Terms used by
meteorologists, forecasters, weather observers, and in weather
Compiled from several
Ablation- The process of being
removed. Snow ablation usually refers to removal by
Absolute Humidity- The density of water
vapor. It is the mass of the water vapor divided by the volume that
Accretion- Growth of
precipitation particles by collision of ice crystals with supercooled
liquid droplets which freeze on impact
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).
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.
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".
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
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.
Aerosol- Particles of matter, solid or liquid,
larger than a molecule but small enough to remain suspended in the
atmosphere (up to 100µ 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.
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
AFOS- the Automation of Field Operations and
Services; AFOS is the computer system that links National Weather Service
offices and other computer networks, such as the NOAA Weather Wire, to
transmit weather information.
AGL- above ground
Air- the mixture of gases that make-up the earth's
Air Mass- A large body of air that has similar
horizontal temperature and moisture characteristics.
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
Air Parcel- An imaginary small body of air
that is used to explain the behavior of air. A parcel is large enough to
contain a very great number of molecules, but small enough so that the
properties assigned to it are approximately uniform throughout.
Air Pollution- The existence
in the air of substances in concentrations that are determined
unacceptable. Contaminants in the air we breathe come mainly from
manufacturing industries, electric power plants, automobiles, buses, and
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.
percentage of light reflected by an object. Snow covered areas have a high
albedo (0.9 or 90%) due to their white color.
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
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
Altitude- Height expressed as the
distance above a reference point, which is normally sea level or ground
Altocumulus- Mid-altitude clouds with a cumuliform
Altostratus- Mid-altitude clouds with a
flat sheet-like shape.
Anabatic- wind flowing up an incline,
such as up a hillside; upslope wind.
front at which the warm is ascending the frontal surface up to high
Anemometer- An instrument that measures wind
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
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.
The deviation of (usually) temperature or precipitation in a given region
over a specified period from the normal value for the same region.
Anticyclone- A large body of air in which the atmospheric
pressure is higher than the pressure in the surrounding air. The
winds blow clockwise around an anticyclone in in the Northern Hemisphere.
Anticyclonic- describes the
movement of air around a high pressure, and rotation about the local
vertical opposite the earth's rotation. This is clockwise in the Northern
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.
- A large overshooting top or penetrating top.
Anvil Rollover - A circular or semicircular lip
of clouds along the underside of the upwind part of a back-sheared anvil,
indicating rapid expansion of the anvil. See cumuliform anvil, knuckles,
Anvil Zits - Frequent (often continuous
or nearly continuous), localized lightning discharges occurring from
within a thunderstorm anvil.
Arctic Air- a mass of very
cold, dry air that usually originates over the Arctic Ocean north of
Canada and Alaska.
Arctic High- a very cold high pressure
that originates over the Arctic Ocean.
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.
general term used to describe areas suffering from lack of rain or
drought. More specifically, a condition in which evaporation exceeds
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
Atmospheric Pressure- (also called air
pressure or barometric pressure) The pressure asserted by the mass of the
column of air directly above any specific
Atmospheric Stability- An indication of how
easily a parcel of air is lifted. If the air is very stable it is
difficult to make the parcel rise. If the air is very unstable the
parcel may rise on its own once started.
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.
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.
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
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
Barogram- The graphic record
of pressure produced by a barograph.
instrument that provides a continuous record of atmospheric
Barometer- An instrument for measuring atmospheric
Barometric pressure- The actual
pressure value indicated by a pressure sensor.
Tendency- The amount and direction of change in barometer
readings over a three-hour period.
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
Bear's Cage- A region
of storm-scale rotation, in a thunderstorm, which is wrapped in heavy
precipitation. This area often coincides with a radar hook echo and/or
mesocyclone, especially one associated with an HP storm. The term reflects
the danger involved in observing such an area visually, which must be done
at close range in low visibility.
Beaufort Scale- A scale
that indicates the wind speed using the effect wind has on certain
Beaver('s) Tail- A particular
type of inflow band with a relatively broad, flat appearance suggestive of
a beaver's tail. It is attached to a supercell's general updraft and is
oriented roughly parallel to the pseudo-warm front, i.e., usually east to
west or southeast to northwest.
Black Ice- thin, new ice
that forms on fresh water or dew covered surfaces; it is common on
roadways during the fall and early winter and appears "black" because of
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
Blizzard warning- Issued when blizzard
condition are expected or are occurring.
Blocking High- A high pressure
area (anticyclone), often aloft, that remains nearly stationary or moves
slowly compared to west-to-east motion. It blocks the movement
eastward movement of low pressure areas (cyclones) at its
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
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
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
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.
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
Bow echo- A radar echo which is
linear but bent outward in a bow shape. Damaging straight-line
winds often occur near the "crest" or center of a bow
echo. Areas of
circulation also can develop at either end of a bow echo, which sometimes
can lead to tornado formation - especially in the left (usually northern)
end, where the circulation exhibits cyclonic rotation.
(or Watch Box) - A severe thunderstorm or tornado watch.
Breezy- Wind in the range of 15 mph to 25 mph
with mild or warm temperatures.
Wind in the range of 15 to 25 mph when the temperature is
Broken Clouds- Clouds which cover between
5/8ths and 7/8ths of the sky.
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
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
Bust- An inaccurate forecast,
usually a situation in which significant weather is expected, but does not
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.
Cold Air Advection
Calm- the absence of apparent motion in
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
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.
Ceiling- The height of
the lowest layer of broken or overcast clouds.
