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New Take
Fri Mar 18, 2005
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 moisture 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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