Dear Tom,
Why, when it starts raining at the beginning of a thunderstorm, are the raindrops the largest? Then smaller drops mix in as the rain continues.
Richard Koffman
Dear Richard,
It’s strictly a matter of raindrop fall speed. Large raindrops fall faster than smaller drops, and so a thunderstorm’s largest raindrops make the descent from the storm’s rain production area to the ground more quickly than smaller drops.
As the rain-generating portion of a storm moves overhead (thunderstorms typically move at 25-35 m.p.h.), big drops arrive first because they are descending more quickly. Fall speed for the largest raindrops (diameter 0.2″) is about 20 m.p.h., whereas medium- sized drops (0.1″) descend at 14 m.p.h.
A large raindrop can tumble 5,000 feet in 2.8 minutes, but a medium drop requires 4.1 minutes to make the same descent.
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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN-TV News at noon and 9 p.m.
Write to: ASK TOM WHY, 2501 Bradley Pl., Chicago, IL 60618 or asktomwhy@wgntv.com (Mail volume precludes personal response.)
WGN-TV meteorologists Steve Kahn, Richard Koeneman and Paul Dailey plus weather producer Bill Snyder contribute to this page.
