Virga is nature’s disappearing act hidden in plain sight
Rain and snow don’t always survive the long trip from cloud to ground
Have you ever seen rain or snow on the radar, only to look out the window and…not see the precipitation precipitating?
Maybe you’ve seen an awesome sky full of wispy curtains draped from the base of the clouds, feathers of grey dancing above the horizon.
Both of these are likely cases of virga—one of those fun weather terms that describes a distinct lack of anything happening.
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Virga is precipitation that falls from the base of a cloud but evaporates before it ever reaches the ground.
It’s possible to see virga hanging beneath the cloud deck if conditions are right and you’ve got good visibility of the horizon.
Shafts of rain or snow falling from the base of a cloud look like fuzzy curtains that vanish to nothing as the precipitation falls into dry air and evaporates before our very eyes.
Virga even “tricks” weather radar from time to time.
Radar works by sending a beam of microwave energy into the atmosphere. The beam bounces off raindrops and snowflakes and reflects back to the radar dish, where it registers the location and intensity of the precipitation.
These radar beams exit the dish at a slight upward angle—and between that tilt and the curvature of the Earth, a radar beam steadily climbs in altitude as it travels farther and farther away from the radar dish.
Radar can detect virga as it’s falling out of the base of the clouds if the radar beam is above the dry layer in the lower atmosphere. This can create the illusion that it’s raining or snowing when you check the radar on your phone, even if nothing is actually reaching the ground at your location.
Virga doesn’t always stay ‘hidden.' Evaporating rain or snow will steadily moisten that dry layer in the atmosphere, sometimes mixing out the dry air completely to allow the precipitation to finally begin reaching the ground.
Header image courtesy of Unsplash.