Weather Whys: What is virga?


Virga is a weather phenomenon that describes a storm or shower that forms as normal, but the rain does not reach the ground. This has to do with different layers in the atmosphere beneath the storm. 

If a storm forms over a saturated layer, that layer cannot absorb any more moisture, so none of the rain evaporates as it falls. This can result in a heavier rain, all of which reaches the ground.

A storm can form over a shallow layer of dry air as well. It initially passes through the saturated layer with no evaporation. It then passes through the small dry layer with a little evaporation. This results in a light rain that reaches the ground.

If a storm forms over a large layer of dry air, this is when we see virga. The rain evaporates on its way through the dry layer, and there is no moisture left by the time it reaches the ground. This creates the gray veils we see, the rain shafts, hanging from clouds but not reaching the ground. Virga! 

Eventually, if rain continues, virga can make way for rain. As rain evaporates into the dry layer, it increases the humidity of the dry air. That is quickened by evaporative cooling. Eventually, it is saturated enough to allow rain to reach the ground. 

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