Listed below is a table of contents of the questions on this subject as they appear in the new online study. Click on the "Enter Online Study" link above to begin preparing for your exam.
From the pull down menu in the online
study select from the following topics:
- All USCG Deck Question Databank
- Navigation General
- Air Masses
- Relative Humidity
- True Wind
- Waves, Swells, Oceanography
- Weather Maps, Reports
- Wind Systems
- Winds, General
If you are studying for a United States Coast Guard Deck License exam which
includes this module, you should work through these questions until you
are scoring 80% or better on them.
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
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
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withdrawn from the body or system concerned.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
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
AGL : above ground level.
Air : the mixture of gases that make : up the earth's
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
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.
: 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
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
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.
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.
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
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
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,
Anvil Zits : Frequent (often continuous or
nearly continuous), localized lightning discharges occurring from within a
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
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
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
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
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
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
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 warning : Issued when blizzard condition are
expected or are occurring.
: 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
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
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
Box (or Watch Box) : A severe thunderstorm or
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
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
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
Cirriform : High altitude ice clouds with a very thin wispy
Cirrocumulus : Cirrus clouds with vertical development.
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
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
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
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.
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
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
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
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
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
Dendrite : hexagonal ice crystals with complex and
often fernlike branches.
Dense Fog : a fog in which the visibility is less than one :
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
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
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
Dew : Moisture from water vapor in the air that has condensed
on objects near the ground, whose temperatures have fallen below the
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
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
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
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
Dry : line Bulge : A bulge in the dry line,
representing the area where dry air is advancing most strongly at lower
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
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
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
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
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
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
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
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
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
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
Frog Storm : the first bad weather in spring after a warm
Front : The boundary or transition zone between two different
air masses. The basic frontal types are cold fronts, warm fronts and
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
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
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
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
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
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
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
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
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
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.
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
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 :
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
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
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 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;
Instability : A state of the atmosphere in which
convection takes place spontaneously, leading to cloud formation and
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
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
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
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
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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
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
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
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
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
Lapse Rate : The change in temperature with altitude in the
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
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
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
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
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
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
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
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
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
Generation RADar. A NWS network of about 140 Doppler radars operating
NGM: Nested Grid Model
generated every 12 hours by NCEP.
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
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
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
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
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
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
Polycrystal : A snowflake composed of many individual
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 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
Prevailing Westerlies : Winds in the middle latitudes
(approximately 30 degrees to 60 degrees) that generally blow from west to
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
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
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
Rain Gauge : An instrument used to measure rainfall
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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,
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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.
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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
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.
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
Watch Box (or Box) : A severe thunderstorm or
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
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
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
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
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.
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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
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
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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