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New Take
Wed Apr 16, 2003
Listen in RealAudio 
Ever been caught in a downpour? Then you've probably wondered how that fleecy white cloud up
there can produce so much water so fast. Hi, I'm Bryan Yeaton for The Weather
Notebook.
Meteorologists have been wondering the same thing. They do know how clouds get started. When
rising air gets saturated with water vapor some of the molecules start glomming onto dust,
salt, and other tiny particles. If the air keeps rising and cooling, the droplets keep
growing, but you could wait days for this process to give you a healthy
raindrop.
Once you do get some good-sized droplets of different sizes they start falling at different
speeds and bump into each other, which helps make them grow even bigger. But, how a cloud
gets from the slow growth mode into the fast lane still isn't clear.
One group of researchers from Israel and Russia think they've found the answer. If you look
at a ray of sunlight just so you can see little eddies tossing bits of dust around. The air
is filled with these turbulent pockets. Now, imagine a set of tiny raindrops being tossed
about the same way. When these droplets are about the width of a fine human hair they can be
flung out of these eddies like they've been tossed by a catapult. On this track they can bump
into other droplets and start to grow.
The scientists who came up with this "sling" theory say it helps explain how some droplets get
the extra oomph that sends them on their way to atmospheric greatness and right into the
middle of your picnic.
Bob Henson from Boulder, Colorado contributed today's piece. Thanks today goes to our special
sponsorship from Davis Instruments, makers of the wireless weather pro home weather station.
Thanks also to Subaru and the National Science Foundation.
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