Ceilometer- A device used to evaluate the height
of clouds or the vertical visibility into a surface-based
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
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 weather systems.
Cirriform- High altitude ice
clouds with a very thin wispy appearance.
Cirrus clouds with vertical development.
Cirrostratus- Cirrus clouds
with a flat sheetlike appearance.
Cirrus- High clouds, usually
above 18,000 feet, composed of ice crystals and appearing in the form of
white, delicate filaments or white or mostly white patches or narrow
Clear- Sky condition of less than 1/10
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
phenomena such as fog, frost, and hail storms. Climate cannot be
considered a satisfactory indicator of actual conditions since it is based
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
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
Climatology- the scientific study of
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
Cloud- A visible cluster of tiny water
and/or ice particles in the atmosphere.
Base- For a given cloud or cloud layer, it is the lowest level in
the atmosphere where cloud particles are visible.
Condensation Nuclei- small particles in the air on which water vapor
condenses and forms cloud droplets.
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.
- 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
Coastal Flood Warning- Issued when there
is widespread coastal flooding expected within 12 hours, more than just
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.
Forecast- A forecast of wind, wave and weather conditions between
the coastline and 25 miles offshore.
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
Cold Advection- (CAA) Transport of cold
air into a region by horizontal winds.
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
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
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
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
Conduction- The transfer of heat by molecular
action between bodies that are in contact.
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
Continental Air Mass- A dry air mass
originating over a large land area.
cloud-like stream formed in cold, clear air behind the engines of an
Convection- The transfer of heat within a the air
by its movement. The term is used specifically to describe vertical
transport of heat and moisture, especially by updrafts and downdrafts in
an unstable atmosphere.
Convective Outlook- A
forecast containing the area(s) of expected thunderstorm occurrence and
expected severity over the contiguous United States, issued several times
daily by the SPC.
Convective Temperature- The
approximate temperature that the air near the ground must warm to in order
for surface-based convection to develop, based on analysis of a
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.
Seas- The combined height of swell and wind waves.
Cooling Degree Day- A form of degree day used to
estimate the required energy for cooling. one cooling degree day occurs
for each degree the daily mean temperature is above 65 degrees Fahrenheit.
Coriolis Force- An apparent force caused by the
rotation of the Earth. In the Northern Hemisphere winds are deflected to
the right, and in the Southern Hemisphere to the left. In synoptic scale
weather systems (hurricanes and large mid-latitude storms), the Coriolis
force causes the air to rotate around a low pressure center in a cyclonic
direction. The air flowing around a hurricane spins counter-clockwise in
the northern hemisphere
Corona- a disk of light surrounding
the sun or moon; this is a result of the diffraction of light by small
CRS- Console Replacement
System. This consists of a computer system and computer voice that
is used to automate NOAA Weather Radio.
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
Cumulus Cloud- A cloud in the
shape of individual detached domes, with a flat base and a bulging upper
portion resembling cauliflower.
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
Cyclogenesis- Development or
intensification of a low-pressure center
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.
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.
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
Degree Day- a measure of the departure of
the daily mean temperature from the normal daily temperature; heating and
cooling Degree Days are the departure of the daily mean temperature from
sixty-five degrees Fahrenheit.
ice crystals with complex and often fernlike branches.
Dense Fog- a fog in which the visibility
is less than one-quarter mile.
Dense Fog Advisory-
Issued when fog is expected to reduce visibility to 1/4 mile or less over
a widespread are for at least 3 hours.
Density Of Air-
The mass of air divided by its volume. The air's density
depends on its temperature, its pressure and how much water vapor is in
Density Altitude- The pressure altitude
corrected for temperature deviations from the standard atmosphere.
It is used by pilots when setting aircraft performance.
Depression- a region of low atmospheric
pressure that is usually accompanied by low clouds and
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
Derechoe- A widespread and usually
fast-moving windstorm associated with convection. Derechoes include any
family of downburst clusters produced by an extratropical MCS, and can
produce damaging straight-line winds over areas hundreds of miles long and
more than 100 miles across.
Dew- Moisture from water vapor
in the air that has condensed on objects near the ground, whose
temperatures have fallen below the dewpoint temperature.
Dew Point- The temperature to
which the air must be cooled for water vapor to condense and form fog or
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)
Cloud motion that appears to differ relative to other nearby
cloud elements, e.g. clouds moving from left to right relative to other
clouds in the foreground or background. Cloud rotation is one example of
differential motion, but not all differential motion indicates rotation.
For example, horizontal wind shear along a gust front may result in
differential cloud motion without the presence of rotation.
Difluence (or Diffluence)- A pattern of wind flow
in which air moves outward (in a "fan-out" pattern) away from a central
axis that is oriented parallel to the general direction of the flow. It is
the opposite of confluence.
Dirty ridge- Most of
the time, upper-level ridges bring fairly clear weather as the storms are
steered around the ridge. Sometimes, however, strong storms undercut the
ridge and create precipitation. Ridges that experience this undercutting
by storms are known as dirty ridges because of the unusual precipitation.
Disturbance- a disruption of the
atmosphere that usually refers to a low pressure area, cool air and
Diurnal- Daily; related to
actions which are completed in the course of a calendar day, and which
typically recur every calendar day (e.g., diurnal temperature rises during
the day, and falls at night).
Divergence- The expansion or
spreading out of a vector field; usually said of horizontal winds. It is
the opposite of convergence.
