Virga

For the neume, see Neume.

Nimbostratus virga

Virga falling from Altocumulus

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. In North America, it is commonly seen in the Western United States and the Canadian Prairies.

Virga can cause varying 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, twig or branch.

Extraterrestrial occurrences

Sulfuric acid rain in the atmosphere of Venus evaporates before reaching the ground due to the high heat near the surface.^[2] Similarly, virga happens on gas giant planets such as Jupiter. In September 2008 NASA's Phoenix lander discovered a snow variety of virga falling from Martian clouds.^[3]

See also

• Aviation safety
• Sun pillar

References

1. ^ Glossary of Meteorology. American Meteorological Society. 2000. ISBN 1878220349. http://amsglossary.allenpress.com/glossary/search?id=virga1. 
2. ^ "Planet Venus: Earth's 'evil twin'". BBC News. 7 November 2005. http://news.bbc.co.uk/2/hi/science/nature/4335628.stm. 
3. ^ "NASA Mars Lander Sees Falling Snow, Soil Data Suggest Liquid Past". 2008-09-29. http://www.nasa.gov/mission_pages/phoenix/news/phoenix-20080929.html. Retrieved 2008-10-03. 

External links

• National Science Digital Library - Virga

• "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.