A Glossary of Weather Terms
National Weather Service
November 1, 1995
Winter & Fall
Blizzard Warning - Issued for winter storm with sustained or frequent winds of 35 mph or higher with considerable falling and/or blowing snow that frequently reduces visibility to 1/4 of a mile or less. These conditions Are expected to prevail for a minimum of 3 hours.
Blowing Snow Advisory - Issued when wind driven snow reduces surface visibility, possibly, hampering traveling. Blowing snow may be falling snow, or snow that has already accumulated but is picked up and blown by strong winds.
Drifting Snow - An uneven distribution of snowfall or existing snow caused by strong surface wind. Drifting snow may occur during or after a snowfall.
Freeze - A condition occurring over a large area when the surface air temperature remains below 32 degrees Fahrenheit for an extended period of time possibly leading to the damage of certain crops.
Freeze Warning - Issued during the growing season when surface temperatures are expected to drop below freezing over a large area for an extended period of time, regardless if frost develops or not.
Freezing Drizzle/Rain Advisory - Issued when freezing rain or freezing drizzle is forecast but a significant amount of it is not expected. However, even small amounts of freezing rain or freezing drizzle may cause traveling problems.
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 Rain - Rain 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 rain can cause traveling problems. Large amounts can pull down power lines and tree branches.
Frost - A covering of ice on exposed surfaces when the air temperature falls below the frost point.
Frost Advisory - Issued during the growing season when widespread frost formation is expected over an extensive area. Surface temperatures are usually in the mid 30s Fahrenheit.
Frost Point - When the temperature to which air must be cooled to in order to be saturated is below freezing.
Frozen Dew - When liquid dew changes into tiny beads of ice. The change occurs after dew formation and then the temperature falls below freezing.
Heating Degree Day - A form of degree day used to estimate the required energy for heating. One heating degree day occurs for each degree the daily mean temperature is below 65 degrees Fahrenheit.
Heavy Snow Warning - Issued when snowfall totaling 6 inches or more in 12 hours or less is expected. Also issued when snowfall totaling 8 inches or more in 24 hours or less is expected.
Ice Crystals - A barely visible crystalline form of ice that has the shape of needles, columns or plates. Ice crystals are so small that they seem to be suspended in air. Ice crystals occur at very low temperatures in a stable atmosphere.
Ice Fog - Fog composed of ice crystals instead of water droplets. Ice fog forms when clear and calm conditions prevail in extremely cold arctic air.
Ice Storm Warning - Issued when damaging accumulations of ice are expected during a freezing rain event. Significant ice accumulations are 1/4 of an inch or greater leading to dangerous walking or driving conditions and the pulling down of power lines and trees.
Snow - Precipitation composed of white or translucent ice crystals. Snow forms in cold clouds by the direct transfer of water vapor to ice.
Snowflake - White ice crystals that have combined in a complex branched hexagonal form.
Snow Advisory - Issued when snowfall is expected to exceed 2 inches but not expected to exceed 5 inches. A snow advisory may also be issued for snowfalls under 2 inches if occurring at the beginning of the snow season or after a prolonged period between snowfall events.
Snow Flurries - Intermittent snowfall of short duration. No measurable accumulation of snow occurs.
Snow Shower - A short duration of light to moderate snowfall. Accumulations of snow are possible.
Sleet - Precipitation consisting of transparent pellets of ice, 5 millimeters or less in diameter. Also called ice pellets. Forms when snow enters a warm layer of air above the surface and melts and then enters a deep layer of sub freezing air on the surface and refreezes.
Sleet Warning - Issued when accumulation of sleet in excess of 1/2 inch is expected. Relatively rare. Usually issued as a winter storm warning for heavy sleet.
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. A wind chill factor of 30 degrees or lower on exposed skin will result in frostbite in a short period of time.
Wind Chill Advisory - Issued when winds of 10 mph or greater are expected to create wind chill factors of 30 degrees below zero or more.
Winter Storm Warning - Issued when hazardous winter weather is occurring, imminent or highly likely over part or all of the area. Hazardous winter weather includes but is not limited to heavy snow, blizzards, ice storms, freezing rain or drizzle and sleet.
Winter Weather Advisory - Issued for winter weather situations that may cause significant inconvenience and, if caution is not exercised, could lead to life threatening situations. Issued when events are occurring, imminent or highly likely to occur. Issued when a combination of 5 inches or less of snow, freezing precipitation and/or sleet is expected. Also issued for a combination of sleet and freezing precipitation light enough not to bring down trees, branches or power lines.
Spring & Summer
Apparent Temperature - See Heat Index.
Coastal Flooding - Prolonged strong onshore flow of wind and/or high astronomical tides causing a rise in sea level that floods coastal areas.
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.
Dew - Water that forms on objects close to the ground when its temperature falls below the dew point of the surface air.
