Weather Resources - Glossary

Wondering what “graupel” is? Or how about “virga”? You have come to the right place. This section contains the definitions of several hundred weather related terms.

Please contact us if you have a suggestion for additional terms.

We have also developed a special glossary section with winter weather terms only.

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Daily Mean - The average temperature for a day computed by averaging either the hourly readings or, more commonly, the maximum and minimum temperatures.

Deeping - Used in describing the history of a low pressure system or an area of cyclonic circulation, it means a decrease in the central pressure of the system. Although it usually describes the action of a pressure system on a constant pressure chart, it also means a surface low is increasing in cyclonic circulation and acquiring more energy. The opposite of filling.

Degree - A measure of temperature difference representing a single division on a temperature scale.
Related terms: Celsius, Fahrentheit, and Kelvin

Density - The ratio of the mass of a substance to the volume it occupies. In oceanography, it is equivalent to specific gravity and represents the ratio of the weight of a given volume of sea water to that of an equal volume of distilled water at 4.0°C or 39.2°F.

Despression - In meteorology, it is another name for an area of low pressure, a low, or trough. It also applies to a stage of tropical cyclone development and is known as a tropical depression to distinguish it from other synoptic features.

Derecho - A line of intense, widespread, and fast-moving thunderstorms that moves across a great distance. They are characterized by damaging straight-line winds over hundreds of miles. Spanish for straight.

Dew - Condensation in the form of small water drops that forms on grass and other small objects near the ground when the temperature has fallen to the dew point, generally during the nighttime hours.

Dew Point - The temperature to which air must be cooled at a constant pressure to become saturated. Precipitation can occur once the temperature drops to the Dew Point.

Diffluence - A rate at which wind flow spreads apart along an axis oriented normal to the flow in question. The opposite of confluence.

Directional Shear - The shear created by a rapid change in wind direction with height.

Divergence - Wind movement that results in a horizontal net outflow of air from a particular region. Divergence at lower levels is associated with a downward movement of air from aloft. Contrast with convergence.

Doppler Radar (LIVE HD Doppler9) - Weather radar that measures direction and speed of a moving object, such as drops of precipitation, by determining whether atmospheric motion is horizontally toward or away from the radar. Using the Doppler effect, it measures the velocity of particles. Named for J. Christian Doppler, an Austrian physicist, who in 1842 explained why the whistle of an approaching train had a higher pitch than the same whistle when the train was going away.

Downburst - A severe localized downdraft from a thunderstorm or shower. This outward burst of cool or colder air creates damaging winds at or near the surface. Sometimes the damage resembles tornadic damage.
Related term: microburst

Downdraft - A sudden descent of cool or cold air to the ground, usually with precipitation, and associated with a thunderstorm or shower.

Drifting Snow - Snow particles blown from the ground by the wind to a height of less than six feet.

Dropsonde - A radiosonde dropped with a parachute from an aircraft rather than lifted by a balloon to measure the atmosphere below.

Drought - Abnormal dry weather for a specific area that is sufficiently prolonged for the lack of water to cause serious hydrological imbalance.

Dry adiabat - The line on a Skew T-Log P chart that depicts the lifting of dry air, or air that is unsaturated. As a parcel rises adiabatically, its pressure decreases and its temperature falls due to the expansion of the air parcel. When an air parcel is unsaturated and rises, then the temperature decreases at a rate of 1°C per 100 meters (5.5°F per 1,000 feet).

Dry bulb thermometer - A thermometer used to measure the ambient temperature. The temperature recorded is considered identical to air temperature. One of the two thermometers that make up a psychrometer.

Dry line - The boundary between the dry desert air mass of the Southwest U.S. and the moist air mass from the Gulf of Mexico. It usually lies north-south across the central and southern High Plains states during spring and summer. The passage of a dry line results in a sharp decrease in humidity, clearing skies, and a wind shift from southeasterly or south to southwesterly or west. Its presence influences severe weather development in the Great Plains.

Dry slot - An area of dry, and usually cloud-free, air that wraps into the southern and eastern sections of a synoptic scale or mesoscale low pressure system. Best seen on a satellite picture, such as a water vapor image.

Dust devil - A small, rapidly rotating column of wind, made visible by the dust, dirt or debris it picks up. It usually occurs in arid or semi-arid areas and is most likely to develop on clear, dry, hot afternoons in response to surface heating.
Related term: whirlwind

Duststorm - A severe weather condition characterized by strong winds and dust-filled air over a large area. Visibility is reduced to between 5/8ths and 5/16ths statute mile. It is reported as "DS" in an observation and on the METAR.

Dynamics - A branch of mechanics that deals with forces and their relations to patterns of motion. In metorology, this relates especially to wind and water patterns.

Portions of the 9NEWS Weather Glossary were taken from the second edition of the Glossary of Meteorology published by the American Meteorological Society (AMS). © 2009 American Meteorological Society