Aug 25, 2010

Q: Sometimes you hear the weather forecaster refer to “virga.” What is it?

A: Virga is a common weather feature, says John Nielsen-Gammon of Texas A&M University. It refers to rain or snow that forms a wispy streak beneath a cloud and appears not to be reaching the ground. “Most of the virga people see today is usually composed of snow, since snow is so much brighter and easier to see than rain,” Nielsen-Gammon explains. “Even in the warm days of summer, the virga coming from the tall anvil-shaped thunderstorms is most likely snow, forming in the frigid temperatures 5-10 miles above the ground. Virga can also come from the base of a thunderstorm and other types of clouds, and it can be mistaken for a tornado at a distance because of its V-shaped appearance.”

Q: Is the rain or snow evaporating before it reaches the ground?

A: “That’s just what the ancient Greeks believed and what most modern textbooks say, but it’s just not so a lot of the time,” Nielsen-Gammon adds. “Recent research suggests that what appears to be the bottom of a shaft of precipitation might just be the level where the snow melts and changes to rain. Snow floats, but raindrops fall. It’s sort of like a big traffic jam in the sky. When all of the snow is moving slowly, it’s all bunched together and blocks the sky. But when the snow melts into rain and everything speeds up, there’s suddenly a lot more space between the drops, and the whole shaft of precipitation becomes harder to see.”

Weather Whys is a service of the Department of Atmospheric Sciences at Texas A&M University.