Weather GLOSSARY (A-I) Terms Explained
Posted: Wed Mar 26, 2003 10:50 am
ACID RAIN: Cloud or rain droplets containing pollutants, such as oxides of sulfur and nitrogen, to make them acidic (eg. pH < 5.6).
ADVECTION: The horizontal transport of air or atmospheric properties. Commonly used with temperatures, i.e., "warm air advection".
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.
AFOS: Automation of Field Operations and Services. Computer system linking NWS offices for the transmission of weather data.
AIR MASS: A large body of air having similar horizontal temperature and moisture characteristics.
ALERT: Automated Local Event Reporting in Real Time. Network of automatic raingauges that transmit via VHF radio link when precipitation occurs. Some sites are also equipped with other sensors such as temperature, wind, pressure, river stage or tide level.
ALBEDO: The percentage of light reflected by an object.
ALTOCUMULUS: Mid-altitude clouds with a cumuliform shape.
ALTOSTRATUS: Mid-altitude clouds with a flat sheet-like shape.
AM: Area Manager.
ANEMOMETER: An instrument that measures wind speed.
ANTICYCLONE: A large area of high pressure around which the winds blow clockwise in the Northern Hemisphere.
ASOS: Automated Surface Observing System. Observes sky conditions, temperature and dewpoint, wind direction and speed, and barometric pressure.
ATWC: Alaska Tsunami Warning Center, located in Palmer, AK.
AVHRR: Advanced Very High Resolution Radiometer. Main sensor on U.S. polar orbiting satellites.
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.
BACKING WINDS: A counterclockwise change in wind direction. Backing winds with height are indicative of cold air advection (CAA).
BAPSU: Bay Area Public Service Unit. Public Service section of the San Francisco Bay Area Weather Service Forecast Office.
BAR: An obstacle formed at the shallow entrance to the mouth of a river or bay.
BAROMETER: An instrument for measuring atmospheric pressure.
BLIZZARD: Snow with winds in excess of 35 mph and visibilities of 1/4 mile or less, for an extended period of time (eg. > 3 hours).
BLOWING DUST: Reduction of visibility by strong winds blowing across dry ground with little or no vegetation. Visibilities of 1/8 mile or less over a widespread area are criteria for a Blowing Dust Advisory.
BROKEN CLOUDS: Clouds which cover between 5/8 and 7/8 of the sky. See Sky Terminology.
CAA: Cold Air Advection
CEILING: The height of the lowest layer of clouds, when the sky is broken or overcast.
CHANCE: A 30, 40 or 50 percent chance of occurrence of measurable precipitation.
CHINOOK WIND: A foehn wind on the east side of the Rocky Mountains.
CIRRIFORM: High altitude ice clouds with a very thin wispy appearance.
CIRROCUMULUS: Cirrus clouds with vertical development.
CIRROSTRATUS: Cirrus clouds with a flat sheetlike appearance.
CIRRUS: High clouds, usually above 18,000 feet, composed of ice crystals.
CLEAR: Sky condition of less than 1/10 cloud coverage.
CLIMATE: The historical record of average daily and seasonal weather events.
CLOSED LOW: See Cutoff Low below.
COASTAL FLOODING: The inundation of land areas along the coast caused by sea water above normal tidal actions.
COASTAL FORECAST: A forecast of wind, wave and weather conditions between the coastline and 60 miles offshore.
COLD FRONT: The boundary between a cold air mass that is advancing and a relatively warmer airmass. Generally characterized by steady precipitation followed by showery precipitation.
COMBINED SEAS: The combined height of swell and wind waves.
CONDENSATION: The process of gas changing to liquid.
CONTINENTAL AIR MASS: A dry air mass originating over a large land area.
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.
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.
DENSE FOG ADVISORY: Issued when fog reduces visibility to 1/8 mile or less over a widespread area. Most common in California's central valley. See tule fog.
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.
DIABLO WIND: Similar to Santa Ana winds in southern California. These winds occur below canyons in the East Bay hills (Diablo range) and in extreme cases can exceed 60 mph. They develop due to high pressure over Nevada and lower pressure along the central California coast.
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.
DOWNBURST: A severe localized downdraft from a thunderstorm.
DRIZZLE: Small, slowly falling water droplets, with diameters between .2 and .5 millimeters.
DUST DEVIL: A small, rapidly rotating wind that is made visible by the dust, dirt or debris it picks up. Also called a whirlwind, it develops best on clear, dry, hot afternoons.
