Meteorology
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Ablation
: The process of being removed. Snow ablation usually refers
to removal by melting.
Absolute Humidity : The density of water vapor.
It is the mass of the water vapor divided by the volume that it
occupies.
Absolute temperature: The temperature
of the centigrade thermometer, increased by 273°, more properly
called the temperature on the absolute or thermodynamic scale. On
the Fahrenheit scale the absolute zero is approximately 459° below
the Fahrenheit zero.
Accretion : Growth of precipitation particles
by collision of ice crystals with supercooled liquid droplets which
freeze on impact
Accessory Clouds : Clouds that are dependent
on a larger cloud system for development and continuance. Accessory
clouds associated with the thunderstorm include roll, shelf, mammatus,
and wall clouds.
Acid Rain : Cloud or rain droplets containing pollutants,
such as oxides of sulfur and nitrogen, to make them acidic (e.g.
pH < 5.6).
Actinometer: An instrument for measuring
the intensity of radiation received from the sun.
Additive data :
A group of coded remarks in a weather observation that
includes pressure tendency, amount of precipitation, and maximum/minimum
temperature during specified periods of time.
Adiabatic : changes in temperature caused by the expansion
(cooling) or compression (warming) of a body of air as it rises
or descends in the atmosphere. The word applied in the science
of thermodynamics to a process during which no heat is communicated
to or withdrawn from the body or system concerned. Adiabatic changes
of atmospheric temperature arc those that occur only in consequence
of compression or expansion accompanying an increase or a decrease
of atmospheric pressure. Such changes are also described as dynamic
heating and cooling.
Adiabatic Process
: The change of temperature of air without transferring heat.
In an adiabatic process compression results in warming, and expansion
results in cooling.
Advection : The horizontal transport of air, moisture
or other atmospheric properties. Commonly used with temperatures,
i.e., "warm air advection". The process of
transfer by horizontal motion, particularly applied to the transfer
of heat by horizontal motion of the air. The transfer of heat from
low to high latitudes is the most obvious example of advection.
Advection Fog : a type of fog that results from the
advection of moist air over a cold surface and the cooling of the
air to its dew point that follows; this type of fog is most common
in coastal regions. Fog resulting from the transfer of warm,
humid air over a cold surface, especially a cold ocean surface,
or (comparatively rarely) from the transport of air that is relatively
very cold over an ocean surface that is relatively very warm.
Advisory : Advisories
are issued for weather situations that cause significant inconveniences
but do not meet warning criteria and, if caution is not exercised,
could lead to life : threatening situations. Advisories are issued
for significant events that are occurring, are imminent, or have
a very high probability of occurrence.
Aerology. The study of the free atmosphere
throughout its vertical extent, as distinguished from investigations
confined to the layer of the atmosphere adjacent to the earth's
surface. Aerological investigations are made directly with pilot
balloons, sounding balloons and by means of airplanes. They are
also made indirectly by visual observations from the ground. Included
in the latter are observations of clouds, meteor trails, the aurora,
etc.
Aerosol : Particles of matter, solid or liquid,
larger than a molecule but small enough to remain suspended in the
atmosphere (up to 100� m diameter). Natural origins include
salt particles from sea spray and clay particles as a result of
weathering of rocks. Aerosols can also originate as a result of
man's activities and in this case are often considered pollutants.
Aerovane : Aerovanes are commonly used at
many weather stations and airports to measure both wind direction
and speed. They are similar to wind vanes and cup anemometers except
have three : bladed propellers attached to the end of the vane.
AFOS : the Automation
of Field Operations and Services; AFOS is the computer system that
links National Weather Service offices and other computer networks,
such as the NOAA Weather Wire, to transmit weather information.
AGL : above ground level.
Air : the mixture of gases that make : up the earth's
atmosphere.
Afterglow. The glow in the western sky
after sunset.
Air mass. A term applied by meteorologists
to an extensive body of air within which the conditions of temperature
and moisture in a horizontal plane are essentially uniform.
A large body of air that has similar horizontal temperature and
moisture characteristics
Air mass property.
Any quality or quantity the nature or value of which can be used
in a characterization of the physical state or condition of an air
mass.
Air : mass Thunderstorm
: Generally, a thunderstorm not associated with a
front or other type of synoptic : scale forcing mechanism. Air mass
thunderstorms typically are associated with warm, humid air in the
summer months; they develop during the afternoon in response to
insolation, and dissipate rather quickly after sunset.
Air Parcel : An imaginary small body of air
that is used to explain the behavior of air. A parcel is large enough
to contain a very great number of molecules, but small enough so
that the properties assigned to it are approximately uniform throughout.
Air Pollution
: The existence in the air of substances in concentrations
that are determined unacceptable. Contaminants in the air we breathe
come mainly from manufacturing industries, electric power plants,
automobiles, buses, and trucks.
Air Pressure : (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.
Airstream : A significant body of air flowing
in the same general circulation.
Albedo : The percentage of light reflected by an object.
Snow covered areas have a high albedo (0.9 or 90%) due to their
white color.
Alberta Clipper : A small, fast : moving
low : pressure system that forms in western Canada and travels southeastward
into the United States. These storms, which generally bring little
precipitation, generally precede an Arctic air mass.
Altimeter : An active instrument (see active
system) used to measure the altitude of an object above a fixed
level.
Altimeter setting : That pressure value to
which an aircraft altimeter scale is set so that it will indicate
the altitude above mean sea : level of an aircraft on the ground
at the location for which the value was determined.
Altitude : Height expressed as the distance
above a reference point, which is normally sea level or ground level.
Altocumulus : Mid : altitude clouds with a cumuliform
shape.
Altostratus : Mid
: altitude clouds with a flat sheet : like shape.
Anabatic : wind flowing
up an incline, such as up a hillside; upslope wind.
Anafront : A front at which the warm is ascending
the frontal surface up to high altitudes.
Anemometer : An instrument that measures wind speed.
Anemoscope. An instrument for indicating
the existence of wind and showing its direction.
Aneroid barometer
: An instrument built around a metal structure that bends
with changing air pressure. These changes are recorded on a pointer
that moves back and forth across a printed scale.
Angular Momentum : the energy of motion of a spinning
body or mass of air or water.
Angular Velocity : the rate at which a spinning body
rotates.
Anomaly : The deviation of (usually) temperature
or precipitation in a given region over a specified period from
the normal value for the same region. The difference between
the mean of any meteorological element, or phase of that element,
over a given time at a particular place, and the mean of the same
clement or phase over the same time for all other points on the
same parallel of latitude.
Anthelion. A rare species of halo, consisting
of a brilliant, usually white, image of the sun opposite the latter
in azimuth. (This term has also been applied to the glory, q. v.).
Anticrepuscular rays. The continuation of
the crepuscular rays, converging toward a point in the sky opposite
to the sun.
Anticyclogenesis. The term applied to the
process which creates or develops a new anticyclone. The word is
applied also to the process which produces an intensification of
a preexisting anticyclone.
Anticyclone : A large
body of air in which the atmospheric pressure is higher than the
pressure in the surrounding air. The winds blow clockwise
around an anticyclone in in the Northern Hemisphere.
Anticyclonic : describes
the movement of air around a high pressure, and rotation about
the local vertical opposite the earth's rotation. This is clockwise
in the Northern Hemisphere.
Anvil Cloud : The flat, spreading top of
a Cb (cumulonimbus), often shaped like an anvil. Thunderstorm anvils
may spread hundreds of miles downwind from the thunderstorm itself,
and sometimes may spread upwind (see back : sheared anvil).
Anvil Crawler : A lightning discharge
occurring within the anvil of a thunderstorm, characterized by one
or more channels that appear to crawl along the underside of the
anvil. They typically appear during the weakening or dissipating
stage of the parent thunderstorm, or during an active MCS.
Anvil Dome : A large overshooting top
or penetrating top.
Anvil Rollover : A circular or semicircular
lip of clouds along the underside of the upwind part of a back :
sheared anvil, indicating rapid expansion of the anvil. See cumuliform
anvil, knuckles, mushroom.
Anvil Zits : Frequent (often continuous
or nearly continuous), localized lightning discharges occurring
from within a thunderstorm anvil.
Arctic Air : a mass of very cold, dry air that usually
originates over the Arctic Ocean north of Canada and Alaska.
Arctic High : a very cold high pressure that originates
over the Arctic Ocean.
Arcus : A low, horizontal cloud formation
associated with the leading edge of thunderstorm outflow (i.e.,
the gust front). Roll clouds and shelf clouds both are types of
arcus clouds.
Aridity : A general term used to describe
areas suffering from lack of rain or drought. More specifically,
a condition in which evaporation exceeds precipitation.
ASOS : Automated Surface Observing System. This system
observes sky conditions, temperature and dewpoint, wind direction
and speed, and barometric pressure, and precipitation.
Atmosphere : The mass of air surrounding the earth
and bound to it more or less permanently by the earth's gravitational
attraction.
Atmospheric Pressure : (also called air pressure
or barometric pressure) The pressure asserted by the mass of the
column of air directly above any specific point.
Atmospheric Stability : An indication of
how easily a parcel of air is lifted. If the air is very stable
it is difficult to make the parcel rise. If the air is very
unstable the parcel may rise on its own once started.
Aurora Borealis : Also known as the northern
lights : The luminous, radiant emission from the upper
atmosphere over middle and high latitudes, and centered around the
earth's magnetic poles. These silent fireworks are often seen on
clear winter nights in a variety of shapes and colors.
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.
VHRR : Advanced Very High Resolution Radiometer. Main
sensor on U.S. polar orbiting satellites.
Avalanche : a large mass of rapidly moving snow down
a steep mountain slope.
AVN : Aviation Model
generated every 12 hours by NCEP.
AWIPS : Advanced
Weather Information Processing System. New NWS computer system integrating
graphics, satellite and radar imagery. The successor to AFOS.
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Back Door Cold Front
: A front that moves east to west in direction rather than
the normal west to east movement. For instance, one that enters
Southern New England from the Gulf of Maine.
Back : building Thunderstorm : A thunderstorm
in which new development takes place on the upwind side (usually
the west or southwest side), such that the storm seems to remain
stationary or propagate in a backward direction.
Back : sheared Anvil : A thunderstorm anvil
which spreads upwind, against the flow aloft. A back : sheared anvil
often implies a very strong updraft and a high severe weather potential.
Backing Wind : Wind which shifts 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). Backing winds with height are indicative of cold air advection
(CAA). The opposite of veering winds.
Ball lightning : A relatively rarely seen
form of lightning, generally consisting of an orange or reddish
ball of the order of a few cm to 30 cm in diameter and of moderate
luminosity, which may move up to 1 m/s horizontally with a lifetime
of a second or two.
Barber Pole : A
thunderstorm updraft with a visual appearance including cloud striations
that are curved in a manner similar to the stripes of a barber pole.
The structure typically is most pronounced on the leading edge of
the updraft, while drier air from the rear flank downdraft often
erodes the clouds on the trailing side of the updraft.
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.
Barogram : The graphic record of pressure
produced by a barograph.
Barograph : An instrument that provides a
continuous record of atmospheric pressure.
Barometer : An instrument for measuring atmospheric
pressure.
Barometric pressure : The actual pressure
value indicated by a pressure sensor.
Barometric Tendency : The amount and direction
of change in barometer readings over a three : hour period.
Barotropic System : A weather system in which
temperature and pressure surfaces are coincident, i.e., temperature
is uniform (no temperature gradient) on a constant pressure surface.
Barotropic systems are characterized by a lack of wind shear, and
thus are generally unfavorable areas for severe thunderstorm development.
Bear's Cage : A region of storm : scale rotation,
in a thunderstorm, which is wrapped in heavy precipitation. This
area often coincides with a radar hook echo and/or mesocyclone,
especially one associated with an HP storm. The term reflects the
danger involved in observing such an area visually, which must be
done at close range in low visibility.
Beaufort Scale : A scale that indicates the wind speed
using the effect wind has on certain familiar objects.
Beaver('s) Tail : A particular type of inflow
band with a relatively broad, flat appearance suggestive of a beaver's
tail. It is attached to a supercell's general updraft and is oriented
roughly parallel to the pseudo : warm front, i.e., usually east
to west or southeast to northwest.
Black Ice : thin, new ice that forms on fresh water
or dew covered surfaces; it is common on roadways during the fall
and early winter and appears "black" because of its transparency.
Blizzard : Includes winter storm conditions of sustained
winds or frequent gusts of 35 mph or more that cause major
blowing and drifting of snow, reducing visibility to less than one
: quarter mile for 3 or more hours. Extremely cold temperatures
often are associated with dangerous blizzard conditions.
Blizzard warning : Issued when blizzard condition
are expected or are occurring.
Blocking High
: A high pressure area (anticyclone), often aloft, that remains
nearly stationary or moves slowly compared to west : to : east motion.
It blocks the movement eastward movement of low pressure areas (cyclones)
at its latitude..
