WeatherShop.com 

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

Note: To view the QuickTime animations referred to in this glossary, you may need to download the QuickTime Plug-in. Click here to download the software.

A

ADVECTION

The horizontal transfer of any property in the atmosphere by movement of air (wind).

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. Take a look at a sample advisory issued by the National Weather Service.

AIR MASS

A large body of air having similar horizontal temperature and moisture characteristics.

ANEMOMETER

An instrument that measures wind speed.

B

BAROMETER

An instrument for measuring atmospheric pressure.

BLIZZARD

Snow with winds in excess of 35 mph and visibility of 1/4 mile or less, for an extended period of time (e.g. > 3 hours).
View a QuickTime animation illustrating the characteristics of a blizzard.

C

 

CEILING

The height of the lowest layer of clouds, when the sky is broken or overcast.

CIRCULATION

The flow or motion of a fluid in or through a given area or volume.

CIRROSTRATUS

Cirriform clouds with a flat sheetlike appearance like a white veil.

CIRRUS

High clouds, usually above 18,000 feet, composed of ice crystals.

CLEAR

Sky condition of less than 1/10 cloud coverage.

COLD FRONT

The boundary between a cold air mass that is advancing and a relatively warmer air mass. Generally characterized by steady precipitation followed by showery precipitation.

CONDENSATION

The process of gas changing to liquid.

CONTINENTAL AIR MASS

A dry air mass originating over a large land area.

CONVECTION

The mass motion within a fluid, resulting in the transport and mixing of the properties of that fluid. It is often used to imply only upward vertical motion. The opposite of subsidence.

CUMULONIMBUS CLOUD

A vertically developed cloud, often capped by an anvil shaped cloud. Also called a thunderstorm cloud, it is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail or gusty winds.

CUMULUS CLOUD

A cloud in the shape of individual detached domes, with a flat base and a bulging upper portion resembling cauliflower.

CUT OFF LOW

An area of low pressure cut off from its associated jet stream.

CYCLONE

An area of low pressure around which winds blow counterclockwise in the Northern Hemisphere. Also the term used for a hurricane in the Indian Ocean and in the Western Pacific Ocean.

D

 

DEW

Moisture that has condensed on objects near the ground, whose temperatures have fallen below the dewpoint temperature.

DEWPOINT

The temperature to which the air must be cooled for water vapor to condense.

DOPPLER RADAR

A type of weather radar that determines whether atmospheric motion is toward or away from the radar. It uses the Doppler effect to measure the velocity of particles suspended in the atmosphere.
View a QuickTime animation explaining doppler radar. (2.2M)

DOWNBURST

A severe localized downdraft from a thunderstorm.
View a QuickTime animation explaining downburst winds. (908K)

DRIZZLE

Small, slowly falling water droplets, with diameters between .2 and .5 millimeters.

E

 

EL NIÑO

A major warming of the equatorial waters in the Pacific Ocean. El Niño events usually occur every 3 to 7 years, and are characterized by shifts in "normal" weather patterns. See Anthony's El Nino Page
View a QuickTime animation explaining the weather patterns behind El Niño. (2.4M)

EVAPORATION

The physical process by which a liquid, such as water, is tranformed into a gaseous state, such as water vapor. It is the opposite physical process of condensation.

F

 

FAIR

Less than 4/10 opaque cloud cover, no precipitation, and no extremes in temperature, visibility or winds.

FLASH FLOOD

A flood that occurs within a few hours (usually less than six) of heavy or excessive rainfall, dam or levee failure.

FLOOD

High flow, overflow or inundation of a normally dry area which causes or threatens damage.

FLOOD STAGE

The level of a river or stream at which considerable inundation of surrounding areas will occur.

FOG

The visible aggregate of minute water droplets suspended in the atmosphere near the earth's surface. Essentially a cloud whose base is at the earth's surface, limiting visibility.

FREEZING LEVEL

The altitude in the atmosphere where the temperature drops to 32F.

FREEZING RAIN

Rain which falls as liquid then freezes upon impact, resulting in a coating of ice on exposed objects.

FRONT

The transition zone between two distinct airmasses. The basic frontal types are cold fronts, warm fronts and occluded fronts.

FROST

The covering of ice that is formed on exposed surfaces whose temperature falls below freezing.

FUJITA SCALE

System developed by Dr. Theodore Fujita to classify tornadoes based on wind damage:

• F0 (weak): 40-72 mph, light damage.
• F1 (weak): 73-112 mph, moderate damage.
• F2 (strong): 113-157 mph, considerable damage.
• F3 (strong): 158-206 mph, severe damage.
• F4 (violent): 207-260 mph, devastating damage.
• F5 (violent): 261-318 mph, (rare) incredible damage.

FUNNEL CLOUD

A rotating, cone-shaped column of air extending downward from the base of a thunderstorm. When it reaches the ground it is called a tornado.

G

 

GALE

Wind speeds from 39 to 54 mph (34 to 47 knots).

GEOSTATIONARY SATELLITE

A satellite that rotates at the same rate as the earth, remaining over the same spot above the equator.
Check out the latest satellite image of the U.S.