Doldrums- the regions on either
side of the equator where air pressure is low and winds are
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.
column of generally cool air that rapidly sinks to the ground, usually
accompanied by precipitation as in a shower or thunderstorm.
Downslope wind- Air that descends an elevated
plain and consequently warms and dries. Occurs when prevailing wind
direction is from the same direction as the elevated terrain and often
produces fair weather conditions.
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.
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
Dry Punch- A surge of drier
air; normally a synoptic-scale or mesoscale process. A dry punch at the
surface results in a dry line bulge.
Dry Slot- A
zone of dry (and relatively cloud-free) air which wraps east- or
northeastward into the southern and eastern parts of a synoptic scale or
mesoscale low pressure system. A dry slot generally is seen best on
Dryline- A boundary which
separates warm, dry air from warm, moist air. The differences in the two
air masses may be significant. The dry line is usually a boundary of
instability along which thunderstorms form.
Dust Devil- A
small, rapidly rotating wind that is made visible by the dust, dirt or
debris it picks up. Also called a whirlwind. Dust devils usually
develop during hot, sunny days over dry and dusty or sandy
Dust Storm- An area where high surface winds have
picked up loose dust, reducing visibility to less than one-half
Dust Plume- A non-rotating "cloud" of dust
raised by straight-line winds. Often seen in a microburst or behind a gust
Dust Whirl- A rotating column of air
rendered visible by dust.
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.
Wave- A wavelike disturbance in the tropical easterly winds that
usually moves from east to west. Such waves can grow into tropical
ECMF- European Center for Meteorology Forecast
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
El Niño- A major warming of the
equatorial waters in the eastern Pacific Ocean. El Niño 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.)
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
Enhanced Wording- An option used by
the SPC in tornado and severe thunderstorm watches when the potential for
strong/violent tornadoes, or unusually widespread damaging straight-line
winds, is high.
Entrance Region- The region
upstream from a wind speed maximum in a jet stream (jet max), in which air
is approaching (entering) the region of maximum winds, and therefore is
accelerating. This acceleration results in a vertical circulation that
creates divergence in the upper-level winds in the right half of the
entrance region (as would be viewed looking along the direction of flow).
This divergence results in upward motion of air in the right rear quadrant
(or right entrance region) of the jet max. Severe weather potential
sometimes increases in this area as a result.
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
process of a liquid changing into a vapor or gas.
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.
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.
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
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
Eye- The low pressure center
of a tropical cyclone. Winds are normally calm and sometimes the sky
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.
Fahrenheit- the standard scale used to measure temperature
in the United States; in which the freezing point of water is thirty-two
degrees and the boiling point is two hundred and twelve
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.
cloud layer that covers between 1/8th and 2/8ths of the
Flanking Line- A line of cumulus connected to
and extending outward from the most active portion of a parent
cumulonimbus, usually found on the southwest side of the storm. The cloud
line has roughly a stair step appearance with the taller clouds adjacent
to the parent cumulonimbus. It is most frequently associated with strong
or severe thunderstorms.
Flash Flood- A flood that occurs
within a few hours (usually less than six) of heavy or excessive rainfall,
dam or levee failure or water released from an ice
Flash Flood Warning- Issued to inform the
public, emergency management, and other cooperating agencies that flash
flooding is in progress, imminent, or highly likely.
Flood Watch- Issued to indicate current or developing hydrologic
conditions that are favorable for flash flooding in and close to the watch
area, but the occurrence is neither certain or imminent.
Flood- a condition that occurs when water overflows the
natural or artificial confines of a stream or river; the water also may
accumulate by drainage over low-lying areas.
Crest- The highest stage or flow occurring in a
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.
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
Foehn- A warm dry wind on the lee side of a
mountain range. The heating and drying are due to adiabatic compression as
the wind descend downslope.
Fog- Water that has condensed
close to ground level, producing a cloud of very small droplets that
reduces visibility to less than one km (three thousand and three hundred
Fogbow- A rainbow that has a white band that appears
in fog, and is fringed with red on the outside and blue on the
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
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.
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.
Level- The altitude in the atmosphere where the temperature drops to
Freezing Nuclei- Particles suspended in the
air around which ice crystals form.
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.
the annual spring rise of streams in cold climates as a result of snow
melt; freshet also refers to a flood caused by rain or melting
Frog Storm- the first bad weather in spring after a
Front- The boundary or transition zone between
two different air masses. The basic frontal types are cold fronts, warm
fronts and occluded fronts.
Frost- The formation
of thin ice crystals on the ground or other surfaces. Frost develops when
the temperature of the exposed surface falls below 32 degrees Fahrenheit
and water vapor is deposited as a solid.
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
Frozen Dew- When liquid dew
changes into tiny beads of ice. The change occurs after dew formation and
then the temperature falls below freezing.
Scale- System developed by Dr. Theodore Fujita to classify
tornadoes based on wind damage. Scale is from F0 for weakest to F5 for
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.
Gale- Sustained wind speeds from 34 to 47 knots (39 to 54
Gale Warning- A marine weather warning for gale force
winds from a non tropical system.
A satellite positioned over the equator that rotates at the same rate as
the earth, remaining over the same spot.
Glaciation- The transformation of cloud particles
from water droplets to ice crystals. Thus, a cumulonimbus cloud is said to
have a "glaciated" upper portion.
Glaze- a layer or coating
of ice that is generally smooth and clear, and forms on exposed objects by
the freezing of liquid raindrops.
warming-A theory that increased concentrations of
greenhouse gases are causing an elevation in the Earth's surface
GOES: Geostationary Operational Environmental
GOES-8- One of the Geostationary
Operational Environmental Satellites. They are owned and run by the
National Oceanic and Atmospheric Administration (NOAA), while NASA designs
and launches them.
