temperature at whichthe equilibriumvapor
pressure between a liquid and its vapor is equal to the
externalpressure on the liquid. The boiling point of pure waterat
standard pressure is 100C or212F.
BOULDER WIND
A local name referring to an extremely strong downslope wind in
the Front Range of theRocky Mountains near Boulder, Colorado.
BOUNDARY LAYERThe lowest layer of the earth's atmosphere,
usually up to 3,300 feet, or one kilometer, fromthe earth's
surface, where the wind is influenced by thefriction of the earth's
surface andthe objects on it.Related terms: surface boundary layer
and friction layer
BOW ECHOA radar echo signature often associated with severe
thunderstorms, especially those thatproduce winddamage. It is bent
outward in a "bow" shape.
BOYLE'S LAWStates that when the temperature is held constant,
the volume of a gas is inverselyproportional to itspressure.
Therefore, if the pressure increases, the volume decreases andvisa
versa. For example, if the volume if halved, then the pressure is
doubled. If thetemperature is held constant, it becomes
anisothermal process. Discovered by RobertBoyle (1627-1691), an
Irish physicist and chemist and co-founder of the Royal
Society.
BRIGHT BANDA narrow, intense radar echo due to
water-coverediceparticles at the melting level wherereflectivity is
at its greatest.
BROKENThe amount ofsky coverfor a cloud layer between 5/8ths and
7/8ths, based on thesummation layer amount for that layer.
BUBBLE HIGHA small high that may be created by precipitation and
vertical instabilityassociated withthunderstormactivity. A product
ofdowndrafts, it is relatively coldand often has the
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characteristics of a different air mass. Convergence along the
leading edge of a bubble highmay help form additional
thunderstorms.Related term: meso high
BUYS BALLOT'S LAWDescribes the relationship of the horizontal
wind direction to the pressure distribution. Inthe Northern
Hemisphere, if one stands with one's back to the wind, the pressure
on one'sleft is lower than the pressure on one's right. It is
reversed in the Southern Hemisphere.
This law was named after the Dutch meteorologist, Buys Ballot,
who developed the formulain 1857.BWER
Acronym for Bounded Weak Echo Region. Refers toradar
echosignatures with lowreflectivity in the center, surrounded by
higher reflectivity. It is associated with strongupdrafts and is
found in the inflow region of a thunderstorm.Related term:
vault
C
CALMAtmospheric conditions devoid ofwind or any other air
motion. In oceanic terms, it is theapparent absence of motion of
the water surface when there is no wind or swell.
CALORIE
In meteorology, it is the amount ofheatrequired to raise the
temperature of one (1) gramofwaterone (1) degree Celsius. It is a
unit of heat energy.
CAPComposed of a layer ofwarmer, dryerair aloft which may
suppress or delay thedevelopment ofthunderstorms. As an air parcel
rises, it becomes cooler relative to theambient, or surrounding,
air in the cap and therefore, less buoyant and unable to
risefurther. Also referred to as a lid.
CAPEAcronym for Convective Available Potential Energy. The
amount of energy available tocreate convection, with higher values
increasing the possibility for severe weather.
CAPE VERDE ISLANDSA group of volcanic islands in the eastern
Atlantic Ocean off the coast of West Africa. ACape Verde hurricane
originates near here.
CARBON DIOXIDE (CO2)A heavy, colorless gas that is the fourth
most abundant constituent of dry air, comprising0.033% of the
total.
CATALINA EDDYA weak low pressurecirculation that may form off
the Southern California coast.
CEILINGThe lowest cloud layer that is reported as brokenor
overcast. If the sky is totallyobscured,then it is the height of
the vertical visibility.Related terms: measured ceiling and
variable ceiling
CEILING LIGHTAn instrument consisting of a drum and an optical
system that projects a narrow verticalbeam of light onto a
cloudbase.
CEILOMETERAn instrument that is used to measure the angular
elevation of a projected light on thebase of a cloud. It measures
the angle of the cloud base included by the observer (ormachine),
the ceiling light and the illuminated spot on the cloud.
CELESTIAL EQUATORThe projection of the plane of the geographical
equator upon the celestial sphere.CELESTIAL SPHERE
The apparent sphere of infinite radius having the earth as its
center. All heavenly bodies(planets, stars, etc.) appear on the
"inner surface" of this sphere and the sun moves alongthe
ecliptic.
CELSIUS TEMPERATURE SCALEA temperature scale where water at sea
level has afreezing pointof 0C (Celsius) and aboiling point of
+100C. More commonly used in areas that observe the metric system
ofmeasurement. Created by Anders Celsius in 1742. In 1948, the
Ninth General Conferenceon Weights and Measures replaced "degree
centigrade" with "degree Celsius."
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Related term: CentigradeCENTRAL PRESSURE
The atmospheric pressure at the center of ahigh orlow. It is the
highest pressure in a highand the lowest pressure in a low,
referring to thesea level pressure of the system on asurface
chart.
