What is Virga & How Does It Occur?

Have you ever taken a peek at the latest radar image and saw a few stray areas of blue and green over your house.  But, then you stepped outside and it wasn’t raining or snowing?  You stood there perplexed for a minute or so and maybe said, “this app stinks!” However, you did see some wisps hanging from the clouds. What were those wisps? What was the radar picking up? The answer to both questions is virga. 

Virga (precipitation evaporating before reaching the ground), as seen from afar. (Source)

Virga is essentially precipitation that evaporates before it reaches the ground. Virga begins as actual precipitation falling from clouds, but encounters a sufficiently thick layer of dry air, causing the precipitation to evaporate. As a result, you see the precipitation as wisps dangling from the clouds. However, these shafts of rain or snow can still be recognized by radars. Why? Well, radars are tilted at slight angles and the farther precipitation is from the radar site, the higher in altitude the radar is “looking.”  So in other words, the radar is correctly picking up rain or snow, but the beam is sampling the precipitation before it evaporates closer to the ground.

Visual depiction of a radar beam and how it detects precipitation above the ground (source)

How do meteorologists avoid confusion from virga? In addition to using radar, they also monitor current observations from vast networks of instruments scattered throughout the country. These weather stations are often a meteorologist’s best friend in situations like these, as they can be used for ground truth as to whether or not the rain or snow is actually reaching the ground. If the shower misses the weather station, webcams can be very helpful in determining if precipitation is falling as they are becoming more and more numerous across the country. There is even an app developed by NOAA NSSL called mPing, which crowd sources precipitation observations for further help with ground truth of rain or snow. All of these sources help a meteorologist reasonably conclude if virga is present.