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Raindrops
Mon Mar 22, 2004
Listen in RealAudio 
Hi, I'm Bryan Yeaton for The Weather Notebook. What is the speed of a falling raindrop? It's a
tricky question because some raindrops don't even fall if the surrounding wind has a
sufficiently strong upward component. In other situations, strong downdrafts may force
raindrops rapidly earthward.
To answer the question, let's consider only raindrops falling through still air. When they
fall, two forces act on the drops. The first is good-old gravity. The second: aerodynamic
resistance, or drag.
Initially, when a raindrop begins falling, it picks up speed because of the pull of gravity.
If it could fall through an airless environment, the raindrop would accelerate until it hit a
surface. Therefore, its speed depends on the distance fallen. A raindrop falling through a
vacuum from 10,000 feet would smash into the ground at over 565 miles per hour. Zoom.
However, in Earth's atmosphere, the drag of surrounding air slows the falling drop. When
aerodynamic resistance eventually equals the weight of the raindrop, the two forces balance
and the drop reaches terminal velocity. It will fall at that speed until it hits the
ground.
Air resistance depends on the raindrop's size, shape, and speed. Most drops are fairly round;
so at high speeds, the air resistance increases approximately with the square of the
velocity.
A large raindrop measuring 5 millimeters across - about the size of a small housefly - falls
at around 20 mph. Falling from 13,000 feet, that raindrop takes approximately seven minutes to
reach ground.
Thanks to our contributing writer, meteorologist Keith Heidorn. Our show is produced with the
help of Subaru and the National Science Foundation.
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