Commonly Used Terms

Advisory - Issued when hazardous weather or hydrologic conditions exist, are imminent or are likely to 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. View Beaufort Wind Scale

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 orographic 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.