Dew Point - The temperature of which air must be cooled for it to be saturated.
Drizzle - Precipitation in the form of liquid drops. It's diameter is less than .5 millimeters. Drizzle falls at a much slower rate than rain does.
Dry Line - 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 Devils - A small but rapidly rotating column of wind of short duration that is made visible by dust, sand, and debris picked up from the ground. Diameter usually ranges from 10 to 100 feet and develop best on clear, dry, hot afternoons. Fairly uncommon in this part of the country.
Excessive Heat Warning - Issued within 12 hours of the onset of the heat conditions listed in the excessive heat watch.
Excessive Heat Watch - Issued for the potential of the following conditions within 12 to 36 hours: heat index of at least 105 degrees Fahrenheit for more than 3 hours per day for 2 consecutive days or heat index more than 115 degrees Fahrenheit for any period of time.
Flash Flood - A flood which is caused by heavy or excessive rainfall in a short period of time, generally under 6 hours, leading to water that rises and falls quite rapidly. The term may also be used to alert the public of non life threatening flooding of small streams, streets, storm drains, and low lying urban areas. A flash flood can also be caused by the failure of a dam or from ice jams on waterways.
Flash Flood Warning - Issued to inform the public, emergency management, and other cooperating agencies that flash flooding is in progress, imminent, or highly likely.
Flash Flood Watch - Issued to indicate current or developing hydrologic conditions that are favorable for flash flooding in and close to the watch area, but the occurrence is neither certain or imminent.
Flood - The condition that occurs when water overflows the artificial or natural boundaries of a stream, river, or other body of water. Also issued for the ponding of water at or near where the rain is falling or has fallen. The term may also be used to alert the public of non life threatening flooding of small streams, streets, storm drains and low lying urban areas. It may also be used if small streams in rural areas reach or exceed bankfull.
Flood Crest - Maximum height of a flood wave as it passes a certain location.
Flood Stage - The level at which a stream, river or other body of water begins to or will begin to leave its banks.
Flood Warning - Issued to inform the public, emergency management, and other cooperating agencies that flooding is in progress, imminent, or highly likely. The
Fujita Tornado Damage Scale - A scale correlating the damage from a tornado with wind speed of the tornado. A copy of the Fujita Tornado Damage Scale is provided with this terminology guide.
Funnel Cloud - A rapidly rotating column of air extending from a cumulonimbus cloud with a circulation that does not reach the ground. once a funnel cloud reaches the ground it is then called a tornado.
Growing Degree Day - A form of degree day to estimate the approximate dates when a crop will be ready to harvest. one growing degree day occurs when the daily mean temperature is one degree above the minimum temperature required for the growth of that specific crop.
Hail - Precipitation in the form of transparent or partially opaque balls or irregular lumps of concentric ice. Hail is normally defined as having a diameter of 5 millimeters or more and is produced by thunderstorms.
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 Index - (Apparent Temperature) - This index is an accurate measure of how hot it feels (in degrees) when moisture (relative humidity) is added to the actual air temperature.
Rain - Precipitation in the form of liquid drops the diameter of which must be .5 millimeters or greater. Implies a steady precipitation that might last for several hours.
Severe Thunderstorm - A thunderstorm with winds of 58 mph or greater and/or with hail 3/4 inches diameter or larger.
Severe Thunderstorm Warning - Issued to warn the public, emergency management and other cooperating agencies when a severe thunderstorm is forecast to occur or is occurring. The warning will include where the storm was occurring, its direction of movement and the primary threat from the storm.
Severe Thunderstorm Watch - Issued when conditions are favorable for the development of severe thunderstorms in and close to a defined area.
Shower - Intermittent rainfall of short duration that falls from a cumulus cloud. Heavy precipitation is possible. Implies more of a scattered rainfall.
Squall Line - A line of solid or nearly solid thunderstorms or strong winds that might extend for several hundred miles.
Supercell - 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 tornados.
Tornado - A rapidly rotating column of air extending from a cumulonimbus cloud with a circulation that reaches the ground. However, the visible portion might not extend all the way to the ground.
Tornado Warning - Issued to warn the public, emergency management and other cooperating agencies when a tornado is forecast to occur or is occurring. The warning will include where the storm was occurring and its direction of movement.
Tornado Watch - Issued when conditions are favorable for the development of severe thunderstorms and possible tornados in and close to a defined area.
Wall Cloud - An area of clouds that extends beneath a severe thunderstorm. If a wall cloud rotates, it might precede tornado development.
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).
Commonly Used Weather Terminology
Advisory - Issued when hazardous weather or hydrologic conditions exist, are imminent or are likely tc occur. Advisories are issued for conditions that may cause major inconvenience and could lead to situations that may threaten life or property.
Anticyclone - An area of high pressure. Winds blow in a clockwise direction in the Northern Hemisphere and in a counter clockwise direction in the Southern Hemisphere. Fair weather is associated with an anticyclone.