CENTRIFUGAL FORCEThe apparent force in a rotating system that
deflects masses radially outward from the axis
of rotation. This force increases towards the equator and
decreases towards the poles.CENTRIPETAL FORCEThe force required to
keep an object moving in a curved or circular path. It is
directedinwards toward the center of the curved path.
CHARLES' LAWStates that when the pressure is held constant, the
volume of a gas varies directly with thetemperature. Therefore, if
the pressure remains constant, the volume of a gas will
increasewith the increase of temperature. It was developed by
Jacques Charles and is also knownas the Charles-Guy-Lussac Law.
CHEMOSPHEREA vaguely defined region of the upper atmosphere in
which photochemical reactions takeplace. It includes the top of
thestratosphere, all of the mesosphere, and sometimes thelower part
of thethermosphere.
CHEYENNE FOGAn upslope fog formed by the westward flow ofair
from the Missouri River Valley,
producing fog on the eastern slopes of the Rockies.CHINOOKA type
offoehn wind. Refers to the warm downslopewind in the Rocky
Mountains that mayoccur after an intensecold spell when the
temperature could rise by 20F to 40F in amatter of minutes.Related
term: Snow Eater and Dave's Dictionary
CHOCOLATTA NORTHA West Indian gale that blows from the
northwest.
CHROMOSPHEREA thin layer of relatively transparent gases above
the photosphere of the sun. It is observedbest during a total
eclipse of the sun.
CIRCULATIONThe flow or motion of a fluid in or through a given
area or volume. In meteorology, it isused to describe the flow
ofair as it moves around a pressure system in the atmosphere.
Itdescribes smaller patterns insemi-permanent pressure systems as
well as the relatively
permanent global currents of air. In oceanic terms, it is used
to describe a water in currentflow within a large area, usually a
closed circular pattern such as in the North Atlantic.
CIRCULATION CELLSLarge areas ofairmovement created by the
rotationof the earth and the transfer ofheatfrom the equator toward
the poles.Circulation is confined to a specific region, such as
thetropics, temperate, or polar, that influences the type ofweather
prevailing there.
CIRRIFORMClouds composed of small particles, mostly ice
crystals. Because the particles are fairlywidely dispersed, this
usually results in relativetransparencyand whiteness,
oftenproducing a halophenomena not observed in other clouds forms.
These clouds generallyhave bases above 20,000 feet in
themid-latitudes, and are classified as high clouds. Theyinclude
all varieties ofcirrus,cirrocumulus, and cirrostratus clouds.
CIRROCUMULUSA cirriformcloud with vertical development,
appearing as a thin sheet of small white puffswhich give it a
rippled effect. It often creates a "mackerel sky", since the
ripples may looklike fish scales. Sometimes it is confused with
altocumulus, however, it has smallerindividual masses and does not
cast a shadow on other elements. It is also the leastcommon cloud
type, often forming from cirrus orcirrostratus, with which it is
associated inthe sky.
CIRROSTRATUSA cirriformcloud that develops from cirrus spreading
out into a thin layer, creating a flatsheetlike appearance. It can
give the sky a slightly milky or veiled look. When viewed fromthe
surface of the earth, theseice crystals can create a halo effect
around the sun or moon.This cloud is a good precursor
ofprecipitation, indicating it may occur within 12 to 24hours.
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CIRRUSOne of the three basic cloud forms (the others
arecumulusand stratus). It is also one ofthe three high cloudtypes.
Cirrus are thin, wispy clouds composed ofice crystals and
oftenappear as veil patches or strands. In the mid-latitudes, cloud
bases are usually foundbetween 20,000 to 30,000 feet, and it is the
highest cloud that forms in the sky, except forthe tops, or anvils,
ofcumulonimbus, which occasionally build to excessive
heights.Related term: Dave's Dictionary
CIVIL TWILIGHTThe time between the moment ofsunset, when the
sun's apparent upper edge is just at thehorizon, until the center
of the sun is 6 directly below the horizon.Related term:
twilight
CLEARThe state of the skywhen no clouds orobscurations are
observed or detected from thepoint ofobservation.
CLEAR AIR TURBULENCEName given to turbulence that may occur in
perfectly clear air without any visual inwarningin the form
ofclouds. It is often found in the vicinity of thejet streamwhere
large shears inthe horizontal and vertical are found, although this
turbulence is not limited just to jetstream locale. Other areas
where it may occur include near mountains, in closed lows aloft,and
in regions ofwind shear. May be referred to as CAT.
CLEAR ICEA glossy, clear, or translucentice formed by the
relatively slow freezing of large
supercooledin water droplets. The droplets spread out over an
object, such as an aircraftwing's leading edge, prior to complete
freezing and forms a sheet of clear ice.Related term: glaze
CLIMATEThe historical record and description of average daily
and in seasonalweatherevents thathelp describe a region. Statistics
are generally drawn over several decades. The word isderived from
the Greek klima, meaning inclination, and reflects the importance
earlyscholars attributed to the sun's influence.