Blowing Dust : dust that is raised by the wind to moderate
heights above the ground to a degree that horizontal visibility
decreases to less than seven miles. Visibilities of 1/8 mile
or less over a widespread area are criteria for a Blowing Dust Advisory.
Blowing Sand : Sand particles picked up from
the surface of the earth by the wind to moderate heights above the
ground, reducing the reported horizontal visibility to less than
7 statute miles.
Blowing Snow : Wind driven snow that reduces
visibility to six miles or less causing significant drifting. Blowing
snow may be snow that is falling and/or loose snow on the ground
picked up by the wind.
Blowing spray : Water droplets torn by the
wind from a body of water, generally from the crests of waves, and
carried up into the air in such quantities that they reduce the
reported horizontal visibility to less than 7 statute miles.
Blustery : Descriptive term for gusty winds
that accompany cold weather.
Bomb Cyclone : An extratropical area of low
pressure in which the central pressure drops at least 24 millibars
in 24 hours.
Boundary Layer : In general, a layer of air
adjacent to a bounding surface. 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.
Bow echo : A radar echo which is linear but
bent outward in a bow shape. Damaging straight : line winds
often occur near the "crest" or center of a bow
echo. Areas of circulation also can develop at either end of a bow
echo, which sometimes can lead to tornado formation :
especially in the left (usually northern) end, where the circulation
exhibits cyclonic rotation.
Box (or Watch Box) : A severe thunderstorm
or tornado watch.
Breezy : Wind in the range of 15 mph to 25
mph with mild or warm temperatures.
Brisk : Wind in the range of 15 to 25 mph
when the temperature is cold.
Broken Clouds : Clouds which cover between
5/8ths and 7/8ths of the sky.
Buoyancy : That property of an object that
enables it to float on the surface of a liquid, or as in the case
with air parcels, to ascend and remain freely suspended in the atmosphere.
Bubble High : A mesoscale area of high pressure,
typically associated with cooler air from the rainy downdraft area
of a thunderstorm or a complex of thunderstorms. A gust front or
outflow boundary separates a bubble high from the surrounding air.
Bulk Richardson Number (or BRN) : A non :
dimensional number relating vertical stability and vertical shear
(generally, stability divided by shear). High values indicate unstable
and/or weakly : sheared environments; low values indicate weak instability
and/or strong vertical shear. Generally, values in the range of
around 50 to 100 suggest environmental conditions favorable for
supercell development.
Bust : An inaccurate forecast, usually
a situation in which significant weather is expected, but does not
occur.
BWER : Bounded Weak Echo Region. (Also
known as a vault.) Radar signature within a thunderstorm characterized
by a local minimum in radar reflectivity at low levels which extends
upward into, and is surrounded by, higher reflectivities aloft.
This feature is associated with a strong updraft and is almost always
found in the inflow region of a thunderstorm. It cannot be seen
visually.
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CAA : Cold Air Advection
Calm : the absence of apparent motion in the air.
Cap (or Capping Inversion) : A layer of relatively
warm air aloft (usually several thousand feet above the ground)
which suppresses or delays the development of thunderstorms. Air
parcels rising into this layer become cooler than the surrounding
air, which inhibits their ability to rise further. As such, the
cap often prevents or delays thunderstorm development even in the
presence of extreme instability.
CAPE : Convective Available Potential Energy.
A measure of the amount of energy available for convection. CAPE
is directly related to the maximum potential vertical speed within
an updraft; thus, higher values indicate greater potential for severe
weather. Observed values in thunderstorm environments often may
exceed 1,000 joules per kilogram (j/kg), and in extreme cases may
exceed 5,000 j/kg. However, as with other indices or indicators,
there are no threshold values above which severe weather becomes
imminent.
Cb : Cumulonimbus cloud
Ceiling : The height of the lowest layer
of broken or overcast clouds.
Ceilometer : A device used to evaluate the
height of clouds or the vertical visibility into a surface : based
obscuration.
Cell : Convection in the form of a single
updraft, downdraft, or updraft/downdraft couplet, typically seen
as a vertical dome or tower as in a cumulus or towering cumulus
cloud. A typical thunderstorm consists of several cells
Celsius : a temperature scale in which zero is the
freezing point of water and one hundred is the boiling point.
Chance : A 30, 40 or 50 percent chance of
occurrence of measurable precipitation.
Chinook Wind : A strong downslope wind that causes
the air to warm rapidly as a result of compressive heating; called
a foehn wind in Europe.
Circulation : The pattern of the movement
of air. General circulation is the flow of air of large, semi :
permanent weather systems, while secondary circulation is the flow
of air of more temporary weather systems.
Cirriform : High altitude ice clouds with a very thin
wispy appearance.
Cirrocumulus : Cirrus clouds with vertical development.
Cirrostratus :
Cirrus clouds with a flat sheetlike appearance.
Cirrus : High
clouds, usually above 18,000 feet, composed of ice crystals and
appearing in the form of white, delicate filaments or white or mostly
white patches or narrow bands.
Clear : Sky condition
of less than 1/10 cloud coverage.
Clear Slot : A local region of clearing skies
or reduced cloud cover, indicating an intrusion of drier air; often
seen as a bright area with higher cloud bases on the west or southwest
side of a wall cloud.
Climate : The prevalent long term weather
conditions in a particular area. Climatic elements include precipitation,
temperature, humidity, sunshine and wind
velocity and phenomena such as fog, frost, and hail storms. Climate
cannot be considered a satisfactory indicator of actual conditions
since it is based upon
a vast number of elements taken as an average.
Climate change : This strictly refers to
all forms of climatic inconsistency. But it is often
used in a more restricted sense to imply a significant change. Within
the media, climate change has been used synonymously with global
warming. Scientists, however, use the term in a wider sense to include
past climate changes also.
Climate Normals : Averages of temperatures, precipitation,
snowfall, etc. made over standard 30 year periods. These normals
span across 3 decades and are rederived every 10 years.
Climatology : the
scientific study of climate.
Closed Low : A low pressure area with a distinct
center of cyclonic circulation which can be completely encircled
by one or more isobars or height contour lines. The term usually
is used to distinguish a low pressure area aloft from a low : pressure
trough. Closed lows aloft typically are partially or completely
detached from the main westerly current, and thus move relatively
slowly.
Cloud : A visible cluster of tiny water and/or
ice particles in the atmosphere.
Cloud Base : For a given cloud or cloud layer,
it is the lowest level in the atmosphere where cloud particles are
visible.
Cloud Condensation Nuclei : small particles in the
air on which water vapor condenses and forms cloud droplets.
Cloud Streets : Rows of cumulus or cumulus
: type clouds aligned parallel to the low : level flow. Cloud streets
sometimes can be seen from the ground, but are seen best on satellite
photographs.
Cloud Tags : Ragged, detached cloud
fragments; fractus or scud.
Cloudburst : A sudden, intense rainfall that
is normally of short duration.
Cloudy : the state of the sky when 7/10ths or more
of the sky is covered by clouds.
Coastal Flood Warning : Issued when there
is widespread coastal flooding expected within 12 hours, more than
just typical overwash.
Coastal Flooding : The inundation of land areas along
the coast caused by sea water above normal tidal actions.
This is often caused by prolonged strong onshore flow of wind and/or
high astronomical tides.
Coastal Forecast : A forecast of wind, wave
and weather conditions between the coastline and 25 miles offshore.
Coastal Waters : include the area from a line approximating
the mean high water along the mainland or island as far out as 25
miles including the bays, harbors and sounds.
Cold Advection : (CAA) Transport of cold
air into a region by horizontal winds.
Cold Air Damming : Cold air damming occurs
when a cold dome of high pressure settles over northeastern New
England. The clockwise circulation around the high pressure center
brings northeasterly winds to the mid Atlantic region. The northeasterly
winds bank cold air against the eastern slopes of the Appalachian
Mountains. Warmer air from the west or southwest is lifted
above the cold air as it moves instead of warming the surface.
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.
Cold Front : A narrow transition zone separating
advancing colder air from retreating warmer air. The air behind
a cold front is cooler and typically drier than the air it is replacing.
Cold Low : a low pressure system with cold air mass
from near the surface to all vertical levels (also called a cold
core low).
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.
Collar Cloud : Frequently used as a synonym
for wall cloud, although it actually is a generally circular ring
of cloud surrounding the upper portion of a wall cloud.
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.
Condensation : The process by which water
vapor becomes a liquid; the opposite of evaporation, which is the
conversion of liquid to vapor.
Condensation Nuclei : Small particles in
the air around which water vapor condenses.
Conduction : The transfer of heat by molecular action
between bodies that are in contact.
Confluence : A pattern of wind flow in which
air flows inward toward an axis oriented parallel to the general
direction of flow. It is the opposite of difluence. Confluence is
not the same as convergence. Winds often accelerate as they enter
a confluent zone, resulting in speed divergence which offsets the
(apparent) converging effect of the confluent flow.
Congestus (or Cumulus Congestus) : A large
cumulus cloud with great vertical development, usually with a cauliflower
: like appearance, but lacking the characteristic anvil shaped top
of a Cb.
Continental Air Mass : A dry air mass originating
over a large land area.
Contrail : A cloud : like stream formed in
cold, clear air behind the engines of an airplane.
Convection : The transfer of heat within a the air
by its movement. The term is used specifically to describe
vertical transport of heat and moisture, especially by updrafts
and downdrafts in an unstable atmosphere.
Convective Outlook : A forecast containing
the area(s) of expected thunderstorm occurrence and expected severity
over the contiguous United States, issued several times daily by
the SPC.
Convective Temperature : The approximate
temperature that the air near the ground must warm to in order for
surface : based convection to develop, based on analysis of a sounding.
Convergence : An atmospheric condition that
exists when the winds cause a horizontal net inflow of air into
a specified region. Divergence is the opposite, where winds cause
a horizontal net outflow of air from a specified region.
Coordinated Universal Time (UTC) : The time
in the zero degree meridian time zone.
Combined Seas : The combined height of swell
and wind waves.
Cooling Degree Day : A form of degree day
used to estimate the required energy for cooling. one cooling degree
day occurs for each degree the daily mean temperature is above 65
degrees Fahrenheit.
Coriolis Force : An apparent force caused
by the rotation of the Earth. In the Northern Hemisphere winds are
deflected to the right, and in the Southern Hemisphere to the left.
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
Corona : a disk of light surrounding the sun or moon;
this is a result of the diffraction of light by small water droplets.
CRS : Console
Replacement System. This consists of a computer system and
computer voice that is used to automate NOAA Weather Radio.
Cumulonimbus Cloud : A vertically developed
cloud, often capped by an anvil shaped cloud. Also called a thunderstorm
cloud, it is frequently accompanied by heavy showers, lightning,
thunder, and sometimes hail or gusty winds.
Cumulus Cloud :
A cloud in the shape of individual detached domes,
with a flat base and a bulging upper portion resembling cauliflower.
Cumulus Congestus : A large cumulus cloud
with great vertical development, usually with a cauliflower : like
appearance, but lacking the characteristic anvil shaped top of a
Cb.
Cut Off Low : An
upper level low pressure system that is no longer in the normal
west to east upper air flow. Usually a cut : off low will lie to
the South of the established upper air flow.
Cyclogenesis : Development or intensification
of a low : pressure center
Cyclone : An area of low pressure around
which winds blow counterclockwise in the Northern Hemisphere. Also
the term used for a hurricane in the Indian Ocean and in the Western
Pacific Ocean.
Cyclonic Circulation (or Cyclonic Rotation) :
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.
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Dart Leader : In
lightning, the leader which, after the first stroke, initiates each
succeeding stroke of a composite flash of lightning.
Debris Cloud : A rotating "cloud"
of dust or debris, near or on the ground, often appearing beneath
a condensation funnel and surrounding the base of a tornado.
Decouple : The tendency for the surface wind
to become much lighter than wind above it at night when the surface
temperature cools.
Degree Day : a measure of the departure of the daily
mean temperature from the normal daily temperature; heating and
cooling Degree Days are the departure of the daily mean temperature
from sixty : five degrees Fahrenheit.
Dendrite : hexagonal ice crystals with complex
and often fernlike branches.
Dense Fog : a fog in which the visibility is less than
one : quarter mile.
Dense Fog Advisory : Issued when fog is expected
to reduce visibility to 1/4 mile or less over a widespread are for
at least 3 hours.
Density Of Air : The mass of air divided
by its volume. The air's density depends on its temperature,
its pressure and how much water vapor is in the air.
Density Altitude : The pressure altitude corrected
for temperature deviations from the standard atmosphere. It
is used by pilots when setting aircraft performance.
Depression : a region
of low atmospheric pressure that is usually accompanied by low clouds
and precipitation.
Depth Hoar : Large (one to several millimeters
in diameter), cohesionless, coarse, faceted snow crystals which
result from the presence of strong temperature gradients within
the snowpack
Derechoe : A widespread and usually fast : moving
windstorm associated with convection. Derechoes include any family
of downburst clusters produced by an extratropical MCS, and can
produce damaging straight : line winds over areas hundreds of miles
long and more than 100 miles across.