GOES

Geostationary Operational Environmental Satellite
View a QuickTime Animation illustrating the uses of GOES satellites. (1.2M)

GREENHOUSE EFFECT

The warming of the atmosphere by the trapping of longwave radiation being radiated to space. The gases most responsible for this effect are water vapor and carbon dioxide.
View a QuickTime animation describing the greenhouse effect. (693K)

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.

H

 

HAIL

Precipitation in the form of balls or irregular lumps of ice.

HALO

The ring of light that seems to encircle the sun or moon when veiled by cirrus clouds. To produce this phenomena, the ice crystals must be in a heterogeneous arrangement to refract the sunlight.

HAZE

Fine dry or wet dust or salt particles in the air that reduce visibility.

HEAT INDEX

An index that combines air temperature and humidity to give an apparent temperature (e.g. how hot it "feels"). Heat Index Chart

HIGH

The center of an area of high pressure, usually accompanied by anticyclonic and outward wind flow. Also known as an anticyclone.

HUMIDITY

The amount of water vapor in the atmosphere. (See relative humidity).

HURRICANE

A severe tropical cyclone with wind speeds in excess of 74 mph (64 knots).
View a QuickTime animation on hurricane formation. (1.9M)

I

 

INDIAN SUMMER

An unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.

ISOBAR

A line of equal barometric pressure on a weather map.

J

 

JET STREAM

Strong winds concentrated within a narrow band in the atmosphere. The jet stream often "steers" surface features such as front and low pressure systems.
View a QuickTime Animation explaining jet streams. (1.8M)

K

KNOT

One nautical mile per hour (1.15 mph).

L

 

LEEWARD

The side of an object that is sheltered from the wind such as a building or a mountain. The opposite of windward.

LIGHTNING

An electrical discharge from a thunderstorm.

LOW

The center of an area of low pressure, usually accompanied by cyclonic and inward wind flow. Also known as a cyclone.

M

 

MARITIME AIR MASS

Moist air mass originating over the ocean.

METEOROLOGY

The study of the atmosphere and atmospheric phenomena.

MILLIBAR

A unit of atmospheric pressure. 1 mb = 100 Pa (pascal). Normal surface pressure is approximately 1013 millibars.

MONSOON

A persistent seasonal wind, often responsible for seasonal precipitation regime.

N

 

NEXRAD

NEXt Generation RADar. A NWS network of about 140 Doppler radars being installed nationwide. See Anthony's FAQ's on NEXRAD
Take a look at a NexRad Image.
Get more information on NexRad.

NHC

National Hurricane Center. The office of the National Weather Service in Miami that is responsible for tracking and forecasting tropical cyclones.

NMC

National Meteorological Center. Central computer and communications facility of the National Weather Service; located in Washington, DC.

NOAA

National Oceanic and Atmospheric Administration. A branch of the US Department of Commerce, NOAA is the parent organization of the National Weather Service.

NWS

National Weather Service.

O

 

OCCLUDED FRONT

A complex frontal system that occurs when a cold front overtakes a warm front. Also known as an occlusion.

OFFSHORE BREEZE

A wind that blows from the land towards a body of water. Also known as a land breeze.

ONSHORE BREEZE

A wind that blows from a body of water towards the land. Also known as a seabreeze.

OUTFLOW

Air that flows outward from a thunderstorm.

OVERCAST

Sky condition when greater than 9/10 of the sky is covered.

OZONE

A form of oxygen containing 3 molecules, usually found in the stratosphere, and responsible for filtering out much of the sun's ultraviolet radiation.

P

PARTLY CLOUDY

Sky condition when between 3/10 and 7/10 of the sky is covered.

PRECIPITATION

Liquid or solid water molecules that fall from the atmosphere and reach the ground.

PRESSURE

The force exerted by the interaction of the atmosphere and gravity. Also known as atmospheric pressure.

R

 

RADAR

An instrument used to detect precipitation by measuring the strength of the electromagnetic signal reflected back. (RADAR= Radio Detection and Ranging)

RAIN

Liquid water droplets that fall from the atmosphere, having diameters greater than drizzle.

RAINBOW

Optical phenomena when light is refracted and reflected by moisture in the air into concentric arcs of color.
View a QuickTime animation explaining how rainbows form. (1.3M)

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).

RIDGE

An elongated area of high pressure in the atmosphere.

S

 

SATURATION POINT

When water vapor in the atmosphere is at its maximum level for the existing temperature.

SCATTERED CLOUDS

Sky condition when between 1/10 and 5/10 are covered.

SAFFIR-SIMPSON SCALE

The Saffir-Simpson hurricane intensity category system was developed to characterize the destructive potential of hurricanes. The Saffir-Simpson system sets the levels for hurricanes to five intensity categories. In addition to maximum sustained wind speed and central pressure, the Saffir-Simpson hurricane categorization includes storm-surge height and coastal destruction potential. More on the Saffir-Simpson Scale.
View a QuickTime Animation describing the characteristics of the five hurricane chategories. (1.9M)

SEVERE THUNDERSTORM

A strong thunderstorm with wind gusts in excess of 58 mph (50 knots) and/or hail with a diameter of 3/4" or more. A thunderstorm with winds greater than 39 mph and/or hail greater than an inch is defined as approaching severe.