Gradient- The time rate or spatial rate
of change of an atmospheric property.
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
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
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
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
Growing Season- The period of time between the last
killing frost of spring and the first killing frost of
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. .
Hail- Precipitation in the
form of balls or irregular lumps of ice produced by liquid precipitation,
freezing and being coated by layers of ice as it is lifted and cooled in
strong updrafts of thunderstorms..
Halo- A ring or arc that
encircles the sun or moon. Halos are caused by the refraction of light
through the ice crystals in cirrus clouds.
freeze where vegetation is killed and the ground surface is frozen
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
Haze- Fine dust or salt particles in the air that
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.
deg f and r=%rel hum
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
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
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
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
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
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
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
Horse Latitudes- Subtropical
regions where anticyclones produce settled weather.
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
Humidity- The amount of water vapor in the
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.
Warning issued when sustained winds of 74 mph (64 knots) or more
are expected within 24 hours. This implies a dangerous storm
Hydrology- The study of the
waters of the earth with relation to the effects of precipitation and
evaporation upon the water in streams, rivers, lakes, and its effect on
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
Hydrosphere- The totality of water
encompassing the Earth, comprising all the bodies of water, ice, and water
vapor in the atmosphere.
Hygrometer- An instrument used to
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.
Crystals- A barely visible crystalline form of ice that has the
shape of needles, columns or plates. Ice crystals are so small that they
seem to be suspended in air. Ice crystals occur at very low temperatures
(around zero degrees F and colder) in a stable atmosphere.
Ice Fog- A suspension of numerous minute ice
crystals in the air, or water droplets at temperatures below 0 Celsius,
based at the Earth's surface, which reduces horizontal visibility.
Usually occurs at -20F and below.
Ice Jam- An accumulation
of broken river ice caught in a narrow channel that frequently produces
local floods during a spring break-up.
pellets- Precipitation of transparent or translucent pellets of
ice, which are round or irregular, rarely conical, and which have a
diameter of 0.2 inch (5
mm), or less. There are two main types.
Hard grains of ice consisting of frozen raindrops and pellets of snow
encased in a thin layer of ice.
Ice Storm- Liquid rain
falling and freezing on contact with cold objects creating ice build-ups
of 1/4th inch or more that can cause severe damage.
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
Indefinite ceiling- The ceiling
classification applied when the reported ceiling value represents the
vertical visibility upward into surface-based
Inflow Bands (or Feeder Bands) -
Bands of low clouds, arranged parallel to the low-level winds and moving
into or toward a thunderstorm.
Inflow Jets -
Local jets of air near the ground flowing inward toward the base of a
Inflow Notch - A radar signature
characterized by an indentation in the reflectivity pattern on the inflow
side of the storm. The indentation often is V-shaped, but this term should
not be confused with V-notch. Supercell thunderstorms often exhibit inflow
notches, usually in the right quadrant of a classic supercell, but
sometimes in the eastern part of an HP storm or in the rear part of a
storm (rear inflow notch).
Inflow Stinger - A
beaver tail cloud with a stinger-like shape.
Radiation- Electromagnetic radiation of lower frequencies and
longer wavelengths than visible light (greater than 0.7 microns (µ 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.
Incoming solar radiation. Solar heating; sunshine.
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 trap atmospheric
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
Isentropic Lift- Lifting of air that is
traveling along an upward-sloping isentropic surface. Situations involving
isentropic lift often are characterized by widespread
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
Isodrosotherm- A line of equal dew point
Isohyet- A line of equal
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
Isotherm- A line of equal temperature on a
January Thaw- A period of mild weather
popularly supposed to recur each year in late January.
streak- A local wind speed maximum within a jet stream.
Jet Stream- Strong winds concentrated within a narrow band
in the upper atmosphere. It normally refers to horizontal,
high-altitude winds. The jet stream often "steers" surface features such
as front and low pressure systems.
Katabatic- Wind blowing
down an incline, such as down a hillside; downslope
Katafront- A front ( usually a cold front)
at which the warm air descents the frontal surface.
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
Kilopascal - The internationally recognized
unit for measuring atmospheric pressure. It is equal to 10
Knot- A measure of
speed. It is one nautical mile per hour (1.15 mph). A nautical mile
is one minute of one degree of latitude.
Lumpy protrusions on the edges, and sometimes the underside, of a
thunderstorm anvil. They usually appear on the upwind side of a
back-sheared anvil, and indicate rapid expansion of the anvil due to the
presence of a very strong updraft. They are not mammatus clouds.
Lake effect- The effect of a lake (usually a
large one) in modifying the weather near the shore and down wind. It is
often refers to the enhanced rain or snow that falls downwind from the
lake. This effect can also result in enhanced snowfall along the
east coast of New England in winter.
Smooth, non-turbulent. Often used to describe cloud formations which
appear to be shaped by a smooth flow of air traveling in parallel layers
La Nina- A cooling of the equatorial waters in
the Pacific Ocean.
Land Breeze- A wind that blows from the
land towards a body of water. Also known as an offshore breeze. It occurs
when the land is cooler than the water.
A tornado that does not arise from organized storm-scale rotation and
therefore is not associated with a wall cloud (visually) or a mesocyclone
(on radar). Landspouts typically are observed beneath Cbs or towering
cumulus clouds (often as no more than a dust whirl), and essentially are
the land-based equivalents of waterspouts.