Backdoor Front - A front that moves east to west in direction rather than the normal west to east movement.
Backing Wind - A wind that changes its direction in a counter clockwise motion. An example would be a Northwest wind changing to a West wind.
Beaufort Wind Scale - A scale classifying wind strength in terms of observable effects both on the sea and over land. Copies of the Beaufort Wind Scale for both land and water are included with this terminology guide.
Ceiling - The height of the lowest layer of broken or overcast cloud layer.
Cold Front - The transition zone where a colder air mass overtakes and replaces a warmer air mass.
Cut-Off Low - An upper level low pressure system that is no longer in the normal west to east upper air flow. Usually a cut-off low will lie to the South of the established upper air flow.
Cyclone - An area of low pressure. Winds blow in a counter clockwise direction in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere. Inclimate weather is associated with a cyclone.
Degree Day - A measure of departure from the mean daily temperature. One degree day occurs when the daily mean temperature is above or below 65 degrees Fahrenheit.
Dense Fog Advisory - Issued when dense fog covers a widespread area and frequently reduces visibility to 1/4 of a mile or less. Dense fog usually results in traveling problems and/or delays.
Downburst - A strong downward rush of air which produces a blast of damaging wind on or close to the surface.
Downslope Flow - 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.
Fog - A cloud, with its base on the surface, reducing visibility. If visibility is frequently reduced to 1/4 of a mile or less, the fog is termed dense fog.
Ground Fog - A shallow layer of fog (less than 20 feet thick) on the ground that reduces visibility more in the horizontal than in the vertical.
High Wind Warning - Issued when sustained winds of 40 mph or greater are occurring or expected to persist for an hour or longer or if winds of 58 mph or greater are expected for any duration.
High Wind Watch - Issued when conditions are favorable for the development of high winds over all of or part of the forecast area but the occurrence is still uncertain. The criteria of a high wind watch are listed under the high wind warning and should include the area affected, the reason for the watch and the potential impact of the winds.
Inversion - A layer of the atmosphere where the temperature increases with height. Surface based inversions occur during long nights when calm conditions and dry air exist.
Isobar - A line on a weather map that connects points of equal pressure. On a weather map, isobars extend around areas of high and low pressure.
Jet Stream - Relatively strong winds that are concentrated in a narrow band in the atmosphere. Jet Streams are usually thousands of kilometers long, hundreds of kilometers wide but only a few kilometers thick. They are usually found between 6 and 10 miles above the surface.
Leeward Side - The side of an object that is facing away from the direction that the wind is blowing.
Lenticular Clouds - A cloud that generally has the form of a smooth lens. They usually appear in formation as the result of oragraphic origin. Viewed from the ground, the clouds appear stationary as the air rushes through them.
Macroburst - A large downdraft of air with an outflow diameter of 2.5 miles or greater and damaging winds lasting from 5 to 20 minutes. May reach tornado intensity.
Microburst - A small downdraft of air with an outflow diameter of less than 2.5 miles with the peak winds lasting from 2 to 5 minutes. May affect aircraft performance.
Occluded Front - A combination of two fronts that forms when a cold front catches up to and overtakes a warm front.
Overrunning - A condition that exists when an air mass moves up and over a denser air mass on the surface. The result is usually low clouds, fog and steady, light precipitation.
Relative Humidity - The ratio of water vapor contained in the air compared to the maximum amount of moisture that the air can hold at that particular temperature and pressure.
Ridge - An elongated area of high pressure.
Stable Air - Air that is colder than its surroundings and as such is resistant to upward movement.
Trough - An elongated area of low pressure.
Unstable Air - Air that is warmer than its surroundings and as such tends to rise, leading to the formation of clouds and possibly precipitation.
Upslope Flow - Air that gradually rises along an elevated plain and consequently cools to the dew point. Occurs when easterly winds prevail and often lead to low clouds, fog and generally steady and light precipitation.
Veering Wind - A wind that changes its direction in a clockwise motion. An example would be a west wind changing to a northwest wind.
Virga - Precipitation that falls from clouds but evaporates in dry air beneath the cloud before reaching the ground. Virga resembles streaks of water extending from the cloud.
Warm front - The transition zone where a warm air mass overtakes and replaces a colder air mass.
Warning - Issued when hazardous weather or hydrologic conditions exist, are imminent or are highly likely to occur. Warnings are issued for conditions that pose a threat to life or property.
Watch - Issued when hazardous weather of hydrologic conditions are possible but its occurrence, location and/or timing is still uncertain. A watch is intended to provide lead time so those who need to set plans in motion may do so.
Wind Advisory - Issued for the occurrence of wind gusts between 40 and 57 mph for any duration.
Windward Side - The side of an object that is facing into the direction that the wind is coming from.
last updated February 19, 1996.