In meteorology, virga is a form of precipitation that falls from clouds, but either evaporates or sublimates before it reaches the ground. The term “virga” comes from the Latin word virga, which means “spring or twig.” The appearance of trailing tendrils of precipitation from clouds gives virga another name: jellyfish clouds. Other words for virga are Fallstreifen, fallstreaks, or precipitation trails.
How Virga Works
Virga starts out as rain or snow. Rain can evaporate before reaching the ground when the air below the clouds is drier or warmer and absorbs the moisture. At high altitudes, virga starts as falling snow or ice crystals. Sometimes snow sublimates into water vapor as pressure changes around the flakes. More often, ice first melts into liquid water and then evaporates.
Usually, virga is not perfectly vertical. It usually falls straight or at an angle and then has a hooked shape near the bottom before it disappears. While wind shear can form the shape, the curl mainly arises from the way evaporation slows the rate of descent of the raindrops or snow crystals.
Virga Clouds
Altocumulus and altostratus clouds are the main types of clouds producing virga. However, it sometimes falls from high-altitude cumuliform clouds, where precipitation falls into drier air layer.
How to See Virga
Virga can occur anywhere, but it is extremely common in temperate zones and deserts. Good places for viewing virga include the prairies of Canada and the United States, the western portion of the United States, North Africa, the Middle East, and Australia.
Viewing is easiest just around sunrise and sunset. Sunlight illuminates the precipitation, which otherwise appears to fade into the sky.
Is Virga Dangerous?
Virga is beautiful, but it can be dangerous. Water has a high heat of vaporization, so the phase transition from liquid water into water vapor removes a lot of heat from the air. The mass of cold air rapidly descends, potentially forming either a wet or dry microburst that poses a risk to aircraft or may even damage property on the ground.
Precipitation that evaporates at a high altitude experiences compression as the air mass falls. The water vapor changes back into a liquid, releasing heat, and causing wind gusts and a downburst. If the heat allows the air to reabsorb the water as vapor, the result can be a heat burst. While rare, heat bursts cause sudden temperature change at the surface and dry, potentially damaging wind. The highest recorded temperatures of a heat burst exceed 50 °C (122 °F). Heat bursts sometimes accompany weakening spring and summer thunderstorms and usually occur at night.
Virga on Other Planets
Earth is not the only place that experiences virga, although it does not always involve water elsewhere in the solar system. On Venus, sulfuric acid rain evaporates before it reaches the ground because the planet’s surface is so hot. NASA’s Phoenix lander observed virga snow falling from clouds on Mars. Jupiter and other gas giants also have virga.
References
- American Meteorological Society. (2000). “Virga.” Glossary of Meteorology. ISBN 1-878220-34-9.
- NASA (2008). “NASA Mars Lander Sees Falling Snow, Soil Data Suggest Liquid Past.” Phoenix Mars Lander.
- Pearce, Robert Penrose (2002). Meteorology at the Millennium. Academic Press. ISBN 978-0-12-548035-2.
- Rincon, Paul (2005). “Planet Venus: Earths ‘evil twin’.” BBC News.