Lapse Rate- The
change in temperature with altitude in the atmosphere.
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
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
Leeward- Situated away from the wind;
downwind - opposite of windward
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
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
Lifting- The forcing of air in a
vertical direction by an upslope in terrain or by the movement of a denser
Lifting Condensation Level - The level
in the atmosphere where a lifted air parcel reaches its saturation point,
and as a result, the water vapor within condenses into water droplets.
Lightning- Any form of visible electrical discharges
produced by thunderstorms.
Likely- In probability of
precipitation statements, the equivalent of a 60 or 70 percent chance.
Loaded Gun (Sounding)- A sounding characterized
by extreme instability but containing a cap, such that explosive
thunderstorm development can be expected if the cap can be weakened or the
air below it heated sufficiently to overcome it.
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
Low- An area of low pressure, usually accompanied
by cyclonic and inward wind flow. Also known as a
Low-level Jet- A region of relatively
strong winds in the lower part of the atmosphere.
Macroburst- Large downburst with a 2.5 mile or greater
outflow diameter and damaging winds lasting 5 to 20 minutes.
Mamma Clouds- Also called mammatus, these clouds
appear as hanging, rounded protuberances or pouches on the under-surface
of a cloud. With thunderstorms, mammatus are seen on the underside of the
anvil. These clouds do not produce tornadoes, funnels, hail, or any other
type of severe weather, although they often accompany severe
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.
Temperature- The average of a series of temperatures taken over a
period of time, such as a day or a month.
Range- In forecasting, (generally) three to seven days in
Mercury Barometer- An instrument that
measures barometric pressure by measuring the level of mercury in a
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.
A mesoscale high pressure area, usually associated with MCSs or their
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.
scale referring to weather systems smaller than synoptic-scale systems but
larger than single storm clouds. Horizontal dimensions generally range
from around 50 miles to several hundred miles. Squall lines are an example
of mesoscale weather systems.
Mesoscale Convective Complex
(MCC)- A large mesoscale convective system, generally round or
oval-shaped, which normally reaches peak intensity at night. The formal
definition includes specific minimum criteria for size, duration, and
eccentricity (i.e., "roundness"), based on the cloud shield as seen on
infrared satellite photographs:
System (MCS)- A complex of thunderstorms which becomes organized
on a scale larger than the individual thunderstorms, and normally persists
for several hours or more. MCSs may be round or linear in shape, and
include systems such as tropical cyclones, squall lines, and MCCs (among
others). MCS often is used to describe a cluster of thunderstorms that
does not satisfy the size, shape, or duration criteria of an MCC.
Mesosphere- A layer of the atmosphere separated
by the ionosphere above and the stratosphere below extending from about
50km-80km above the Earth's surface. The air temperature in mesosphere
decreases with height.
META- The mesoscale ETA
model. A mathematical model of the atmosphere run on a computer that
makes forecasts out to 30 hours.
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
Millibar- A metric unit of atmospheric
pressure. 1 mb = 100 Pa (pascal). Normal surface pressure is approximately
Minimum Temperature- The lowest
temperature during a specified time period.
Consists of microscopic water droplets suspended in the air which
produce a thin grayish veil over the landscape. It reduces visibility to a
lesser extent than fog.
Mixing- Air movements (usually
vertical) that make the properties of the air with a parcel
homogeneous. It may result in a lapse rate approaching the moist or
dry adiabatic rate.
Model- A mathematical
representation of a process, system, or object developed to understand its
behavior or to make predictions. The representation always involves
certain simplifications and assumptions.
Moderate Risk (of
severe thunderstorms)- Severe thunderstorms are expected to
affect between 5 and 10 percent of the area.
Advection- Transport of moisture by horizontal winds.
Moisture Convergence- A measure of the degree to
which moist air is converging into a given area, taking into account the
effect of converging winds and moisture advection. Areas of persistent
moisture convergence are favored regions for thunderstorm development, if
other factors (e.g., instability) are favorable.
persistent seasonal wind, often responsible for seasonal precipitation
regime. It is most commonly used to describe meteorological changes in
southern and eastern Asia.
System of winds that blow downhill during the
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.
Range Forecast model generated every 12 hours by NCEP.
Mean sea level.
MSLP- Mean sea level pressure.
Muggy- Colloquially descriptive of warm and
especially humid weather.
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.
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
Mushroom - A thunderstorm with a
well-defined anvil rollover, and thus having a visual appearance
resembling a mushroom.
NCDC: National Climatic Data Center.
Located in Asheville, North Carolina, the agency that archives climatic
and forecast data from the National Weather Service.
NCEP: National Centers for Environmental
Prediction. Central computer and communications facility of the National
Weather Service; located in Washington, DC.
Negative Tilt Trough- An upper
level system which is tilted to the west with increasing latitude (i.e.,
with an axis from southeast to northwest). A negative-tilt trough often is
a sign of a developing or intensifying system.
NEXRAD: NEXt Generation RADar. A NWS
network of about 140 Doppler radars operating nationwide.
NGM: Nested Grid Model generated every
12 hours by NCEP.
NHC: National Hurricane Center. The
office of the National Weather Service in Miami that is responsible for
tracking and forecasting tropical cyclones.
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.
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.
long-term average value of a meteorological element for a certain area.
For example, "temperatures are normal for this time of year" Usually
averaged over 30 years.