Dew : Moisture from water vapor in the air that has
condensed on objects near the ground, whose temperatures have fallen
below the dewpoint temperature.
Dew Point : The
temperature to which the air must be cooled for water vapor to condense
and form fog or clouds.
Diamond Dust : A fall of non : branched (snow
crystals are branched) ice crystals in the form of needles, columns,
or plates. (same as ice crystals)
Differential Motion : Cloud motion that appears
to differ relative to other nearby cloud elements, e.g. clouds moving
from left to right relative to other clouds in the foreground or
background. Cloud rotation is one example of differential motion,
but not all differential motion indicates rotation. For example,
horizontal wind shear along a gust front may result in differential
cloud motion without the presence of rotation.
Difluence (or Diffluence) : A pattern of
wind flow in which air moves outward (in a "fan : out"
pattern) away from a central axis that is oriented parallel to the
general direction of the flow. It is the opposite of confluence.
Dirty ridge : Most of the time, upper : level
ridges bring fairly clear weather as the storms are steered around
the ridge. Sometimes, however, strong storms undercut the ridge
and create precipitation. Ridges that experience this undercutting
by storms are known as dirty ridges because of the unusual precipitation.
Disturbance : a disruption of the atmosphere
that usually refers to a low pressure area, cool air and inclement
weather.
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 : The expansion or spreading out of a vector
field; usually said of horizontal winds. It is the opposite of convergence.
Doldrums : the regions on either side of the equator
where air pressure is low and winds are light.
Doppler Radar : A type of weather radar that determines
whether atmospheric motion is toward or away from the radar. It
determines the intensity of rainfall and uses the Doppler effect
to measure the velocity of droplets in the atmosphere.
Downburst : A strong downdraft resulting in an outward
burst of damaging winds on or near the ground. Downburst winds can
produce damage similar to a strong tornado.
Downdraft : A column of generally cool air
that rapidly sinks to the ground, usually accompanied by precipitation
as in a shower or thunderstorm. .
Downslope wind : Air that descends an elevated
plain and consequently warms and dries. Occurs when prevailing wind
direction is from the same direction as the elevated terrain and
often produces fair weather conditions.
Downstream : In the same direction as a stream or
other flow, or toward the direction in which the flow is moving.
Drifting snow : Uneven distribution of snowfall caused
by strong surface winds. Drifting snow does not reduce visibility.
Dry Adiabat : A line of constant potential
temperature on a thermodynamic chart.
Dry Line : A boundary separating moist and
dry air masses, and an important factor in severe weather frequency
in the Great Plains.
Dry : line Bulge : A bulge in the dry line,
representing the area where dry air is advancing most strongly at
lower levels
Drizzle : Small, slowly falling water droplets, with
diameters between .2 and .5 millimeters.
Drought : Abnormally dry weather in a region
over an extended period sufficient to cause a serious hydrological
(water cycle) imbalance in the affected area. This can cause such
problems as crop damage and water : supply shortage.
Dry Punch : A surge of drier air; normally
a synoptic : scale or mesoscale process. A dry punch at the surface
results in a dry line bulge.
Dry Slot : A zone of dry (and relatively
cloud : free) air which wraps east : or northeastward into
the southern and eastern parts of a synoptic scale or mesoscale
low pressure system. A dry slot generally is seen best on satellite
photographs.
Dryline : A boundary which separates warm, dry air
from warm, moist air. The differences in the two air masses may
be significant. The dry line is usually a boundary of instability
along which thunderstorms form.
Dust Devil : A small, rapidly rotating wind that is
made visible by the dust, dirt or debris it picks up. Also called
a whirlwind. Dust devils usually develop during hot, sunny
days over dry and dusty or sandy areas.
Dust Storm : An area where high surface winds have
picked up loose dust, reducing visibility to less than one : half
mile.
Dust Plume : A non : rotating "cloud"
of dust raised by straight : line winds. Often seen in a microburst
or behind a gust front.
Dust Whirl : A rotating column of air rendered
visible by dust.
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.
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Easterly Wave : A wavelike
disturbance in the tropical easterly winds that usually moves from
east to west. Such waves can grow into tropical depressions.
ECMF : European Center for Meteorology Forecast model.
Eddy : A small volume of air that behaves
differently from the predominant flow of the layer in which it exists,
seemingly having a life of its own. An example of such would be
a tornado, which has its own distinct rotation, but is different
than the large : scale flow of air surrounding the thunderstorm
in which the tornado is born.
El Ni�o : A
major warming of the equatorial waters in the eastern Pacific Ocean.
El Ni�o events usually occur every 3 to 7 years, and are
related to shifts in global weather patterns. (Spanish for the "Christ
Child", named this because it often begins around Christmas.)
Enhanced greenhouse effect : The natural
greenhouse effect has been enhanced by man's emissions of greenhouse
gases. Increased concentrations of carbon dioxide, methane and nitrous
oxide trap more infra : red radiation, so heating up the atmosphere.
Enhanced Wording : An option used by the
SPC in tornado and severe thunderstorm watches when the potential
for strong/violent tornadoes, or unusually widespread damaging straight
: line winds, is high.
Entrance Region : The region upstream from
a wind speed maximum in a jet stream (jet max), in which air is
approaching (entering) the region of maximum winds, and therefore
is accelerating. This acceleration results in a vertical circulation
that creates divergence in the upper : level winds in the right
half of the entrance region (as would be viewed looking along the
direction of flow). This divergence results in upward motion of
air in the right rear quadrant (or right entrance region) of the
jet max. Severe weather potential sometimes increases in this area
as a result.
ENSO : El Nino : Southern Oscillation.
Equilibrium Level (or EL) : On a sounding,
the level above the level of free convection (LFC) at which the
temperature of a rising air parcel again equals the temperature
of the environment.
ETA : "Eta" (from Greek) model generated
every 12 hours by NCEP
Evaporation : the process of a liquid changing into a
vapor or gas.
Excessive Heat Warning : Issued within 12
hours of the onset of the following conditions: heat index of at
least 105 degrees Fahrenheit for more than 3 hours per day for 2
consecutive days or heat index more than 115 degrees Fahrenheit
for any period of time.
Excessive Heat Watch : Issued for the potential of
the following conditions within 12 to 36 hours: heat index of at
least 105 degrees Fahrenheit for more than 3 hours per day for 2
consecutive days or heat index more than 115 degrees Fahrenheit
for any period of time.
Exit Region : The region downstream from
a wind speed maximum in a jet stream (jet max), in which air is
moving away from the region of maximum winds, and therefore is decelerating.
This deceleration results in divergence in the upper : level winds
in the left half of the exit region (as would be viewed looking
along the direction of flow). This divergence results in upward
motion of air in the left front quadrant (or left exit region) of
the jet max. Severe weather potential sometimes increases in this
area as a result.
Extended Outlook : a basic forecast of general weather
conditions three to five days in the future.
Extratropical cyclone : A storm that forms
outside the tropics, sometimes as a tropical storm or hurricane
changes. See table below for differences between extratropical and
tropical cyclones.
Eye : The
low pressure center of a tropical cyclone. Winds are normally calm
and sometimes the sky clears.
Eye wall : The
ring of thunderstorms that surrounds a storm's eye. The heaviest
rain, strongest winds and worst turbulence are normally in the eye
wall.
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Fahrenheit : the standard
scale used to measure temperature in the United States; in which
the freezing point of water is thirty : two degrees and the boiling
point is two hundred and twelve degrees.
Fair : describes weather in which there is less than
4/10ths of opaque cloud cover, no precipitation, and there is no
extreme visibility, wind or temperature conditions.
Fall Wind : a strong, cold, downslope wind.
Feeder Bands : Lines or bands of low : level
clouds that move (feed) into the updraft region of a thunderstorm,
usually from the east through south (i.e., parallel to the inflow).
This term also is used in tropical meteorology to describe spiral
: shaped bands of convection surrounding, and moving toward, the
center of a tropical cyclone.
Fetch : The area
in which ocean waves are generated by the wind. Also refers to the
length of the fetch area, measured in the direction of the wind.
Few : A cloud layer that covers between 1/8th
and 2/8ths of the sky.
Flanking Line : A line of cumulus connected
to and extending outward from the most active portion of a parent
cumulonimbus, usually found on the southwest side of the storm.
The cloud line has roughly a stair step appearance with the taller
clouds adjacent to the parent cumulonimbus. It is most frequently
associated with strong or severe thunderstorms.
Flash Flood : A flood that occurs within a few hours
(usually less than six) of heavy or excessive rainfall, dam or levee
failure or water released from an ice jam.
Flash Flood Warning : Issued to inform the
public, emergency management, and other cooperating agencies that
flash flooding is in progress, imminent, or highly likely.
Flash Flood Watch : Issued to indicate current
or developing hydrologic conditions that are favorable for flash
flooding in and close to the watch area, but the occurrence is neither
certain or imminent.
Flood : a condition that occurs when water overflows
the natural or artificial confines of a stream or river; the water
also may accumulate by drainage over low : lying areas.
Flood Crest : The highest stage or flow occurring
in a flood.
Flood Warning : Issued when there is expected
inundation of a normally dry area near a stream, other water course;
or unusually severe ponding of water.
Flood Stage : The stage at which water overflowing the
banks of a river, stream or body of water begins to cause damage.
Flurries : Light
snow falling for short durations. No accumulation or just a light
dusting is all that is expected.
Foehn : A warm dry wind on the lee side of a mountain
range. The heating and drying are due to adiabatic compression as
the wind descend downslope.
Fog : Water that has condensed close to ground level,
producing a cloud of very small droplets that reduces visibility
to less than one km (three thousand and three hundred feet).
Fogbow : A rainbow that has a white band that appears
in fog, and is fringed with red on the outside and blue on the inside.
Forecast : A forecast provides a description
of the most significant weather conditions expected during the current
and following days. The exact content depends upon the intended
user, such as the Public or Marine forecast audiences.
Fractus : Ragged, detached cloud fragments
Freeze : Occurs when the surface air temperature
is expected to be 32 degrees Fahrenheit or below over a widespread
area for a significant period of time.
Freeze Warning : Issued during the growing
season when surface temperatures are expected to drop below freezing
over a large area for an extended period of time, regardless if
frost develops or not.
Freezing : The change in a substance from
a liquid to a solid state.
Freezing Drizzle : Drizzle that falls in
liquid form and then freezes upon impact with the ground or an item
with a temperature of 32 degrees Fahrenheit or less, possibly producing
a thin coating of ice. Even in small amounts, freezing drizzle may
cause traveling problems.
Freezing fog : A suspension of numerous minute
ice crystals in the air, or water droplets at temperatures below
0 Celsius, based at the Earth's surface, which reduces horizontal
visibility; also called ice fog.
Freezing Level : The altitude in the atmosphere where
the temperature drops to 32F.
Freezing Nuclei : Particles suspended in
the air around which ice crystals form.
Freezing Rain : Rain that freezes on objects such
as trees, cars and roads, forming a coating or glaze of ice. Temperatures
at higher levels are warm enough for rain to form, but surface temperatures
are below 32 degrees Fahrenheit, causing the rain to freeze on impact.
Freshet : the annual spring rise of streams in cold
climates as a result of snow melt; freshet also refers to a flood
caused by rain or melting snow.
Frog Storm : the first bad weather in spring after
a warm period.
Front : The boundary or transition zone between two
different air masses. The basic frontal types are cold fronts, warm
fronts and occluded fronts.
Frost : The formation of thin ice crystals
on the ground or other surfaces. Frost develops when the temperature
of the exposed surface falls below 32 degrees Fahrenheit and water
vapor is deposited as a solid.
Frost Advisory : Issued during the growing
season when widespread frost formation is expected over an extensive
area. Surface temperatures are usually in the mid 30s Fahrenheit.
Frost Point : When the temperature to which
air must be cooled to in order to be saturated is below freezing.
Frozen Dew : When liquid dew changes into
tiny beads of ice. The change occurs after dew formation and then
the temperature falls below freezing.
Fujita Scale : System developed by
Dr. Theodore Fujita to classify tornadoes based on wind damage.
Scale is from F0 for weakest to F5 for strongest tornadoes.
Fujiwhara effect : The Fujiwhara effect describes
the rotation of two storms around each other.
Funnel Cloud : A rotating, cone : shaped column of
air extending downward from the base of a thunderstorm but not touching
the ground. When it reaches the ground it is called a tornado.
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Gale : Sustained wind speeds from
34 to 47 knots (39 to 54 mph).
Gale Warning : A marine weather warning for gale force
winds from a non tropical system.
Geostationary Satellite : A satellite positioned over
the equator that rotates at the same rate as the earth, remaining
over the same spot.
Glaciation : The transformation of cloud
particles from water droplets to ice crystals. Thus, a cumulonimbus
cloud is said to have a "glaciated" upper portion.
Glaze : a layer or coating of ice that is generally
smooth and clear, and forms on exposed objects by the freezing of
liquid raindrops.
