Earth and Sky科普英语：Raindrops 雨滴
From a light rain to a heavy downpour, raindrops fall to Earth's surface. And if you watch over time, you'll see that some raindrops are small and some much larger. Find out why -- on today's Earth and Sky.
DB: This is Earth and Sky, with a question from a listener: "What determines the size of raindrops? Sometimes it's a real fine mist, and other times it's big drops."
JB: A mist isn't technically rain, although many people use the word to indicate a light rain. A mist or fog is a cloud on the ground -- and clouds are made up of very tiny water droplets. These droplets are smaller than the radius of a human hair -- too small to fall through the atmosphere as rain.
DB: It takes about a million tiny droplets of mist to make one average-sized raindrop. The drops become raindrops by colliding and coalescing with each other in the cloud. If the drops get large enough, they fall as rain. The longer the drops stay in the clouds, the larger they're able to grow.
JB: And that's where the type of cloud that produces the raindrop comes in. Inside of high puffy cumulus clouds, winds are strong. Drops are buffeted by the winds and suspended in the cloud long enough to grow very large.
DB: Then there are stratus clouds -- the relatively low, often uniformly gray clouds. Winds inside these clouds are relatively weak. Raindrops that fall from these clouds may be very small -- because they fall before they've have a chance to grow.
JB: That's today's show, made possible by the National Science Foundation. And, by the way, to ask your Earth or sky question come see us at earthsky.org. I'm Joel Block, with Deborah Byrd, for Earth and Sky.
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