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Dry Thunder
Fri Aug 20, 2004
Listen in RealAudio 
When you hear the word "thunderstorm" in a forecast, you automatically think of
thunder and lightning, plus wind and rain. Lots and lots of rain. But not all
thunderstorms come with cloudbursts, and that is troubling to those watching for, or
fighting forest and grass fires. Hi, I’m Bryan Yeaton, and this is The Weather Notebook.
It is rare for a thunderstorm to form and not produce rain. But in some thunderstorms,
the rain that falls never reaches the ground. This is a dry thunderstorm. Dry
thunderstorms are more frequent in the western regions of the continent where the air
near the surface is often desert-dry and the cumulonimbus cloud base is high in
altitude. These thunderstorms produce the same rain, hail, and even snow in their
upper reaches as wet thunderstorms do, but once the raindrops exit the cloud, they
evaporate on their way down.
We can see evidence of this if we look under the towering cloud and see streaks or a
wispy area below the base that terminates before reaching the ground; this is called
virga. How far down the virga extends depends on the dryness of the air below cloud
base and the size of the raindrops falling out. Small drops falling through very dry air
disappear quickly. Even hail can melt and evaporate before reaching the surface.
Although dry thunderstorms produce no surface rain, they still produce lightning that
does reach the ground, and gusty, erratic winds. And this is what wildland fire fighters
and spotters fear. The splash of a good drenching rain—one that goes all the way to
the ground—is the music they want to hear.
Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook
is generously funded by the National Science Foundation and Subaru of America.
Thanks to Trish Anderton, who puts our show together.
Today's Links
Virga:
http://www.noaa.gov/questions/question_081701.html
Virga photo:
http://ccc.atmos.colostate.edu/~hail/cool/rain/pages/nd_virga.htm
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