Northern Lights- Also
known as the aurora borealis. The luminous, radiant emission from
the upper atmosphere over middle and high latitudes, and centered around
the earth's magnetic poles. These silent fireworks are often seen on clear
winter nights in a variety of shapes and colors.
A short-term weather forecast, generally out to six hours or
NSSL- The National Severe Storms
Nucleus- a particle of any nature upon
which molecules of water or ice accumulate.
Forecasting- Forecasting the weather through digital computations
carried out by supercomputers.
NWS- National Weather Service.
Obscuration- Any phenomenon in the atmosphere,
other than precipitation, that reduces the horizontal visibility in the
Occluded Front- A complex frontal system that
occurs when a cold front overtakes a warm front. Also known as an
Offshore Breeze- A wind that blows from the land
towards a body of water. Also known as a land breeze.
Offshore Forecast- A marine weather
forecast for the waters between 60 and 250 miles off the coast.
Omega- A term used to describe vertical motion in the
atmosphere. The "omega equation" used in numerical weather models is
composed of two terms, the "differential vorticity advection" term and the
"thickness advection" term. Put more simply, omega is determined by the
amount of spin (or large scale rotation) and warm (or cold) advection
present in the atmosphere. On a weather forecast chart, high values of
omega (or a strong omega field) relate to upward vertical motion in the
atmosphere. If this upward vertical motion is strong enough and in a
sufficiently moist airmass, precipitation results.
Breeze- A wind that blows from a body of water towards the land. Also
known as a seabreeze.
Orographic- Related to, or
caused by, physical geography (such as mountains or sloping terrain).
Orographic Lift- The lifting of air as it passes over
terrain features such hills or mountains. This can create orographic
clouds and/or precipitation.
Orphan Anvil- An
anvil from a dissipated thunderstorm, below which no other clouds remain.
Outflow- Air that flows outward from a thunderstorm.
Outflow Boundary- A storm-scale or mesoscale
boundary separating thunderstorm-cooled air (outflow) from the surrounding
air; similar in effect to a cold front, with passage marked by a wind
shift and usually a drop in temperature.
Winds- Winds that blow down fjords and inlets from the land to
Overcast- Sky condition when greater than 9/10 of
the sky is covered by clouds.
condition that exists when a relatively warm air mass moves up and over a
colder and denser air mass on the surface. The result is usually low
clouds, fog and steady, light precipitation.
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
Ozone Hole- A thinning of the ozone layer
over Antarctica, which occurs each spring.
Sky condition when between 3/10 and 7/10 of the sky is covered.
Used more frequently at night.
Similar to partly cloudy. Used to emphasize daytime
Patches- Used with fog to denote random
occurrence over relatively small areas.
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
Polar Air- A mass of very cold, very
dry air that forms in polar regions.
The semi-permanent, semi-continuous front that encircles the northern
hemisphere separating air masses of tropical and polar
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.
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
presence of substances in the atmosphere, resulting from man-made
activities or from natural processes, causing adverse effects to man and
Polycrystal- A snowflake
composed of many individual ice crystals.
of Precipitation. Probability forecasts are subjective estimates of the
chances of encountering measurable precipitation at some time during the
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
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.
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.
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
Precipitation- Liquid or solid water that falls
from the atmosphere and reaches the ground.
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
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.
Gradient- The rate of decrease of pressure with distance at a
Pressure Gradient Force- Force acting
on air that causes it to move from areas of higher pressure to areas of
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.
The character and amount of atmospheric pressure change during a
specified period of time, usually the 3-hour period preceding an
Pressure Unsteady- A pressure that
fluctuates by 0.03 inch of mercury or more from the mean pressure during
the period of measurement.
Winds in the middle latitudes (approximately 30 degrees to 60 degrees)
that generally blow from west to east.
Wind- The direction from which the wind blows most frequently in
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
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
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
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.
Quantitative Precipitation Forecast
Quality Of Snow- The amount of ice in a snow sample
expressed as a percent of the weight of the sample.
An instrument used to detect precipitation by measuring the strength of
the electromagnetic signal reflected back. (RADAR= Radio Detection and
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).
Fog- Fog produced over the land by the cooling of the lower atmosphere
as it comes in contact with the ground. Also known as ground
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
Radiosonde- An instrument attached to a weather
balloon that transmits pressure, humidity, temperature and winds as it
ascends to the upper atmosphere.
Rain- Liquid water
droplets that fall from the atmosphere, having diameters greater than
drizzle (0.5 mm).
Rain Foot- A horizontal bulging
near the surface in a precipitation shaft, forming a foot-shaped
prominence. It is a visual indication of a wet microburst.
Rain-Free Base- A horizontal, dark cumulonimbus
base that has no visible precipitation beneath it. This structure usually
marks the location of the thunderstorm updraft. Tornadoes most commonly
develop (1) from wall clouds that are attached to the rain-free base, or
(2) from the rain-free base itself. This is particularly true when the
rain-free base is observed to the south or southwest of the precipitation
Rain Gauge- An instrument used to measure
Rain Shadow- The region on the
lee side of a mountain or mountain range where the precipitation is
noticeably less than on the windward
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
Rankine Temperature Scale- A temperature
scale with the degree of the Fahrenheit temperature scale and the zero
point of the Kelvin temperature scale.
Rawinsonde- A balloon
that is tracked by radar to measure wind speeds and wind directions in the
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
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).
Retrograde Motion)- Movement of a weather system in a direction
opposite to that of the basic flow in which it is embedded, usually
referring to a closed low or a longwave trough which moves westward.