EBB CURRENT: Movement of a tidal current away from shore or down a tidal river or estuary.
ECMF: European Centre for Meteorology Forecast model.
EL NINO: A major warming of the equatorial waters in the Pacific Ocean. El Nino events usually occur every 3 to 7 years, and are characterized by shifts in "normal" weather patterns. See El Nino and La Nina page
ENSO: El Nino-Southern Oscillation.
ETA: "Eta" (from Greek) model generated every 12 hours by NCEP
FAIR: Less than 4/10 opaque cloud cover, no precipitation, and no extremes in temperature, visibility or winds.
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.
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.
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: 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. Scale is from F0 for weakest to F5 for strongest tornadoes.
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.
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.
GOES: Geostationary Operational Environmental Satellite
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.
GROUND FOG: Fog produced over the land by the cooling of the lower atmosphere as it comes in contact with the ground. Also known as radiation fog, and in parts of California as tule fog.
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.
HAIL: Precipitation in the form of balls or irregular lumps of ice.
HALOS: Rings or arcs that seem to encircle the sun or moon. They are caused by the refraction of light through the ice crystals in cirrus clouds.
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 (eg. how hot it "feels"). Here is a heat index formula originally from Weatherwise magazine. It gives valid results above 70 deg. F.
(-42.379+2.04901523*t+10.14333127*r-.22475541*t*r-(6.83783e-3)*t^2-(5.48 1717e-2)*r^2+(1.22874e-3)*t^2*r+(8.5282e-4)*t*r^2-(1.99e-6)*t^2*r^2)
t=temp deg f and r=%rel hum
See Heat Index Chart
HIC: Hydrologist In Charge.
HIGH: The center of an area of high pressure, usually accompanied by anticyclonic and outward wind flow. Also known as an anticyclone.
HIGH WIND: Sustained winds greater than or equal to 40 mph or gust greater than or equal to 58 mph.
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).
INDIAN SUMMER: An unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.
INVERSION: An increase in temperature with height. The reverse of the normal cooling with height in the atmosphere.
ISOBAR: A line of equal barometric pressure on a weather map.
ITCZ: Inter-tropical Convergence Zone. The region where the northeasterly and southeasterly tradewinds converge, forming an often continuous band of clouds or thunderstorms near the equator.
ADVECTION: The horizontal transport of air or atmospheric properties. Commonly used with temperatures, i.e., "warm air advection".
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.
AFOS: Automation of Field Operations and Services. Computer system linking NWS offices for the transmission of weather data.
AIR MASS: A large body of air having similar horizontal temperature and moisture characteristics.
ALERT: Automated Local Event Reporting in Real Time. Network of automatic raingauges that transmit via VHF radio link when precipitation occurs. Some sites are also equipped with other sensors such as temperature, wind, pressure, river stage or tide level.
ALBEDO: The percentage of light reflected by an object.
ALTOCUMULUS: Mid-altitude clouds with a cumuliform shape.
ALTOSTRATUS: Mid-altitude clouds with a flat sheet-like shape.
AM: Area Manager.
ANEMOMETER: An instrument that measures wind speed.
ANTICYCLONE: A large area of high pressure around which the winds blow clockwise in the Northern Hemisphere.
ASOS: Automated Surface Observing System. Observes sky conditions, temperature and dewpoint, wind direction and speed, and barometric pressure.
ATWC: Alaska Tsunami Warning Center, located in Palmer, AK.
AVHRR: Advanced Very High Resolution Radiometer. Main sensor on U.S. polar orbiting satellites.
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.
BACKING WINDS: A counterclockwise change in wind direction. Backing winds with height are indicative of cold air advection (CAA).
BAPSU: Bay Area Public Service Unit. Public Service section of the San Francisco Bay Area Weather Service Forecast Office.
BAR: An obstacle formed at the shallow entrance to the mouth of a river or bay.
BAROMETER: An instrument for measuring atmospheric pressure.
BLIZZARD: Snow with winds in excess of 35 mph and visibilities of 1/4 mile or less, for an extended period of time (eg. > 3 hours).
BLOWING DUST: Reduction of visibility by strong winds blowing across dry ground with little or no vegetation. Visibilities of 1/8 mile or less over a widespread area are criteria for a Blowing Dust Advisory.
BROKEN CLOUDS: Clouds which cover between 5/8 and 7/8 of the sky. See Sky Terminology.