Global warming : A theory that increased
concentrations of greenhouse gases are causing an elevation in the
Earth's surface temperature.
GOES: Geostationary Operational Environmental Satellite
GOES : 8 : One of the Geostationary Operational
Environmental Satellites. They are owned and run by the National
Oceanic and Atmospheric Administration (NOAA), while NASA designs
and launches them.
Gradient : The time rate or spatial rate of change
of an atmospheric property.
Graupel : Small pellets of ice created when
supercooled water droplets coat, or rime, a snowflake. The pellets
are cloudy or white, not clear like sleet, and often are mistaken
for hail.
Gravity Wave : A wave disturbance in which
buoyancy acts as the restoring force on parcels displaced from hydrostatic
equilibrium. Waves on the ocean are examples of gravity waves.
Greenhouse Effect : The warming of the atmosphere by
the trapping of longwave radiation (heat) being radiated to space.
The gases most responsible for this effect are water vapor and carbon
dioxide.
Ground Fog : Shallow fog (less than twenty feet deep)
produced over the land by the cooling of the lower atmosphere as
it comes in contact with the ground. Also known as radiation fog.
Growing Degree Day : A form of degree day
to estimate the approximate dates when a crop will be ready to harvest.
one growing degree day occurs when the daily mean temperature is
one degree above the minimum temperature required for the growth
of that specific crop.
Growing Season : The period of time between the last
killing frost of spring and the first killing frost of autumn.
Gust : A brief sudden increase in wind speed. Generally
the duration is less than 20 seconds and the fluctuation greater
than 10 mph.
Gust Front : The
leading edge of the downdraft from a thunderstorm. A gust front
may precede the thunderstorm by several minutes and have winds that
can easily exceed 80 mph.
Gustnado (or Gustinado) : Gust front tornado.
A small tornado, usually weak and short - lived, that occurs along
the gust front of a thunderstorm. Often it is visible only as a
debris cloud or dust whirl near the ground.
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Hail : Precipitation in the form
of balls or irregular lumps of ice produced by liquid precipitation,
freezing and being coated by layers of ice as it is lifted and cooled
in strong updrafts of thunderstorms..
Halo : A ring or arc that encircles the sun or moon.
Halos are caused by the refraction of light through the ice crystals
in cirrus clouds.
Hard Freeze : freeze where vegetation is killed and
the ground surface is frozen solid.
Harmattan : a hot, dry, and dusty northeasterly or
easterly wind that occurs in West Africa north of the equator and
is caused by the outflow of air from subtropical high pressure areas.
Haze : Fine dust or salt particles in the air that
reduce visibility.
Heat Advisory : Issued within 12 hours of
the onset of the following conditions: heat index of at least 105
degrees but less than 115 degrees for less than 3 hours per day.
Nighttime lows remain above 80 degrees for 2 consecutive days.
Heat Balance : The equilibrium existing between
the radiation received and emitted by a planetary system.
Heat Index : An index
that combines air temperature and humidity to give an apparent temperature
(how hot it feels). Here is a heat index formula originally from
Weatherwise magazine. It gives valid results above 70 deg. F.
Heat Island : A dome of elevated temperatures
over an urban area caused by the heat absorbed by structures and
pavement.
Heat Lightning : Lightning that can be seen, but
is too far away for the thunder to be heard.
Heating Degree Day : A form of degree day used to
estimate the required energy for heating. One heating degree day
occurs for each degree the daily mean temperature is below 65 degrees
Fahrenheit.
Heavy snow : Depending on the region of the USA,
this generally means that four or more inches of snow has accumulated
in 12 hours, or six or more inches of snow in 24 hours.
Heavy Snow Warning : Older terminology replaced
by winter storm warning for heavy snow. Issued when 7 or more inches
of snow or sleet is expected in the next 24 hours. A warning
is used for winter weather conditions posing a threat to life and
property.
Heavy Surf : the result of large waves breaking on
or near the shore resulting from swells or produced by a distant
storm.
Helicity : A property of a moving fluid which
represents the potential for helical flow (i.e. flow which follows
the pattern of a corkscrew) to evolve. Helicity is proportional
to the strength of the flow, the amount of vertical wind shear,
and the amount of turning in the flow (i.e. vorticity).
High : An area of high pressure, usually accompanied
by anticyclonic and outward wind flow. Also known as an anticyclone.
High Risk (of severe thunderstorms) : Severe
weather is expected to affect more than 10 percent of the area.
High Wind Warning : Issued when sustained
winds from 40 to 73 mph are expected for at least 1 hour; or any
wind gusts are expected to reach 58 mph or more.
High Wind Watch : Issued when conditions
are favorable for the development of high winds over all of or part
of the forecast area but the occurrence is still uncertain. The
criteria of a high wind watch are listed under the high wind warning
and should include the area affected, the reason for the watch and
the potential impact of the winds.
Hodograph : A plot representing the vertical
distribution of horizontal winds, using polar coordinates. A hodograph
is obtained by plotting the end points of the wind vectors at various
altitudes, and connecting these points in order of increasing height.
Hook Echo : A radar pattern sometimes observed
in the southwest quadrant of a tornadic thunderstorm. Appearing
like a fishhook turned in toward the east, the hook echo is precipitation
aloft around the periphery of a rotating column of air 2 : 10 miles
in diameter.
Horse Latitudes : Subtropical regions where anticyclones
produce settled weather.
Hot Spot : Typically large areas of pavement,
these "hot spots" are heated much quicker by the sun than
surrounding grasses and forests. As a result, air rises upwards
from the relatively hot surface of the pavement, reaches its condensation
level, condenses, and forms a cloud above the "hot spot".
Humidity : The amount of water vapor in the atmosphere.
Hurricane : A severe tropical cyclone with sustained
winds over 74 mph (64 knots). Normally applied to such
storms in the Atlantic Basin and the Pacific Ocean east of the International
Date Line.
Hurricane Warning : Warning issued when sustained
winds of 74 mph (64 knots) or more are expected within 24 hours.
This implies a dangerous storm surge.
Hydrology :
The study of the waters of the earth with relation to the effects
of precipitation and evaporation upon the water in streams, rivers,
lakes, and its effect on land surfaces.
Hydrologic Cycle : The composite picture of the interchange
of water substance between the earth, the atmosphere and the seas
which includes the change of state and vertical and horizontal transport.
Hydrosphere : The totality of water encompassing
the Earth, comprising all the bodies of water, ice, and water vapor
in the atmosphere.
Hygrometer : An instrument used to measure humidity.
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Ice age : Periods in
the history of the earth characterized by a growth of the ice caps
towards the equator and a general lowering of global surface temperatures,
especially in temperate mid : latitudes. The most recent ice age
ended about 10,000 years ago. Ice advances in this period are known
to have altered the whole pattern of global atmospheric circulation.
Ice Crystals : A barely visible crystalline
form of ice that has the shape of needles, columns or plates. Ice
crystals are so small that they seem to be suspended in air. Ice
crystals occur at very low temperatures (around zero degrees F and
colder) in a stable atmosphere.
Ice Fog : A suspension of numerous minute
ice crystals in the air, or water droplets at temperatures below
0 Celsius, based at the Earth's surface, which reduces horizontal
visibility. Usually occurs at : 20F and below.
Ice Jam : An accumulation of broken river ice caught
in a narrow channel that frequently produces local floods during
a spring break : up.
Ice pellets : Precipitation of transparent
or translucent pellets of ice, which are round or irregular, rarely
conical, and which have a diameter of 0.2 inch (5
mm), or less. There are two main types. Hard grains of ice
consisting of frozen raindrops and pellets of snow encased in a
thin layer of ice.
Ice Storm : Liquid rain falling and freezing on contact
with cold objects creating ice build : ups of 1/4th inch or more
that can cause severe damage.
Ice storm warning : Older terminology replaced by
winter storm warning for severe icing. Issued when 1/2 inch
or more of accretion of freezing rain is expected. This may
lead to dangerous walking or driving conditions and the pulling
down of power lines and trees. A warning is used for winter weather
conditions posing a threat to life and property.
Indefinite ceiling : The ceiling classification
applied when the reported ceiling value represents the vertical
visibility upward into surface : based obscuration.
Inflow Bands (or Feeder Bands) : Bands
of low clouds, arranged parallel to the low : level winds and moving
into or toward a thunderstorm.
Inflow Jets : Local jets of air near
the ground flowing inward toward the base of a tornado.
Inflow Notch : A radar signature characterized
by an indentation in the reflectivity pattern on the inflow side
of the storm. The indentation often is V : shaped, but this term
should not be confused with V : notch. Supercell thunderstorms often
exhibit inflow notches, usually in the right quadrant of a classic
supercell, but sometimes in the eastern part of an HP storm or in
the rear part of a storm (rear inflow notch).
Inflow Stinger : A beaver tail cloud
with a stinger : like shape.
Infra : Red Radiation : Electromagnetic radiation
of lower frequencies and longer wavelengths than visible light (greater
than 0.7 microns (� m)). Solar ultra : violet radiation is
absorbed by the Earth's surface and re : emitted as infra : red
radiation.
Indian Summer : An unseasonably warm period near the
middle of autumn, usually following a substantial period of cool
weather.
Insolation : Incoming solar radiation. Solar
heating; sunshine.
Instability : A state of the atmosphere in
which convection takes place spontaneously, leading to cloud formation
and precipitation.
Intertropical Convergence Zone (ITCZ) : The
region where the northeasterly and southeasterly trade winds converge,
forming an often continuous band of clouds or thunderstorms near
the equator.
Inversion : An increase in temperature with height.
The reverse of the normal cooling with height in the atmosphere.
Temperature inversions trap atmospheric
pollutants in the lower troposphere, resulting in higher concentrations
of pollutants at ground levels than would usually be experienced.
Ionosphere : Also known as the thermosphere.
A layer in the atmosphere above the mesosphere extending from about
80km above the Earth's surface. It can be considered a distinct
layer due to a rise in air temperature with increasing height. Atmospheric
densities here are very low.
Iridescence : Brilliant patches of green or pink sometimes
seen near the edges of high : or medium : level clouds.
Isentropic Lift : Lifting of air that is
traveling along an upward : sloping isentropic surface. Situations
involving isentropic lift often are characterized by widespread
stratiform clouds and precipitation.
Isentropic Surface : A two : dimensional
surface containing points of equal potential temperature.
Isobar : A line of equal barometric pressure on a weather
map.
Isodrosotherm : A line of equal dew point
temperature.
Isohyet : A line of equal precipitation amounts.
Isopleth : General term for a line of equal
value of some quantity. Isobars, isotherms, etc. all are examples
of isopleths.
Isotach : A line of equal wind speed.
Isotherm : A line of equal temperature on a weather
map.
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January Thaw : A period
of mild weather popularly supposed to recur each year in late January.
Jet streak : A local wind speed maximum
within a jet stream.
Jet Stream : Strong winds concentrated within a narrow
band in the upper atmosphere. It normally refers to horizontal,
high : altitude winds. The jet stream often "steers" surface
features such as front and low pressure systems.
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Katabatic : Wind blowing down
an incline, such as down a hillside; downslope wind.
Katafront : A front ( usually a cold front)
at which the warm air descents the frontal surface.
Kelvin Temperature Scale : A temperature
scale in which 0 degrees is the point at which all molecular motion
ceases (absolute zero).
Killing Frost : Frost severe enough to end the growing
season. This usually occurs at temperatures below 28F.
Kilopascal : The internationally recognized
unit for measuring atmospheric pressure. It is equal to 10 millibars.
Knot : A measure of speed. It is one nautical
mile per hour (1.15 mph). A nautical mile is one minute of
one degree of latitude.
Knuckles : Lumpy protrusions on the edges,
and sometimes the underside, of a thunderstorm anvil. They usually
appear on the upwind side of a back : sheared anvil, and indicate
rapid expansion of the anvil due to the presence of a very strong
updraft. They are not mammatus clouds.
Lake effect : The
effect of a lake (usually a large one) in modifying the weather
near the shore and down wind. It is often refers to the enhanced
rain or snow that falls downwind from the lake. This effect
can also result in enhanced snowfall along the east coast of New
England in winter.
Laminar : Smooth, non : turbulent. Often
used to describe cloud formations which appear to be shaped by a
smooth flow of air traveling in parallel layers or sheets.
La Nina : A cooling of the equatorial waters in the
Pacific Ocean.
Land Breeze : A wind that blows from the land towards
a body of water. Also known as an offshore breeze. It occurs when
the land is cooler than the water.
Landspout : A tornado that does not arise
from organized storm : scale rotation and therefore is not associated
with a wall cloud (visually) or a mesocyclone (on radar). Landspouts
typically are observed beneath Cbs or towering cumulus clouds (often
as no more than a dust whirl), and essentially are the land : based
equivalents of waterspouts.
Lapse Rate : The change in temperature with altitude
in the atmosphere.
