January 29, 2001
Virga is precipitation which evaporates before reaching the ground. In this remarkable shot (taken by Charles Jackson), you can see virga falling from a hole in an altocumulus cloud deck. How's this possible? Basically, the cloud area producing the virga has lost its moisture and is no longer visible. Virga occurs with many different cloud types, and can be in the form of either rain or snow. (Its name comes from the Latin word for "streak.") It can look like wisps, streamers, or curtains, but only rarely does it fall from the middle of a uniform cloudbank as you see here.
What causes virga? It occurs when the surface air is dry, causing the precipitation to evaporate in mid-air. This means that virga is most commonly seen in desert areas, such as the southwestern US. It makes sense that the air is drier beneath a cloud than inside it, thus most all precipitation falling into this drier air initially evaporates, though it's not always visible. Virga can show up as precipitation on weather radar, giving forecasters one more headache to deal with. So if the weather map shows rain, but it's not raining, go outside and look up: if you're really lucky, perhaps you'll see a scene like this!