CAA: Cold Air Advection
CEILING: The height of the lowest layer of clouds, when the sky is broken or overcast.
CHANCE: A 30, 40 or 50 percent chance of occurrence of measurable precipitation.
CHINOOK WIND: A foehn wind on the east side of the Rocky Mountains.
CIRRIFORM: High altitude ice clouds with a very thin wispy appearance.
CIRROCUMULUS: Cirrus clouds with vertical development.
CIRROSTRATUS: Cirrus clouds with a flat sheetlike appearance.
CIRRUS: High clouds, usually above 18,000 feet, composed of ice crystals.
CLEAR: Sky condition of less than 1/10 cloud coverage.
CLIMATE: The historical record of average daily and seasonal weather events.
CLOSED LOW: See Cutoff Low below.
COASTAL FLOODING: The inundation of land areas along the coast caused by sea water above normal tidal actions.
COASTAL FORECAST: A forecast of wind, wave and weather conditions between the coastline and 60 miles offshore.
COLD FRONT: The boundary between a cold air mass that is advancing and a relatively warmer airmass. Generally characterized by steady precipitation followed by showery precipitation.
COMBINED SEAS: The combined height of swell and wind waves.
CONDENSATION: The process of gas changing to liquid.
CONTINENTAL AIR MASS: A dry air mass originating over a large land area.
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.
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.
DENSE FOG ADVISORY: Issued when fog reduces visibility to 1/8 mile or less over a widespread area. Most common in California's central valley. See tule fog.
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.
DIABLO WIND: Similar to Santa Ana winds in southern California. These winds occur below canyons in the East Bay hills (Diablo range) and in extreme cases can exceed 60 mph. They develop due to high pressure over Nevada and lower pressure along the central California coast.
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.
DOWNBURST: A severe localized downdraft from a thunderstorm.
DRIZZLE: Small, slowly falling water droplets, with diameters between .2 and .5 millimeters.
DUST DEVIL: A small, rapidly rotating wind that is made visible by the dust, dirt or debris it picks up. Also called a whirlwind, it develops best on clear, dry, hot afternoons.
EBB CURRENT: Movement of a tidal current away from shore or down a tidal river or estuary.
ECMF: European Centre for Meteorology Forecast model.
EL NINO: A major warming of the equatorial waters in the Pacific Ocean. El Nino events usually occur every 3 to 7 years, and are characterized by shifts in "normal" weather patterns. See El Nino and La Nina page
ENSO: El Nino-Southern Oscillation.
ETA: "Eta" (from Greek) model generated every 12 hours by NCEP
FAIR: Less than 4/10 opaque cloud cover, no precipitation, and no extremes in temperature, visibility or winds.
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.
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.
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: 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. Scale is from F0 for weakest to F5 for strongest tornadoes.
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.
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.
GOES: Geostationary Operational Environmental Satellite
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.
GROUND FOG: Fog produced over the land by the cooling of the lower atmosphere as it comes in contact with the ground. Also known as radiation fog, and in parts of California as tule fog.
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.
HAIL: Precipitation in the form of balls or irregular lumps of ice.
HALOS: Rings or arcs that seem to encircle the sun or moon. They are caused by the refraction of light through the ice crystals in cirrus clouds.
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 (eg. how hot it "feels"). Here is a heat index formula originally from Weatherwise magazine. It gives valid results above 70 deg. F.
(-42.379+2.04901523*t+10.14333127*r-.22475541*t*r-(6.83783e-3)*t^2-(5.48 1717e-2)*r^2+(1.22874e-3)*t^2*r+(8.5282e-4)*t*r^2-(1.99e-6)*t^2*r^2)
t=temp deg f and r=%rel hum
See Heat Index Chart
HIC: Hydrologist In Charge.
HIGH: The center of an area of high pressure, usually accompanied by anticyclonic and outward wind flow. Also known as an anticyclone.
HIGH WIND: Sustained winds greater than or equal to 40 mph or gust greater than or equal to 58 mph.
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).
INDIAN SUMMER: An unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.
INVERSION: An increase in temperature with height. The reverse of the normal cooling with height in the atmosphere.
ISOBAR: A line of equal barometric pressure on a weather map.
ITCZ: Inter-tropical Convergence Zone. The region where the northeasterly and southeasterly tradewinds converge, forming an often continuous band of clouds or thunderstorms near the equator.