Return Flow- South winds on the back (west) side
of an eastward-moving surface high pressure system. Return flow over the
central and eastern United States typically results in a return of moist
air from the Gulf of Mexico (or the Atlantic Ocean).
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.
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
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
River Flood Warning- Issued when main
stem rivers (such as the Merrimack, Charles, Connecticut, etc) are
expected to reach a level above flood stage.
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
Rope (or Rope Funnel) - A
narrow, often contorted condensation funnel usually associated with the
decaying stage of a tornado.
Rope Cloud - In
satellite meteorology, a narrow, rope-like band of clouds sometimes seen
on satellite images along a front or other boundary.
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
RUC- Rapid Update Cycle, a numerical
model run at NCEP that focuses on short-term (up to 12 h) forecasts and
small-scale (mesoscale) weather features. Forecasts are prepared every 3
hours for the contiguous United States.
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.
Damage Potential Scale- A scale that measures hurricane
intensity, developed by Herbert Saffir and Robert
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
Santa Ana Winds- Relatively warm, dry winds that
blow into Southern California coastal areas from an anticyclone located
over the high deserts of California or Nevada. The warmth and dryness are
due to compressional heating.
Satellite Photo- A
photograph of the earth taken by weather satellites that shows areas of
Saturation- A condition of the atmosphere
in which a certain volume of air holds the maximum water vapor it can hold
at a specific temperature.
Saturation Vapor Pressure
(water)- The maximum amount of water vapor necessary to keep
moist air in equilibrium with a surface of pure water. This is the maximum
water vapor the air can hold for any given combination of temperature and
Scattered- A cloud layer that covers between 3/8ths
and 1/2 of the sky.
Scud Clouds- Small, ragged,
low cloud fragments that are unattached to a larger cloud base and often
seen with and behind cold fronts and thunderstorm gust fronts. Such clouds
generally are associated with cool moist air, such as thunderstorm
Secondary Cold Front- A front that
follows a primary cold front and ushers in even colder air.
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
Sea-level Pressure- The pressure value
obtained by the theoretical reduction or increase of barometric pressure
Sensible Heat- The excess radiative
energy that has passed from the Earth's surface to the atmosphere through
advection, conduction, and convection processes.
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.
Warning- Issued when thunderstorms are expected to have wind
gusts to 58 mph or above or hail 3/4 inch or more in
Severe Thunderstorm Watch- Issued when
conditions are favorable for the development of severe thunderstorms in
and close to a defined area.
Shallow fog- Fog in
which the visibility at 6 feet above ground level is 5/8ths of a mile or
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
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'.
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
Sky Cover- The amount of the sky
which is covered by clouds or obscurations in contact with the
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.
Risk (of severe thunderstorms)- Severe thunderstorms are expected
to affect between 2 and 5 percent of the area. A slight risk generally
implies that severe weather events are expected to be isolated.
Sling Psychrometer- A psychrometer in which the
wet and dry bulb thermometers are mounted upon a frame connected to a
handle. The psychrometer may be whirled by hand in order to provided
the necessary ventilation.
In probability of precipitation statements, usually equivalent to a 20
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.
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
Snow Flurries- Light snow showers,
usually of an intermittent nature and short duration with no measurable
Snow Grains- Precipitation of very
small, white, opaque grains of ice.
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
Snow Shower- Snow falling at varying
intensities for brief periods of time. Some accumulation is
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
Snowflake- White ice crystals that
have combined in a complex branched hexagonal form.
Energy- The energy produced by the
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
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
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.
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
Spin-up- A small-scale vortex
initiation, such as what may be seen when a gustnado, landspout, or
suction vortex forms.
Spray- An ensemble of water
droplets torn by the wind from an extensive body of water, generally from
the crests of waves, and carried up into the air in such quantities that
it reduces the horizontal visibility.
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
Stability- An indication of how
easily a parcel of air is lifted. If the air is very stable it is
difficult to make the parcel rise. If the air is very unstable the
parcel may rise on its own once started.
Stable Air- Air
with little or no tendency to rise, that is usually accompanied by clear
Standard Atmosphere- A hypothetical
vertical distribution of the atmospheric temperature, pressure, and
density, which by international agreement is considered to be
representative of the atmosphere for pressure-altimeter calibrations and
other purposes (29.92INS or 1013hPa).
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.
Identifier- A group of four alphabetic characters used to
identify a location that makes weather
Station Pressure- The pressure that
is read from a barometer but is not adjusted to sea
Stationary Front- The boundary between cool and warm
air masses in that are not moving.
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
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
relative to a moving thunderstorm, usually referring to winds, wind shear,
Storm-scale- Referring to weather
systems with sizes on the order of individual thunderstorms. See synoptic
Straight Line Winds- Thunderstorm winds
most often found with the gust front. They originate from downdrafts and
can cause damage which occurs in a "straight line", as opposed to tornadic
wind damage which has circular characteristics.
Stratiform- Having extensive horizontal
development, as opposed to the more vertical development characteristic of
convection. Stratiform clouds cover large areas but show relatively little
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.
Grooves or channels in cloud formations, arranged parallel to the
flow of air and therefore depicting the airflow relative to the parent
Sublimation- The change from ice directly
to water vapor or from water vapor to ice with out going through the
liquid water phase.
