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In meteorology, virga is an observable streak or shaft of precipitation that falls from a cloud but evaporates before reaching the ground.1 At high altitudes the precipitation falls mainly as ice crystals before melting and finally evaporating; this is usually due to compressional heating, because the air pressure increases closer to the ground. It is very common in the desert and in temperate climates. It is also common in the Southern United States during summer.
Virga can cause very interesting weather effects, because as rain is changed from liquid to vapor form, it removes heat from the air due to the high heat of vaporization of water. In some instances, these pockets of colder air can descend rapidly, creating a dry microburst which can be extremely hazardous to aviation. Conversely, precipitation evaporating at high altitude can compressionally heat as it falls, and result in a gusty downburst which may substantially and rapidly warm the surface temperature. This fairly rare phenomenon, a heat burst, also tends to be of exceedingly dry air.
Virga also has a role in seeding storm cells whereby small particles from one cloud are blown into neighboring supersaturated air and act as nucleation particles for the next thunderhead cloud to begin forming.citation needed
Virga can produce dramatic and beautiful scenes, especially during a red sunset. The red light can be caught by the streamers of falling precipitation, and winds may push the bottom ends of the virga so it falls at an angle, making the clouds appear to have commas attached.
The word virga is derived from Latin meaning twig or branch and a popular backronym in meteorology is "Variable Intensity Rain Gradient Aloft."citation needed
Extraterrestrial versions
Sulfuric acid rain in the atmosphere of Venus evaporates before reaching the ground due to the immense heat near the surface. 2 Similarly, virga happens on gas giant planets such as Jupiter.citation needed In September 2008 NASA's Phoenix lander discovered a snow variety of virga falling from Martian clouds. 3
See also
References
- ^ Glossary of Meteorology, American Meteorological Society. 2000. ISBN 1878220349, http://amsglossary.allenpress.com/glossary/search?id=virga1.
- ^ "Planet Venus: Earth's 'evil twin'", BBC News (7 November 2005).
- ^ "NASA Mars Lander Sees Falling Snow, Soil Data Suggest Liquid Past" (2008-09-29). Retrieved on 2008-10-03.
External links
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Wikimedia Commons has media related to: Virga |
- National Science Digital Library - Virga
- Picture: Virga near Denver, Colorado.
- "Viewing the Vagaries and Verities of Virga" Alistair B. Fraser and Craig F. Bohren, Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania, 2 November 1992 and 25 January 1993.
Wikipedia content modification information:
- This page was last modified on 2 November 2008, at 07:23.
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