Latent Heat : The heat energy that must be absorbed
when a substance changes from solid to liquid and liquid to gas,
and which is released when a gas condenses and a liquid solidifies.
Layer : An array of clouds and/or obscurations
whose bases are at approximately the same level.
Left Front Quadrant (or Left Exit Region) : The
area downstream from and to the left of an upper : level jet max
(as would be viewed looking along the direction of flow). Upward
motion and severe thunderstorm potential sometimes are increased
in this area relative to the wind speed maximum.
Left Mover : A thunderstorm which moves to
the left relative to the steering winds, and to other nearby thunderstorms;
often the northern part of a splitting storm.
Leeward : Situated away from the wind; downwind
: opposite of windward
Lenticular Clouds : A cloud that generally
has the form of a smooth lens. They usually appear in formation
as the result of orographic origin. Viewed from the ground, the
clouds appear stationary as the air rushes through them.
Lifted Index (or LI) : 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 if thunderstorms develop they are
more likely to be stronger.
Lifting : The forcing of air in a vertical
direction by an upslope in terrain or by the movement of a denser
air mass.
Lifting Condensation Level : The level
in the atmosphere where a lifted air parcel reaches its saturation
point, and as a result, the water vapor within condenses into water
droplets.
Lightning : Any form of visible electrical discharges
produced by thunderstorms.
Likely : In probability of precipitation statements,
the equivalent of a 60 or 70 percent chance.
Loaded Gun (Sounding) : A sounding characterized
by extreme instability but containing a cap, such that explosive
thunderstorm development can be expected if the cap can be weakened
or the air below it heated sufficiently to overcome it.
Longwave Trough : A trough in the prevailing
westerly flow aloft which is characterized by large length and (usually)
long duration. Generally, there are no more than about five longwave
troughs around the Northern Hemisphere at any given time. Their
position and intensity govern general weather patterns (e.g., hot/cold,
wet/dry) over periods of days, weeks, or months.
Low : An area of low pressure, usually accompanied
by cyclonic and inward wind flow. Also known as a cyclone.
Low : level Jet : A region of relatively
strong winds in the lower part of the atmosphere.
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Macroburst : Large downburst
with a 2.5 mile or greater outflow diameter and damaging winds lasting
5 to 20 minutes.
Mamma Clouds : Also called mammatus, these
clouds appear as hanging, rounded protuberances or pouches on the
under : surface of a cloud. With thunderstorms, mammatus are seen
on the underside of the anvil. These clouds do not produce tornadoes,
funnels, hail, or any other type of severe weather, although they
often accompany severe thunderstorms.
Maritime Air Mass : An air mass that forms over water.
It is usually humid, and may be cold or warm.
Maximum Temperature : The highest temperature
during a specified time period.
Mean Sea Level (MSL) : 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.
Mean Temperature : The average of a series
of temperatures taken over a period of time, such as a day or a
month.
Medium Range : In forecasting, (generally)
three to seven days in advance.
Mercury Barometer : An instrument that measures
barometric pressure by measuring the level of mercury in a column.
Meridional flow : A type of atmospheric circulation
pattern in which the north and south component of motion is unusually
pronounced. Opposite of zonal flow.
Mesocyclone : A storm : scale region of rotation,
typically around 2 : 6 miles in diameter and often found in the
right rear flank of a supercell (or often on the eastern, or front,
flank of an HP storm). The circulation of a mesocyclone covers an
area much larger than the tornado that may develop within it.
Mesohigh : A mesoscale high pressure area,
usually associated with MCSs or their remnants.
Mesolow (or Sub : synoptic Low) : A
mesoscale low : pressure center. Severe weather potential often
increases in the area near and just ahead of a mesolow.
Mesonet : A regional network of observing
stations (usually surface stations) designed to diagnose mesoscale
weather features and their associated processes.
Mesoscale : Size scale referring to weather
systems smaller than synoptic : scale systems but larger than single
storm clouds. Horizontal dimensions generally range from around
50 miles to several hundred miles. Squall lines are an example of
mesoscale weather systems.
Mesoscale Convective Complex (MCC) : A large
mesoscale convective system, generally round or oval : shaped, which
normally reaches peak intensity at night. The formal definition
includes specific minimum criteria for size, duration, and eccentricity
(i.e., "roundness"), based on the cloud shield as seen
on infrared satellite photographs:
Mesoscale Convective System (MCS) : A complex
of thunderstorms which becomes organized on a scale larger than
the individual thunderstorms, and normally persists for several
hours or more. MCSs may be round or linear in shape, and include
systems such as tropical cyclones, squall lines, and MCCs (among
others). MCS often is used to describe a cluster of thunderstorms
that does not satisfy the size, shape, or duration criteria of an
MCC.
Mesosphere : A layer of the atmosphere separated
by the ionosphere above and the stratosphere below extending from
about 50km : 80km above the Earth's surface. The air temperature
in mesosphere decreases with height.
META : The mesoscale ETA model. A mathematical
model of the atmosphere run on a computer that makes forecasts out
to 30 hours.
Metamorphism : Changes in the structure and
texture of snow grains which results from variations in temperature,
migration of liquid water and water vapor, and pressure within the
snow cover
METAR : A weather observation near ground
level. It may include date and time, wind, visibility, weather and
obstructions to vision, sky condition, temperature and dew point,
sea level pressure, precipitation amount and other data used for
aircraft operations.
Meteorologist : A person who studies
meteorology. Some examples include research meteorologist, climatologist,
operational meteorologist, TV meteorologist.
Meteorology : 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.
Microburst : A strong localized downdraft from a thunderstorm
with peak gusts lasting 2 to 5 minutes.
Microclimate : A local climate that differs from the
main climate around it.
Mid : Latitudes : The areas in the northern
and southern hemispheres between the tropics and the Arctic and
Antarctic circles.
Millibar : A metric unit of atmospheric pressure. 1 mb
= 100 Pa (pascal). Normal surface pressure is approximately 1013
millibars.
Minimum Temperature : The lowest temperature
during a specified time period.
Mist : Consists of microscopic water droplets
suspended in the air which produce a thin grayish veil over the
landscape. It reduces visibility to a lesser extent than fog.
Mixing : Air movements (usually vertical) that make
the properties of the air with a parcel homogeneous. It may
result in a lapse rate approaching the moist or dry adiabatic rate.
Model : A mathematical representation of
a process, system, or object developed to understand its behavior
or to make predictions. The representation always involves certain
simplifications and assumptions.
Moderate Risk (of severe thunderstorms) : Severe
thunderstorms are expected to affect between 5 and 10 percent of
the area.
Moisture Advection : Transport of moisture
by horizontal winds.
Moisture Convergence : A measure of the degree
to which moist air is converging into a given area, taking into
account the effect of converging winds and moisture advection. Areas
of persistent moisture convergence are favored regions for thunderstorm
development, if other factors (e.g., instability) are favorable.
Monsoon : A persistent seasonal wind, often responsible
for seasonal precipitation regime. It is most commonly used to describe
meteorological changes in
southern and eastern Asia.
Mountain Breeze : System of winds that blow
downhill during the night.
Morning Glory : An elongated cloud
band, visually similar to a roll cloud, usually appearing in the
morning hours, when the atmosphere is relatively stable. Morning
glories result from perturbations related to gravitational waves
in a stable boundary layer.
MOS : Model Output Statistics.
MRF : Medium Range Forecast model generated every 12
hours by NCEP.
MSL : Mean sea level.
MSLP : Mean sea level pressure.
Muggy : Colloquially descriptive of warm
and especially humid weather.
Multicell Cluster Thunderstorm : A thunderstorm
consisting of two or more cells, of which most or all are often
visible at a given time as distinct domes or towers in various stages
of development.
Multivortex Tornado : A tornado in which
two or more condensation funnels or debris clouds are present at
the same time, often rotating about a common center or about each
other. Multiple : vortex tornadoes can be especially damaging.
Mushroom : A thunderstorm with a well
: defined anvil rollover, and thus having a visual appearance resembling
a mushroom.
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NCDC: National Climatic Data Center. Located
in Asheville, North Carolina, the agency that archives climatic
and forecast data from the National Weather Service.
NCEP: National Centers
for Environmental Prediction. Central computer and communications
facility of the National Weather Service; located in Washington,
DC.
Negative Tilt Trough :
An upper level system which is tilted to the west with
increasing latitude (i.e., with an axis from southeast to northwest).
A negative : tilt trough often is a sign of a developing or intensifying
system.
NEXRAD: NEXt Generation
RADar. A NWS network of about 140 Doppler radars operating nationwide.
NGM: Nested Grid Model
generated every 12 hours by NCEP.
NHC: National Hurricane
Center. The office of the National Weather Service in Miami that
is responsible for tracking and forecasting tropical cyclones.
NOAA : National Oceanic and Atmospheric Administration.
A branch of the U.S. Department of Commerce, NOAA is the parent
organization of the National Weather Service.
NOAA WEATHER WIRE (NWWS): A computer dissemination network
that sends National Weather Service products to the media and public.
NOAA WEATHER RADIO (NWR): Continuous, 24 hour a day VHF broadcasts
of weather observations and forecasts directly from National Weather
Service offices. A special tone allows certain receivers to alarm
when watches or warnings are issued.
Nocturnal Related to nighttime, or occurring at
night.
Nor'easter : A low : pressure disturbance
forming along the South Atlantic coast and moving northeast along
the Middle Atlantic and New England coasts to the Atlantic Provinces
of Canada. It usually causes strong northeast winds with rain or
snow. Also called a Northeaster or Coastal Storm.
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.
Northern Lights : Also known as the aurora
borealis. The luminous, radiant emission from the upper atmosphere
over middle and high latitudes, and centered around the earth's
magnetic poles. These silent fireworks are often seen on clear winter
nights in a variety of shapes and colors.
Nowcast : A short : term weather forecast,
generally out to six hours or less.
NSSL : The National Severe Storms Laboratory.
Nucleus : a particle of any nature upon which
molecules of water or ice accumulate.
Numerical Forecasting : Forecasting the weather
through digital computations carried out by supercomputers.
NWP : Numerical Weather Prediction.
NWS : National Weather Service.
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Obscuration : Any
phenomenon in the atmosphere, other than precipitation, that reduces
the horizontal visibility in the atmosphere.
Occluded Front : A complex frontal system that occurs
when a cold front overtakes a warm front. Also known as an occlusion.
Offshore Breeze : A wind that blows from the land towards
a body of water. Also known as a land breeze.
Offshore Forecast : A
marine weather forecast for the waters between 60 and 250 miles
off the coast.
Omega : A term used to describe vertical motion in
the atmosphere. The "omega equation" used in numerical
weather models is composed of two terms, the "differential
vorticity advection" term and the "thickness advection"
term. Put more simply, omega is determined by the amount of spin
(or large scale rotation) and warm (or cold) advection present in
the atmosphere. On a weather forecast chart, high values of omega
(or a strong omega field) relate to upward vertical motion in the
atmosphere. If this upward vertical motion is strong enough and
in a sufficiently moist airmass, precipitation results.
Onshore Breeze : A wind that blows from a body of water
towards the land. Also known as a seabreeze.
Orographic : Related to, or caused by, physical
geography (such as mountains or sloping terrain).
Orographic Lift : The lifting of air as it passes over
terrain features such hills or mountains. This can create orographic
clouds and/or precipitation.
Orphan Anvil : An anvil from a dissipated
thunderstorm, below which no other clouds remain.
Outflow : Air that flows outward from a thunderstorm.
Outflow Boundary : A storm : scale or mesoscale
boundary separating thunderstorm : cooled air (outflow) from the
surrounding air; similar in effect to a cold front, with passage
marked by a wind shift and usually a drop in temperature.
Outflow Winds : Winds that blow down fjords
and inlets from the land to the sea.
Overcast : Sky condition when greater than 9/10 of
the sky is covered by clouds.
Overrunning : A condition that exists when
a relatively warm air mass moves up and over a colder and denser
air mass on the surface. The result is usually low clouds, fog and
steady, light precipitation.
Overshooting Top (or Penetrating Top) : A
dome : like protrusion above a thunderstorm anvil, representing
a very strong updraft and hence a higher potential for severe weather
with that storm.
Ozone : A form of oxygen in which the molecule is made
of 3 atoms instead of the usual two. Ozone is usually
found in the stratosphere, and responsible for filtering out much
of the sun's ultraviolet radiation. It is also a primary component
of smog.
Ozone Hole : A thinning of the ozone layer
over Antarctica, which occurs each spring.
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Partly Cloudy : Sky condition
when between 3/10 and 7/10 of the sky is covered. Used more
frequently at night.
Partly Sunny : Similar to partly cloudy.
Used to emphasize daytime sunshine.
Patches : Used with fog to denote random
occurrence over relatively small areas.
Pendant Echo : Radar signature generally
similar to a hook echo, except that the hook shape is not as well
defined.
Permafrost : A soil layer below the surface of tundra
regions that remains frozen permanently.
Polar Air : A mass of very cold, very dry
air that forms in polar regions.