SHOWER

Precipitation that is intermittent, both in time, space or intensity.

SLEET

A type of frozen precipitation, consisting of small transparent pellets.

SLIGHT CHANCE

In probability of precipitation statements, usually equivalent to a 20 percent chance.

SMALL CRAFT

Generally a vessel under 65 feet in length.

SMALL CRAFT ADVISORY

Winds in excess of 20 knots (23 mph), and less than 34 knots (39 mph), 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.

SNOW

Frozen precipitation composed of ice particles in complex hexagonal patterns.

SNOW ADVISORY

An advisory issued when snow is expected to create hazardous or restricted travel conditions, but not as severe as expected with a winter storm.

SNOW FLURRIES

Light snow showers, usually of an intermittent nature with no measurable accumulation.

STATIONARY FRONT

A transition zone between airmasses, with neither advancing upon the other.

STORM

In marine usage, winds 48 knots (55 mph) or greater.

STORM SURGE

A rise of the sea, preceding a storm (usually a hurricane) due to the winds of the storm and low atmospheric pressure.
View a QuickTime Animation on the destructive potential of storm surge. (1.4M)

STRATUS

Very flat low level clouds.

SUBSIDENCE

A descending motion of the air in the atmosphere that usually extends over a rather broad area.

SUBTROPICAL JET

The branch of the jet stream that is found in the lower latitudes.

SUSTAINED WINDS

The wind speed obtained by averaging the observed values over a one minute period.

SWELLS

Ocean waves of regular and longer duration than wind waves.

T

 

THERMAL

Small rising column of air due to surface heating.

THUNDER

The sound caused by a lightning stroke as it heats the air and causes it to rapidly expand.
View a QuickTime animation illustrating how thunder develops. (1.9M)

THUNDERSTORM

A storm with lightning and thunder, produced by a cumulonimbus cloud, usually producing gusty winds, heavy rain and sometimes hail.

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.
View a QuickTime animation on tornado formation. (646K)

TRADE WINDS

Persistent tropical winds that blow from the subtropical high pressure centers towards the equatorial low.

TROPICAL DEPRESSION

Tropical mass of thunderstorms with a cyclonic wind circulation and winds between 20 and 34 knots.

TROPICAL DISTURBANCE

An organized mass of tropical thunderstorms, with a slight cyclonic circulation, and winds less than 20 knots.

TROPICAL STORM

An organized cyclone in the tropics with wind speed between 35 and 64 knots.

TROUGH

An elongated area of low pressure at the surface or aloft.

TSUNAMI

An ocean wave generated by a submarine earthquake, volcano or landslide. (Also known as a seismic seawave, and incorrectly as a tidal wave).

TURBULENCE

Disrupted flow in the atmosphere that produces gusts and eddies.

TYPHOON

A hurricane that forms in the Western Pacific Ocean.

U

 

UPWELLING

The rising of cold water from the deeper areas of the ocean to the surface. This phenomena often occurs along the California coast during the summer.

V

 

VISIBILITY

The horizontal distance an observer can see and identify a prominent object.

VORTICITY

A measure of the amount of "spin" (or rotation) in the atmosphere.

W

 

WARM FRONT

A boundary between a warm airmass that is replacing a cooler airmass.

WARNING STAGE

The level of a river or stream which may cause minor flooding, and at which concerned interests should take action.

WARNING

Forecast issued when a particular hazard is "imminent" or already occurring (e.g., tornado warning, flash flood warning). Take a look at a sample warning issued by the National Weather Service.

WATCH

Forecast issued well in advance to alert the public of the possibility of a particular hazard (eg. tornado watch, flash flood watch). Take a look at a sample watch issued by the National Weather Service.

WATERSPOUT

A column of rotating air over a body of water (ie. a tornado over the water).

WIND ADVISORY

Sustained winds 25 to 39 mph and/or gusts to 57 mph. Issuance is normally site specific. However, winds of this magnitude occurring over an area that frequently experiences such winds would not require the issuance of this advisory.

WIND SHEAR

The change of wind speed or direction with distance, usually vertical.

WIND VANE

An instrument that determines the direction from which a wind is blowing.

WIND CHILL FACTOR

The amount of cooling one "feels" due to the combination of wind and temperature. Wind Chill Chart
View an informative QuickTime animation on the wind chill factor. (1.2M)

WINTER STORM

A heavy snow event. In the Sierra Nevada below 7000 feet, a snow accumulation of >6 inches/12 hrs or >12 inches/24 hrs; and above 7000 feet, >8 inches/12 hrs or >18 inches/24 hrs.

Return To Top

Sources: National Weather Service Glossary, Jan Null, Lead Forecaster
NOAA Technical Memorandum NWS SR-145, A Comprehensive Glossary of Weather Terms for Storm Spotters, M. Branick (1995)
Online Guide to Meteorology, University of Illinois at Urbana-Champaign