Monday, March 9, 2015

Virga Bomb

Add another term to your weather vocabulary: Virga Bomb.

Recently we’ve heard about the polar vortex and the derecho. Now comes virga bomb.

Virga is from the Latin, meaning “twig” or “branch”. I first heard of virga when I moved to New Mexico in 2013. Virga is rain that falls from a cloud but evaporates before it hits the ground. It looks like vertical wispy trails under a cloud.

Over this past weekend, I heard about virga bombs on the TV news. So what are they?

When the rain falls from the cloud and evaporates, it cools the air under the cloud. Cool air is denser than warm air, so the colder air sinks. When conditions are right, the cold air can fall very rapidly, creating a microburst. When the microburst hits the ground, the air has nowhere to go except horizontally along the terrain, causing high, gusty winds.

And that’s a virga bomb!