Virga is the name given to precipitation (usually rain) that evaporates or sublimes before it hits the ground. It tends to look like wispy gray streaks hanging underneath the base of a cloud. For this reason, you may also hear virga referred to as "fallstreaks." The storms that are associated with virga produce trace amounts of ground level precipitation only.
Why the funny name? Keeping in the tradition of clouds whose names are Latin, the term is derived from the Latin word virga, meaning "twig" or "branch," likely referring to the thin delicate streaks it produces.
Relative Humidity is Under 50 Percent
Virga is produced when precipitation falls from high clouds into extremely dry air (low humidity) and high air temperatures below. (Virga is commonly seen across the desert region of the Western United States, an area prone to both low humidity and high temperatures.) As the liquid raindrops or ice crystals hit the warm, dry air they absorb the high levels of heat energy which energizes the movement of their water molecules, transforming them straight into water vapor (sublimation).
Eventually, as more and more precipitation evaporates into the air, the air becomes moister (RH rises). If precipitation is light, it can take several hours for the air to saturate. As the air saturates first aloft, then down to the surface, a kind of "moist pathway" is carved out that precipitation can follow to the surface as rain or snow.
Virga On Radar
Like all light precipitation, virga shows up on the radar as shades of light green (rain) or light blue (snow). However, with virga, while the radar may detect it, your eyes won't. If you've ever watched your radar screen and seen the leading edge of a rain or snow band over your location but not seen any rain or snow actually falling outside your door, then you've been tricked by virga before. This is common in winter, especially when waiting for the start of a snowstorm. We've all heard our meteorologist say " It's already snowing in the upper air, but the air at the surface is too dry to see it."
Virga vs. Rain Shafts
It's easy to mistake virga for a distant rain shaft (a dark curtain of rainfall extending from the base of a thunderstorm down to the ground). What's the biggest give away for virga? If it's virga, it won't reach the ground.
Commas in the Sky
It is also theorized that virga is partially responsible for creating hole-punch clouds. In addition, virga high in the atmosphere can reflect sunlight creating brilliant sun pillars and other atmospheric optics associated with sunlight.
Edited by Tiffany Means