CLIMATE ANALYSIS CENTER (CAC)The U.S. National Weather
Servicedivision that applies new technology and approaches tothe
analysis, diagnosis, and projection of short term climate
fluctuations on a regional andglobal basis.For further information,
contact the CAC, located in Camp Spring, Maryland.
CLIMATE PREDICTION CENTER (CPC)A branch of the National Centers
for Environmental Prediction,the Center maintains a
continuous watch on short-term climate fluctuations and
diagnoses and predicts them.For further information, contact the
CPC, located in Washington, D.C.
CLIMATOLOGYThe study ofclimate. It includes climatic data, the
analysis of the causes of the differencesin climate, and the
application of climatic data to the solution of specific design
oroperational problems.
CLINOMETERAn instrument used to measure angles of inclination.
Used in conjunction with a ceilinglight, it determinescloud height
at night, based on the angle of a projected light on theclouds, the
observer, and the ceiling light.
CLOSED LOWA region oflow pressure distinguished by a center of
counterclockwise circulation (in theNorthern Hemisphere), and is
surrounded by one or more isobarsor height contours.Closed lows
aloft (i.e., above the surface) may become disconnected from the
primarywesterly flowand thus progress eastward more slowly. It is
important to note that a cutofflow is a closed low, but not all
closed lows are cutoff lows.
CLOUDA visible collection of minute particle matter, such as
water droplets and/or ice crystals, inthe freeair. A cloud forms in
the atmosphere as a result ofcondensation ofwater
vapor.Condensation nuclei, such as in smoke ordust particles, form
a surface upon which watervapor can condense.Related term: Dave's
Dictionary
CLOUD BANKA well-defined cloudmass that can be observed at a
distance. It covers thehorizon, but isnot directly overhead.
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CLOUDBURSTA sudden, heavyrainfall of a showery nature.Related
term: downburst
COALESCENCEThe merging of twowater drops into a single larger
drop.
COLDA condition marked by low or decidedly subnormal
temperature. The lack ofheat.
Related term: Dave's DictionaryCOLD ADVECTIONThe horizontal
movement ofcolder air into a location. Contrast withwarm
advection.
COLD AIR FUNNELFunnel clouds, usually short-lived, that develop
from relatively smallshowers orthunderstorms when the air aloft is
very incold. Cold air funnels may touch down briefly,but in general
are less violent than most other types oftornadoes.
COLD CORE THUNDERSTORMSThunderstorms formed primarily due to
steeplapse rates, especially when very coldairaloft overlies warmer
surface air.
COLD FRONTThe leading edge of an advancing coldair mass that is
under running and displacing thewarmer air in its path. Generally,
with the passage of a cold front, the temperature
andhumiditydecrease, the pressurerises, and the wind shifts
(usually from the southwest tothe northwest in the Northern
Hemisphere). Precipitation is generally at and/or behind the
front, and with a fast-moving system, asquall linemay develop
ahead of the front.Related terms: occluded front and warm frontCOLD
HIGH
A high pressure system that has its coldest temperatures at or
near the center ofcirculation, and horizontally, is thermally
barotropic. It is shallow in nature, as circulationdecreases with
height. Associated with cold Arctic air, it is usuallystationary.
Also known asa cold core high. Contrast with awarm high.
COLD LOWA low pressure system that has its coldest temperatures
at or near the center ofcirculation,and is thermally barotropicwith
respect to a horizontal plane. Also known as a cold corelow. Acut
off low is an example, where an isolated pool of colder air is
located south of themain westerlies.
COLD WAVEA rapid fall in temperature within twenty-four hours to
temperatures requiring substantiallyincreased protection to
agriculture, industry, commerce, and social activities.
National
Weather Servicecriteria includes the rate of temperature fall
and theminimum to which itfalls, depending on the region of the
country and time of the inyear. The Weather Channeluses the
following criteria for a cold wave: a cold spell of two days or
more with belownormal temperatures in at least fifteen states, with
at least five of them more than fifteendegrees below normal.
COLLADAA strong, steadywindblowing from the north or northwest
in the upper part of the Gulf ofCalifornia and from the northeast
in the lower part.
COLORADO LOWA low pressure disturbance that forms in the leeof
the Rocky Mountains, usually insoutheastern Colorado.
COMMA CLOUDA feature seen on satellite images with a distinctive
comma-shape. This is indicative of asynopticcloud pattern
associated with large, well-developedlow pressure systems.
CONDENSATIONThe process by which water vaporundergoes a change
in state from a gas to a liquid. It isthe opposite physical process
ofevaporation.
CONDENSATION FUNNELA funnel-shaped cloud consisting of
condensedwater dropsthat has possible rotation.
CONDENSATION NUCLEIA particle upon which condensation ofwater
vapor occurs. It may be either in a solid orliquid state.
CONDITIONAL INSTABILITY
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