Polar front : The semi : permanent, semi
: continuous front that encircles the northern hemisphere separating
air masses of tropical and polar origin.
Polar Stratospheric Clouds (PSCs) : High
altitude clouds that form in the stratosphere above Antarctica during
the Southern Hemisphere winter. Their presence seems to initiate
the ozone loss experienced during the ensuing Southern Hemisphere
spring.
Polar vortex : A circumpolar wind circulation
which isolates the Antarctic continent during the cold Southern
Hemisphere winter, heightening ozone depletion.
Pollutant : Strictly too much of any substance
in the wrong place or at the wrong time is a pollutant. More specifically,
atmospheric pollution may be defined as the
presence of substances in the atmosphere, resulting from man : made
activities or from natural processes, causing adverse effects to
man and the environment.
Polycrystal : A snowflake composed of many
individual ice crystals.
POP : Probability of Precipitation. Probability forecasts
are subjective estimates of the chances of encountering measurable
precipitation at some time during the forecast period.
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 Area : The area on a sounding representing
the layer in which a lifted parcel would be warmer than the environment;
thus, the area between the environmental temperature profile and
the path of the lifted parcel.
Positive : tilt Trough : An upper level system
which is tilted to the east with increasing latitude (i.e., from
southwest to northeast). A positive : tilt trough often is a sign
of a weakening weather system, and generally is less likely to result
in severe weather than a negative : tilt trough if all other factors
are equal.
Potential Temperature : The temperature a
parcel of dry air would have if brought adiabatically (i.e., without
transfer of heat or mass) to a standard pressure level of 1000 mb.
Precipitation : Liquid or solid water that falls from
the atmosphere and reaches the ground.
Precipitation Shaft : A visible column of
rain and/or hail falling from a cloud base. When viewed against
a light background, heavy precipitation appears very dark gray,
sometimes with a turquoise tinge. This turquoise tinge has been
commonly attributed to hail, but its actual cause is unknown.
Pressure : The force exerted by the
interaction of the atmosphere and gravity. Also known as atmospheric
pressure.
Pressure Change : The net difference between
pressure readings at the beginning and ending of a specified interval
of time.
Pressure Falling Rapidly : A decrease in
station pressure at a rate of 0.06 inch of mercury or more per hour
which totals 0.02 inch or more.
Pressure Gradient : The rate of decrease
of pressure with distance at a fixed level.
Pressure Gradient Force : Force acting on
air that causes it to move from areas of higher pressure to areas
of lower pressure.
Pressure Rising Rapidly : An increase in
station pressure at a rate of 0.06 inch of mercury or more per hour
which totals 0.02 inch or more.
Pressure Tendency. The character and amount of
atmospheric pressure change during a specified period of time, usually
the 3 : hour period preceding an observation.
Pressure Unsteady : A pressure that fluctuates
by 0.03 inch of mercury or more from the mean pressure during the
period of measurement.
Prevailing Westerlies : Winds in the middle
latitudes (approximately 30 degrees to 60 degrees) that generally
blow from west to east.
Prevailing Wind : The direction from which
the wind blows most frequently in any location.
Profiler : An instrument designed to measure
horizontal winds directly above its location, and thus measure the
vertical wind profile. Profilers operate on the same principles
as Doppler radar.
Psychrometer : An instrument used for measuring
the water vapor content of the atmosphere. It consists of
two thermometers, one of which is an ordinary glass thermometer,
while the other has its bulb covered with a jacket of clean muslin
which is saturated with distilled water prior to use.
Pulse Storm : A thunderstorm within which
a brief period (pulse) of strong updraft occurs, during and immediately
after which the storm produces a short episode of severe weather.
These storms generally are not tornado producers, but often produce
large hail and/or damaging winds. See overshooting top, cyclic storm.
PVA : Positive Vorticity Advection.
Advection of higher values of vorticity into an area, which often
is associated with upward motion (lifting) of the air. PVA typically
is found in advance of disturbances aloft (i.e., shortwaves), and
is a property which often enhances the potential precipitation.
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QPF: Quantitative Precipitation
Forecast
Quality Of Snow : The amount of ice in a snow sample
expressed as a percent of the weight of the sample.
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Radar : An instrument used to detect
precipitation by measuring the strength of the electromagnetic signal
reflected back. (RADAR= Radio Detection and Ranging)
Radiation : Energy emitted in the form of
electromagnetic waves. Radiation has differing characteristics depending
upon the wavelength. Radiation from the Sun has a short wavelength
(ultra : violet) while energy re : radiated from the Earth's surface
and the atmosphere has a long wavelength (infra : red).
Radiation Fog : Fog produced over the land by the cooling
of the lower atmosphere as it comes in contact with the ground.
Also known as ground fog.
Radiational Cooling : Cooling process of
the Earth's surface and adjacent air, which occurs when infrared
(heat) energy radiates from the surface of the Earth upward through
the atmosphere into space. Air near the surface transfers its thermal
energy to the nearby ground through conduction, so that radiative
cooling lowers the temperature of both the surface and the lowest
part of the atmosphere.
Radiosonde : An instrument attached to a weather balloon
that transmits pressure, humidity, temperature and winds as it ascends
to the upper atmosphere.
Rain : Liquid water droplets that fall from the atmosphere,
having diameters greater than drizzle (0.5 mm).
Rain Foot : A horizontal bulging near the
surface in a precipitation shaft, forming a foot : shaped prominence.
It is a visual indication of a wet microburst.
Rain : Free Base : A horizontal, dark cumulonimbus
base that has no visible precipitation beneath it. This structure
usually marks the location of the thunderstorm updraft. Tornadoes
most commonly develop (1) from wall clouds that are attached to
the rain : free base, or (2) from the rain : free base itself. This
is particularly true when the rain : free base is observed to the
south or southwest of the precipitation shaft.
Rain Gauge : An instrument used to measure
rainfall amounts.
Rain Shadow : The region on the lee side
of a mountain or mountain range where the precipitation is noticeably
less than on the windward side.
Rainbow : Optical phenomena when light
is refracted and reflected by moisture in the air into concentric
arcs of color. Raindrops act like prisms, breaking the light into
the colors of a rainbow, with red on the outer, and blue on the
inner edge.
Rankine Temperature Scale : A temperature scale with
the degree of the Fahrenheit temperature scale and the zero point
of the Kelvin temperature scale.
Rawinsonde : A balloon that is tracked by radar to
measure wind speeds and wind directions in the atmosphere.
Reflectivity : Radar term referring to the
ability of a radar target to return energy; used to estimate precipitation
intensity and rainfall rates.
Refraction : The bending of light as it passes
through areas of different density, such as from air through ice
crystals.
Relative Humidity : The amount of water vapor in the
air, compared to the amount the air could hold if it was totally
saturated. (Expressed as a percentage).
Retrogression (or Retrograde Motion) : Movement
of a weather system in a direction opposite to that of the basic
flow in which it is embedded, usually referring to a closed low
or a longwave trough which moves westward.
Return Flow : South winds on the back (west)
side of an eastward : moving surface high pressure system. Return
flow over the central and eastern United States typically results
in a return of moist air from the Gulf of Mexico (or the Atlantic
Ocean).
RFC : River Forecast Center. The Northeast River
Forecast Center is located in Taunton, MA..
Ridge : An elongated area of high pressure in the atmosphere.
Opposite of a trough.
Right Entrance Region (or Right Rear Quadrant) :
The area upstream from and to the right of an upper : level
jet max (as would be viewed looking along the direction of flow).
Upward motion and severe thunderstorm potential sometimes are increased
in this area relative to the wind speed maximum.
Right Mover : A thunderstorm that moves appreciably
to the right relative to the main steering winds and to other nearby
thunderstorms. Right movers typically are associated with a high
potential for severe weather. (Supercells often are right movers.)
Rime : Tiny balls of ice that form when tiny
drops of water (usually not precipitation) freeze on contact with
the surface.
River Flood Warning : Issued when main stem
rivers (such as the Merrimack, Charles, Connecticut, etc) are expected
to reach a level above flood stage.
Roll Cloud : A relatively rare, low : level
horizontal, tube : shaped accessory cloud completely detached from
the cumulonimbus base. When present, it is located along the gust
front and most frequently observed on the leading edge of a line
of thunderstorms. The roll cloud will appear to be slowly "rolling"
about its horizontal axis. Roll clouds are not and do not produce
tornadoes.
Rope (or Rope Funnel) : A narrow, often
contorted condensation funnel usually associated with the decaying
stage of a tornado.
Rope Cloud : In satellite meteorology,
a narrow, rope : like band of clouds sometimes seen on satellite
images along a front or other boundary.
Rope Stage : The dissipating stage
of a tornado, characterized by thinning and shrinking of the condensation
funnel into a rope (or rope funnel). Damage still is possible during
this stage.
Rossby Waves : Long waves that form in air
or water that flows almost parallel to the equator, which results
form the effect of the earth's rotation.
Rotor Cloud. A turbulent cloud formation found
in the lee of some large mountain barriers. The air in the cloud
rotates around an axis parallel to the mountain range.
RUC : Rapid Update Cycle, a numerical model run at
NCEP that focuses on short : term (up to 12 h) forecasts and small
: scale (mesoscale) weather features. Forecasts are prepared every
3 hours for the contiguous United States.
Runway Visual Range (RVR) : An instrumentally
: derived value, based on standard calibrations, that represents
the horizontal distance a pilot may see down the runway from the
approach end.
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Saffir : Simpson Hurricane Damage Potential
Scale : A scale that measures hurricane intensity,
developed by Herbert Saffir and Robert Simpson.
St. Elmo's Fire : A luminous, and often audible,
electric discharge that is intermediate in nature between a spark
discharge and a point discharge (with its diffuse, quiescent,
and non : luminous character). It occurs from objects, especially
pointed ones, when the electric field strength near their surfaces
attains a value near 100,000 volts per m. Aircraft flying through
active electrical storms often develop corona discharge streamers
from antennas and propellers, and even from the entire fuselage
and wing structure. It is seen also, during stormy weather, emanating
from the yards and masts of ships at sea.
Sandstorm : Particles of sand carried aloft
by a strong wind. The sand particles are mostly confined to the
lowest ten feet, and rarely rise more than fifty feet above the
ground.
Santa Ana Winds : Relatively warm, dry winds that blow
into Southern California coastal areas from an anticyclone located
over the high deserts of California or Nevada. The warmth and dryness
are due to compressional heating.
Satellite Photo : A photograph of the earth
taken by weather satellites that shows areas of cloud.
Saturation : A condition of the atmosphere
in which a certain volume of air holds the maximum water vapor it
can hold at a specific temperature.
Saturation Vapor Pressure (water) : The maximum
amount of water vapor necessary to keep moist air in equilibrium
with a surface of pure water. This is the maximum water vapor the
air can hold for any given combination of temperature and pressure
Scattered : A cloud layer that covers between 3/8ths
and 1/2 of the sky.
Scud Clouds : Small, ragged, low cloud fragments
that are unattached to a larger cloud base and often seen with and
behind cold fronts and thunderstorm gust fronts. Such clouds generally
are associated with cool moist air, such as thunderstorm outflow.
Secondary Cold Front : A front that follows
a primary cold front and ushers in even colder air.
Sea Breeze : A wind that blows from a sea or ocean
towards a land mass. Also known as an onshore breeze. It occurs
when the land is warmer than the water.
Sea : level Pressure : The pressure value
obtained by the theoretical reduction or increase of barometric
pressure to sea : level.
Sensible Heat : The excess radiative energy
that has passed from the Earth's surface to the atmosphere through
advection, conduction, and convection processes.
Severe Thunderstorm : A strong thunderstorm with wind
gusts in excess of 58 mph (50 knots) and/or hail with a diameter
of 3/4" or more. A thunderstorm with winds greater than 39
mph and/or hail greater than � inch is defined as approaching
severe.
Severe Thunderstorm Warning : Issued
when thunderstorms are expected to have wind gusts to 58 mph or
above or hail 3/4 inch or more in diameter.
Severe Thunderstorm Watch : Issued when conditions
are favorable for the development of severe thunderstorms in and
close to a defined area.
Shallow fog : Fog in which the visibility
at 6 feet above ground level is 5/8ths of a mile or more.
Shear (Wind Shear) : 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.
Shelf Cloud : A low : level horizontal accessory
cloud that appears to be wedge : shaped as it approaches. It is
usually attached to the thunderstorm base and forms along the gust
front. The leading edge of the shelf is often smooth and at times
layered or terraced. It is most often seen along the leading edge
of an approaching line of thunderstorms, accompanied by gusty straight
winds as it passes overhead and followed by precipitation. The underside
is concave upward, turbulent, boiling, or wind : torn. Tornadoes
rarely occur with the shelf cloud.
Short : Fuse Warning : A warning issued by
the NWS for a local weather hazard of relatively short duration.
Short : fuse warnings include tornado warnings, severe thunderstorm
warnings, and flash flood warnings. Tornado and severe thunderstorm
warnings typically are issued for periods of an hour or less, flash
flood warnings typically for three hours or less.