Subsidence- Downward moving (sinking)
air over a broad area that is associated with warming air and little cloud
Subtropical Jet- The branch of the jet stream
that is found in the lower latitudes.
storm- A low pressure system that develops in subtropical waters
(north of 20 north degrees latitude) and initially has non-tropical
features (see table below for a list of tropical features) but does have
some element of a tropical cyclone's cloud structure (located close to the
center rather than away from the center of circulation).
Suction Vortex (sometimes Suction Spot) - A small
but very intense vortex within a tornado circulation. Several suction
vortices typically are present in a multiple-vortex tornado. Much of the
extreme damage associated with violent tornadoes (F4 and F5 on the Fujita
scale) is attributed to suction vortices.
Thunderstorm- A severe thunderstorm whose updrafts and downdrafts
are in near balance allowing the storm to maintain itself for several
hours. Supercells often produce large hail and tornadoes.
Supercooled Water- Water that stays in liquid form if
undisturbed even though it has been cooled to a temperature below its
normal freezing point.
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.
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
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.
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
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 Niño.
Temperate Zone- The area of the
globe between the tropics and the polar
Temperature- a measure of the warmth or coldness of
an object or substance with reference to a standard
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
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.
storm with lightning and thunder, produced by a cumulonimbus cloud,
usually producing gusty winds, heavy rain and sometimes hail.
Tilted Storm or Tilted Updraft- A thunderstorm or
cloud tower which is not purely vertical but instead exhibits a slanted or
tilted character. It is a sign of vertical wind shear, a favorable
condition for severe storm development.
Topography- Generally, the lay-out of the major
natural and man-made physical features of the earth's surface. Bridges,
highways, trees, rivers and fields are all components that make up this
Tornadic Activity- The occurrence or
disappearance of tornadoes, funnel clouds, or
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
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
Tornado Family - A series of
tornadoes produced by a single supercell, resulting in damage path
segments along the same general line.
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
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
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.)
Persistent tropical winds that blow from the subtropical high pressure
centers towards the equatorial low. They blow northeasterly in the
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
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.
Point- The intersection point between two boundaries (dry line,
outflow boundary, cold front, warm front etc.), often a focus for
Tropical Air- An air mass that
has warm temperatures and high humidities and develops over tropical or
Tropical Depression- Tropical mass of
thunderstorms with a cyclonic wind circulation and winds near the surface
between 23 mph and 39 mph.
Tropical Disturbance- An organized mass
of thunderstorms in the tropics than lasts for more than 24 hours, has a
slight cyclonic circulation, and winds less than 23 mph.
Tropical Storm- An organized low
pressure system in the tropics with wind speeds between 38 and 74 mph.
Tropical Storm Warning- A warning issued
when sustained winds of 39 to 73 mph (34 to 63 knots) are expected within
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.
area of the globe from latitudes 23.5 degrees north to 23.5 degrees
Tropopause- The boundary between
troposphere and the stratosphere. It is usually characterized by an
abrupt change in temperature with height from positive (decreasing
temperature with height) to neutral or negative (temperature constant or
increasing with height).
Troposphere - The layer
of the atmosphere from the earth's surface up to the tropopause,
characterized by decreasing temperature with height. It's the layer of the
atmosphere where most of the weather occurs.
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.
colloquial term for a tornado.
Typhoon- A hurricane that
forms in the Western Pacific Ocean.
UKMET: United Kingdom
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
Unstable Air- Air that rises easily
and can form clouds and rain.
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
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
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.
UVV- Upward Vertical Velocity.
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
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.
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).
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
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.
Precipitation falling from the base of a cloud and evaporating before it
reaches the ground.
Volcanic Ash- Fine particles
of rock powder that originate from a volcano and that may remain suspended
in the atmosphere for long periods.
(Short for vorticity maximum), a center, or maximum, in the
vorticity field of an airmass.
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.
atmospheric feature that tends to rotate. It has vorticity and
usually has closed streamlines.
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.
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.
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.
Stage- The level of a river or stream which may cause minor flooding,
and at which concerned interests should take action.
Watch- Forecast issued
well in advance to alert the public of the possibility of a particular
weather related hazard (e.g. tornado watch, flash flood watch). The
occurrence, location and timing may still be
Watch Box (or Box) - A severe
thunderstorm or tornado watch.
Water- a transparent,
odorless, tasteless liquid; composed of hydrogen and
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
Waterspout- A rapidly rotating
column of air extending from a cumulonimbus cloud with a circulation that
reaches the surface of the water, (i.e. a tornado over water).
Wave- In meteorology any pattern identifiable on
a weather map that has a cyclic pattern, or, a small cyclonic circulation
in the early stages of development that moves along a cold
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
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,
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
Wind Aloft- The wind speeds and wind directions
at various levels in the atmosphere above the area of
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
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.
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.
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.
The direction from which the wind is blowing.
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
Windward- Upwind, or the direction from
which the wind is blowing; the opposite of leeward.
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.
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.
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
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
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.
Yellow Wind- a strong, cold, dry west
wind of eastern Asia that blows across the plains during winter and
carries a yellow dust from the desert.
Youg- a hot wind
during unsettled summer weather in the Mediterranean.
Ice- newly formed flat, sea or lake ice generally between two and
eight inches thick.
Zigzag Lightning- ordinary lightning of
a cloud-to-ground discharge that appears to have a single lightning
Zodiac- the position of the sun throughout a year
as it appears to move through successive star groups or
Zonal Flow (Zonal Wind)- Large-scale
atmospheric flow in which the east-west component (i.e., latitudinal) is
Zone Of Maximum Precipitation- The belt of
elevation at which the annual precipitation is greatest in a mountain
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.