Shortwave (Shortwave Trough) : A disturbance
in the mid or upper part of the atmosphere which induces upward
motion ahead of it. If other conditions are favorable, the upward
motion can contribute to thunderstorm development ahead of a shortwave.
Shortwave Radiation : The radiation received
from the sun and emitted in the spectral wavelengths less than 4
microns. It is also called 'solar radiation'.
Shower : Precipitation that is intermittent, both in
time, space or intensity.
Sky Condition : The state of the sky in terms
of such parameters as sky cover, layers and associated heights,
ceiling, and cloud types.
Sky Cover : The amount of the sky which is
covered by clouds or obscurations in contact with the surface.
Sleet : Rain drops that freeze into ice pellets
before reaching the ground. Sleet usually bounces when hitting a
surface and does not stick to objects. Forms when snow enters
a warm layer of air above the surface and melts and then enters
a deep layer of sub freezing air near the surface and refreezes.
Slight Risk (of severe thunderstorms) : Severe
thunderstorms are expected to affect between 2 and 5 percent of
the area. A slight risk generally implies that severe weather events
are expected to be isolated.
Sling Psychrometer : A psychrometer in which
the wet and dry bulb thermometers are mounted upon a frame connected
to a handle. The psychrometer may be whirled by hand in order
to provided the necessary ventilation.
Slight Chance : In probability of precipitation statements,
usually equivalent to a 20 percent chance.
Small Craft Advisory : A marine advisory for winds
25 to 33 knots (29 to 38 mph) or seas of 5 feet or more, that
may cause hazardous conditions for operators of small vessels.
Smog : Pollution formed by the interaction of pollutants
and sunlight (photochemical smog), usually restricting visibility,
and occasionally hazardous to health.
Smoke : A suspension in the air of small
particles produced by combustion. A transition to haze may occur
when smoke particles have traveled great distances (25 to 100 statute
miles or more) and when the larger particles have settled out and
the remaining particles have become widely scattered through the
atmosphere.
Snow : Frozen precipitation composed of ice particles
in complex hexagonal patterns. Snow forms in cold clouds
by the direct transfer of water vapor to ice.
Snow Advisory : Older
terminology replaced by winter weather advisory. An advisory issued
when 4, 5, or 6 inches of snow or sleet is expected in 24 hours.
It is expected to create hazardous or restricted travel conditions,
but not as severe as expected with a winter storm.
Snow Depth : The vertical height of frozen
precipitation on the ground. For this purpose, frozen precipitation
includes ice pellets, glaze, hail, any combination of these, and
sheet ice formed directly or indirectly from precipitation.
Snow Flurries : Light
snow showers, usually of an intermittent nature and short duration
with no measurable accumulation.
Snow Grains : Precipitation of very small,
white, opaque grains of ice.
Snow Pellets : Precipitation of white, opaque
grains of ice. The grains are round or sometimes conical. Diameters
range from about 0.08 to 0.2 inch (2 to 5 mm).
Snow Shower : Snow falling at varying intensities
for brief periods of time. Some accumulation is possible.
Snow Squalls : Intense, but of limited duration,
periods of moderate to heavy snowfall, accompanied by strong, gusty
surface winds and possible lightning.
Snowburst : Very intense shower of snow,
often of short duration, that greatly restricts visibility and produces
periods of rapid snow accumulation.
Snowfall : The depth of new snow that has
accumulated since the previous day or since the previous observation.
Snowflake : White ice crystals that have
combined in a complex branched hexagonal form.
Solar Energy : The energy produced by the
sun.
Sounder : A special kind of radiometer that
measures changes in atmospheric temperature with height, as well
as the content of various chemical species in the atmosphere at
various levels. The High Resolution Infrared Radiation Sounder (HIRS),
found on NOAA polar : orbiting satellites, is a passive instrument.
See passive system.
Sounding : A plot of the vertical profile
of temperature and dew point (and often winds) above a fixed location
( example). Soundings are used extensively in weather forecasting,
e.g., to determine instability, locate temperature inversions etc.
Southern Oscillation : A periodic reversal of the pressure
pattern across the tropical Pacific Ocean during El Nino events.
Special Marine Warning : Issued for brief
or sudden occurrence of sustained wind or frequent gusts of 34 knots
or more. This is usually associated with severe thunderstorms
or waterspouts.
SPC : Storm Prediction Center. Located in Norman, OK.
This office is responsible for monitoring and forecasting severe
convective weather in the continental U.S. This includes the issuance
of Tornado and Severe Thunderstorm Watches.
Speed Shear : The component of wind shear
which is due to a change in wind speed with height, e.g., southwesterly
winds of 20 mph at 10,000 feet increasing to 50 mph at 20,000 feet.
Speed shear is an important factor in severe weather development,
especially in the middle and upper levels of the atmosphere.
Spin : up : A small : scale vortex initiation,
such as what may be seen when a gustnado, landspout, or suction
vortex forms.
Spray : An ensemble of water droplets torn
by the wind from an extensive body of water, generally from the
crests of waves, and carried up into the air in such quantities
that it reduces the horizontal visibility.
Squall : A strong wind characterized by a
sudden onset in which the wind speed increases at least 16 knots
and is sustained at 22 knots or more for at least one minute.
Squall Line : Any non : frontal line or narrow band
of active thunderstorms. The term is usually used to describe solid
or broken lines of strong or severe thunderstorms.
Stability : An indication of how easily a
parcel of air is lifted. If the air is very stable it is
difficult to make the parcel rise. If the air is very unstable
the parcel may rise on its own once started.
Stable Air : Air with little or no tendency to rise,
that is usually accompanied by clear dry weather.
Standard Atmosphere : A hypothetical vertical
distribution of the atmospheric temperature, pressure, and density,
which by international agreement is considered to be representative
of the atmosphere for pressure : altimeter calibrations and other
purposes (29.92INS or 1013hPa).
Standing Lenticular Cloud : A, more or less,
isolated cloud with sharp outlines that is generally in the form
of a smooth lens or almond. These clouds often form on the lee side
of and generally parallel to mountain ranges. Depending on their
height above the surface, they may be reported as stratocumulus
standing lenticular cloud (SCSL); altocumulus standing lenticular
cloud (ACSL); or cirrocumulus standing lenticular cloud (CCSL).
Statement : Provides the public with information concerning
the status of existing warnings.
Station Identifier : A group of four alphabetic
characters used to identify a location that makes weather observations.
Station Pressure : The pressure that is read
from a barometer but is not adjusted to sea level.
Stationary Front : The boundary between cool and warm
air masses in that are not moving.
Stationary wave : Wave (flow pattern with
periodicity in time and/or space) that is fixed relative to Earth.
Steam fog : Fog that is formed when water
vapor is added to air which is much colder than the vapor's source.
This is most common when very cold air drifts across relatively
warm water.
Steering Winds (Steering Currents) :
A prevailing synoptic scale flow which governs the movement
of smaller features embedded within it.
Storm : In marine usage, winds 48 knots (55 mph) or
greater.
Storm Surge : A
rise of the sea level alone the shore that builds up as a storm
(usually a hurricane) moves over water. It is a result of the winds
of the storm and low atmospheric pressures.
Storm Track : the path that a low pressure area follows.
Storm Warning : A marine wind warning for sustained
winds greater of 48 knots (55 mph) or more from a non : tropical
system.
Storm : relative : Measured relative to a
moving thunderstorm, usually referring to winds, wind shear, or
helicity.
Storm : scale : Referring to weather systems
with sizes on the order of individual thunderstorms. See synoptic
scale, mesoscale.
Straight Line Winds : Thunderstorm winds most often
found with the gust front. They originate from downdrafts and can
cause damage which occurs in a "straight line", as opposed
to tornadic wind damage which has circular characteristics.
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.
Stratocumulus : Low : level clouds, existing
in a relatively flat layer but having individual elements. Elements
often are arranged in rows, bands, or waves.
Stratosphere : : The layer of atmosphere
above the troposphere and below the mesosphere (between 10 km and
50 km) generally characterized by an increase in
temperature with height.
Stratus : A flat, 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.
Striations : Grooves or channels in cloud
formations, arranged parallel to the flow of air and therefore depicting
the airflow relative to the parent cloud.
Sublimation : The change from ice directly
to water vapor or from water vapor to ice with out going through
the liquid water phase.
Subsidence : Downward moving (sinking) air over a broad
area that is associated with warming air and little cloud formation.
Subtropical Jet : The branch of the jet stream that
is found in the lower latitudes.
Subtropical storm : A low pressure system
that develops in subtropical waters (north of 20 north degrees latitude)
and initially has non : tropical features (see table below for a
list of tropical features) but does have some element of a tropical
cyclone's cloud structure (located close to the center rather than
away from the center of circulation).
Suction Vortex (sometimes Suction Spot) : A
small but very intense vortex within a tornado circulation. Several
suction vortices typically are present in a multiple : vortex tornado.
Much of the extreme damage associated with violent tornadoes (F4
and F5 on the Fujita scale) is attributed to suction vortices.
Supercell Thunderstorm : A severe thunderstorm
whose updrafts and downdrafts are in near balance allowing the storm
to maintain itself for several hours. Supercells often produce large
hail and tornadoes.
Supercooled Water : Water that stays in liquid form
if undisturbed even though it has been cooled to a temperature below
its normal freezing point.
Supersaturation : The condition which occurs
in the atmosphere when the relative humidity is greater than 100
percent.
Surface Hoar : The deposition (sublimation)
of ice crystals on a surface which occurs when the temperature of
the surface is colder than the air above and colder than the frost
point of that air.
Surface Pressure : The pressure that is read
from a barometer but is not adjusted to sea level.
Sustained Winds : The wind speed obtained by averaging
the observed values over a one minute period.
SWEAT Index : Severe Weather ThrEAT index.
A stability index developed by the Air Force which incorporates
instability, wind shear, and wind speeds.
Synoptic Chart : Chart showing meteorological
conditions over a region at a given time; weather map.
Synoptic Scale (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.
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TAF : A weather forecast
for aircraft operations at an airport.
Tail Cloud : A low tail : shaped cloud extending
outward from the northern quadrant of a wall cloud. Motions in the
tail cloud are toward the wall cloud with rapid updraft at the junction
of tail and wall cloud. This horizontal cloud is not a funnel or
tornado.
Tail : end Charlie : The thunderstorm at
the southernmost end of a squall line or other line or band of thunderstorms.
Teleconnection : A strong statistical
relationship between weather in different parts of the globe. For
example, there appears to be a teleconnection between the tropics
and North America during El Ni�o.
Temperate Zone : The area of the globe between
the tropics and the polar regions.
Temperature : a measure of the warmth or coldness of
an object or substance with reference to a standard value.
Terrestrial Radiation : The total infrared
radiation emitted by the Earth.
Thermal : A small rising parcel of warm air
produced when the earth's surface is unevenly heated.
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.
Thermometer : An instrument for measuring
temperature.
Theta : e (or Equivalent Potential Temperature) :
The temperature a parcel of air would have if a) it was lifted until
it became saturated, b) all water vapor was condensed out, and c)
it was returned adiabatically (i.e., without transfer of heat or
mass) to a pressure of 1000 millibars.
Theta : e Ridge : An axis of relatively high
values of theta : e. Severe weather and excessive rainfall often
occur near or just upstream from a theta : e ridge.
Thunder : The sound caused by a lightning stroke as
it heats the air and causes it to rapidly expand.
Thunderstorm : A storm with lightning and thunder,
produced by a cumulonimbus cloud, usually producing gusty winds,
heavy rain and sometimes hail.
Tilted Storm or Tilted Updraft : A thunderstorm
or cloud tower which is not purely vertical but instead exhibits
a slanted or tilted character. It is a sign of vertical wind shear,
a favorable condition for severe storm development.
Topography : Generally, the lay : out of
the major natural and man : made physical features of the earth's
surface. Bridges, highways, trees, rivers and fields are all components
that make up this topography.
Tornadic Activity : The occurrence or disappearance
of tornadoes, funnel clouds, or waterspouts.
Tornado : A violent rotating column of air, in contact
with the ground, pendant from a cumulonimbus cloud. A tornado does
not require the visible presence of a funnel cloud. It has
a typical width of tens to hundreds of meters and a lifespan of
minutes to hours.
Tornado Alley : The area of the United States
in which tornadoes are most frequent. It encompasses the great lowland
areas of the Mississippi, the Ohio, and lower Missouri River Valleys.
Although no state is entirely free of tornadoes, they are most frequent
in the Plains area between the Rocky Mountains and Appalachians
Tornado Family : A series of tornadoes
produced by a single supercell, resulting in damage path segments
along the same general line.
Tornado Warning : Issued when there is likelihood
of a tornado within the given area based on radar or actual sighting.
It is usually accompanied by conditions indicated for Severe Thunderstorm
Warning.
Total : Totals Index : A stability index
and severe weather forecast tool, equal to the temperature at 850
mb plus the dew point at 850 mb, minus twice the temperature at
500 mb.
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.)
Trade Winds : Persistent tropical winds that blow from
the subtropical high pressure centers towards the equatorial low.
They blow northeasterly in the Northern Hemisphere.
Transverse Bands : Bands of clouds oriented
perpendicular to the flow in which they are embedded. They often
are seen best on satellite photographs. When observed at high levels
(i.e., in cirrus formations), they may indicate severe or extreme
turbulence.
Transverse Rolls : Elongated low : level
clouds, arranged in parallel bands and aligned parallel to the low
: level winds but perpendicular to the mid : level flow.
Triple Point : The intersection point between
two boundaries (dry line, outflow boundary, cold front, warm front
etc.), often a focus for thunderstorm development.
Tropical Air : An air mass that has warm temperatures
and high humidities and develops over tropical or sub : tropical
areas.
Tropical Depression : Tropical mass of thunderstorms
with a cyclonic wind circulation and winds near the surface between
23 mph and 39 mph.
Tropical Disturbance : An
organized mass of thunderstorms in the tropics than lasts for more
than 24 hours, has a slight cyclonic circulation, and winds less
than 23 mph.
Tropical Storm :
An organized low pressure system in the tropics with wind speeds
between 38 and 74 mph.
Tropical Storm Warning : A warning
issued when sustained winds of 39 to 73 mph (34 to 63 knots) are
expected within 24 hours.
Tropical wave : A kink or bend in the normally
straight flow of surface air in the tropics which forms a low pressure
trough, or pressure boundary, and showers and thunderstorms. Can
develop into a tropical cyclone.
Tropics : The area of the globe from latitudes
23.5 degrees north to 23.5 degrees south.
Tropopause : The boundary between troposphere
and the stratosphere. It is 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).
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 : An elongated area of relatively low atmospheric
pressure surface or aloft. Usually not associated with a closed
circulation, and thus used to distinguish from a closed low. The
opposite of ridge.
Turbulence : Disrupted flow in the atmosphere that
produces gusts and eddies. At times this can be violent and
can cause the up and down movement of a plane.
Turkey Tower : A narrow, individual cloud
tower that develops and falls apart rapidly.
TVS : Tornadic Vortex Signature. Doppler
radar signature in the radial velocity field indicating intense,
concentrated rotation : more so than a mesocyclone.
Twister : A colloquial term for a tornado.
Typhoon : A hurricane that forms in the Western Pacific
Ocean.
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UKMET: United Kingdom forecast model.
Ultraviolet radiation : The energy range
just beyond the violet end of the visible spectrum. Although ultraviolet
radiation constitutes only about 5 percent of the total energy emitted
from the sun, it is the major energy source for the stratosphere
and mesosphere, playing a dominant role in both energy balance and
chemical composition.
Unstable Air : Air that rises easily and can form clouds
and rain.
Updraft : A small : scale current of rising
air. This is often associated with cumulus and cumulonimbus clouds.
Upper Level System : A general term for any
large : scale or mesoscale disturbance capable of producing upward
motion (lift) in the middle or upper parts of the atmosphere.
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.
Upstream : Toward the source of the flow,
or located in the area from which the flow is coming.
UTC : Coordinated Universal Time. The time
in the zero degree meridian time zone.
UVI : Ultraviolet Index
UVV : Upward Vertical Velocity.
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Valley Breeze : System
of winds that blow uphill during the day.
Vapor Pressure : The pressure exerted by
water vapor molecules in a given volume of air
Variable Ceiling : A ceiling of less than
3,000 feet which rapidly increases or decreases in height by established
criteria during the period of observation.
Veering Wind : Wind which changes 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).
Veering winds with height are indicative of warm air advection (WAA).
Vertical Shear : The rate of change of wind
speed or direction, with a given change in height.
Vertically : stacked System : A low : pressure
system, usually a closed low or cutoff low, which is not tilted
with height, i.e., located similarly at all levels of the atmosphere.
Vicinity : A proximity qualifier used to
indicate weather phenomena observed between 5 and 10 statute miles
of the usual point of observation but not at the
station.
VIL : Vertically : Integrated Liquid water.
A property computed by RADAP II and WSR : 88D units that takes into
account the three : dimensional reflectivity of an echo. The maximum
VIL of a storm is useful in determining its potential severity,
especially in terms of maximum hail size.
Visibility : The greatest horizontal distance an observer
can see and identify a prominent object.
Virtual temperature : The temperature a parcel
of air would have if the moisture in it were removed and its specific
heat was added to the parcel.
Virga : Precipitation falling from the base of a cloud
and evaporating before it reaches the ground.
Volcanic Ash : Fine particles of rock powder
that originate from a volcano and that may remain suspended in the
atmosphere for long periods.
Vort Max : (Short for vorticity maximum),
a center, or maximum, in the vorticity field of an airmass.
Vorticity : A measure of the local rotation
in a fluid flow. In weather analysis and forecasting, it usually
refers to the vertical component of rotation (i.e., rotation about
a vertical axis) and is used most often in reference to synoptic
scale or mesoscale weather systems. By convention, positive values
indicate cyclonic rotation.
Vortex : An atmospheric feature that tends
to rotate. It has vorticity and usually has closed streamlines.
Return to Top
WAA: Warm Air Advection
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 : A local and often abrupt lowering
of a rain : free cumulonimbus base into a low : hanging accessory
cloud, from 1 to 4 miles in diameter. The wall cloud is usually
situated in the southwest portion of the storm below an intense
updraft marked by the main cumulonimbus cloud and associated with
a very strong or severe thunderstorm. When seen from several miles
away, many wall clouds exhibit rapid upward motion and rotation
in the same sense as a tornado, except with considerably lower speed.
A rotating wall cloud usually develops before tornadoes or funnel
clouds by a time which can range from a few minutes up to possibly
an hour.
Warm Advection : Transport of warm air into
an area by horizontal winds. Low : level warm advection sometimes
is referred to (erroneously) as overrunning.
Warm Front : A narrow transitions zone separating
advancing warmer air from retreating cooler air. The air behind
a warm front is warmer and typically more humid than the air it
is replacing.
Warning : Forecast
issued when a particular weather or flood hazard is "imminent"
or already occurring (e.g., tornado warning, flash flood warning).
A warning is used for conditions posing a threat to life or property.
Warning Stage : The level of a river or stream which
may cause minor flooding, and at which concerned interests should
take action.
Watch :
Forecast issued well in advance to alert the public
of the possibility of a particular weather related hazard (e.g.
tornado watch, flash flood watch). The occurrence, location and
timing may still be uncertain.
Watch Box (or Box) : A severe thunderstorm
or tornado watch.
Water : a transparent, odorless, tasteless liquid;
composed of hydrogen and oxygen.
Water Equivalent : The liquid content of
solid precipitation that has accumulated on the ground (snow depth).
The accumulation may consist of snow, ice formed by freezing precipitation,
freezing liquid precipitation, or ice formed by the refreezing of
melted snow.
Water Vapor : Water substance in a gaseous state that
comprises one of the most important of all the constituents of the
atmosphere.
Waterspout : A rapidly
rotating column of air extending from a cumulonimbus cloud with
a circulation that reaches the surface of the water, (i.e. a tornado
over water).
Wave : In meteorology any pattern identifiable
on a weather map that has a cyclic pattern, or, a small cyclonic
circulation in the early stages of development that moves along
a cold front.
Wave Crest : The highest point in a wave.
Wave Trough : The lowest point in a wave.
Wavelength : Physical distance of one period
(wave repeat).
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 balloon filled with
helium or hydrogen that carries a radiosonde (weather instrument)
aloft to measure temperature pressure and humidity as the balloon
rises through the air. It is attached to a small parachute so that
when the balloon inevitably breaks, the radiosonde doesn't hurtle
back to earth dangerously quickly.
Weather Synopsis : a description of weather patterns
affecting a large area.
Wedge (or Wedge Tornado) : A large
tornado with a condensation funnel that is at least as wide (horizontally)
at the ground as it is tall (vertically) from the ground to cloud
base.
WFO: Weather Forecast Office. The Southern New England
WFO is located in Taunton, MA. Other WFOs for the Northeast area
located in Albany, NY, Gray, ME, and Upton, NY.
Whiteout : A condition caused by falling
and/or blowing snow that reduces visibility to nothing or zero miles;
typically only a few feet. Whiteouts can occur rapidly often blinding
motorists and creating chain : reaction crashes involving multiple
vehicles. Whiteouts are most frequent during blizzards.
Wind : Air in motion relative to the surface of the
earth.
Wind Advisory : Issued for sustained winds 31 to 39
mph for at least 1 hour or any gusts 46 to 57 mph. However,
winds of this magnitude occurring over an area that frequently experiences
such winds would not require the issuance a wind advisory.
Wind Aloft : The wind speeds and wind directions at
various levels in the atmosphere above the area of surface.
Wind Chill : The additional cooling effect
resulting from wind blowing on bare skin. The wind chill is based
on the rate of heat loss from exposed skin caused by the combined
effects of wind and cold. The (equivalent) wind chill temperature
is the temperature the body "feels" for a certain combination
of wind and air temperature
Wind Chill Factor : The apparent temperature which
describes the cooling effect on exposed skin by the combination
of temperature and wind, expressed as the loss of body heat. Increased
wind speed will accelerate the loss of body heat. The formula
to calculate wind chill is: WC=.0817(3.71 V^.5 + 5.81 :
.25 v)(T : 91.4)+91.4 where V=wind speed in MPH and T=temperature
F.
Wind Chill Advisory : Issued when the wind
chill index is expected to be between : 25F and : 39F
for at least 3 hours. This is using the wind chill of the
sustained wind, not gusts.
Wind Chill Warning : Issued when life : threatening
wind chills of : 40F or colder are expected for at least 3
hours. This is using the wind chill of the sustained
wind, not gusts.
Wind Direction : The direction from which the wind
is blowing.
Wind Shear : 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 Speed : The rate at which air is moving
horizontally past a given point. It may be a 2 : minute average
speed (reported as wind speed) or an instantaneous speed (reported
as a peak wind speed, or gust).
Wind Vane : An instrument
that determines the direction from which a wind is blowing.
Wind Wave : A wave that is caused by the action of
wind on the surface of water.
Windward : Upwind, or the direction from
which the wind is blowing; the opposite of leeward.
Winter Storm : A heavy snow event. A snow accumulation
of more than 6 inches in 12 hrs or more than 12 inches in
24 hrs.
Winter Storm Watch : A significant winter
storm may affect your area, but its occurrence, location and timing
are still uncertain. A winter storm watch is issued to provide 12
to 36 hours notice of the possibility of severe winter weather.
A watch will often be issued when neither the path of a developing
winter storm nor the consequences of the weather
event are as yet well defined. Ideally, the winter storm watch will
eventually be upgraded to a warning when the nature and location
of the developing weather event becomes more apparent. A winter
storm watch is intended to provide enough lead time so those who
need to set plans in motion can do so.
Winter Storm Warning : Issued when 7 or more
inches of snow or sleet is expected in the next 24 hours, or 1/2
inch or more of accretion of freezing rain is expected. A
warning is used for winter weather conditions posing a threat to
life and property.
Winter Weather Advisory : Issued when 4,
5, or 6 inches of snow or sleet is expected in 24 hours; or any
accretion of freezing rain or freezing drizzle is expected on road
surfaces; or when blowing or drifting snow is expected to occasionally
reduce visibility to 1/4 mile or less.
Wiresonde : an atmospheric sounding instrument that
is used to obtain temperature and humidity information between ground
level and height of a few thousand feet; this instrument is supported
by a captive balloon while traveling from the ground level.
World Meteorological Organization (WMO) : A
specialized UN agency responsible for the establishment of meteorological
stations and networks, and the monitoring of meteorological observations.
Wrapping Gust Front : A gust front which
wraps around a mesocyclone, cutting off the inflow of warm moist
air to the mesocyclone circulation and resulting in an occluded
mesocyclone.
WSR : 88D : Weather Surveillance Radar
: 1988 Doppler; NEXRAD unit.
Return to Top
Yellow Wind : a strong, cold,
dry west wind of eastern Asia that blows across the plains during
winter and carries a yellow dust from the desert.
Youg : a hot wind during unsettled summer weather in
the Mediterranean.
Young Ice : newly formed flat, sea or lake ice generally
between two and eight inches thick.
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Zigzag Lightning : ordinary
lightning of a cloud : to : ground discharge that appears to have
a single lightning channel.
Zodiac : the position of the sun throughout a year
as it appears to move through successive star groups or constellations.
Zonal Flow (Zonal Wind)
: Large : scale atmospheric flow in which the east : west
component (i.e., latitudinal) is dominant.
Zone Of Maximum Precipitation : The belt of elevation
at which the annual precipitation is greatest in a mountain region.
Zulu time : Same as UTC, Universal Coordinated
Time. Is is called Zulu because Z is often appended to the
time to distinguish it from local time.
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