Dear Tom,
On a vacation in the Rockies, we noticed that in quiet weather when the western United States was free of storm systems, thunderstorms always developed over the mountains in the afternoon, but not over the plains around them. Any explanation?
The Heider family
Dear Heider family,
The answer is differential heating. Thunderstorms form when sun warms the ground, the ground warms the overlying air and, after sufficient warming, currents of heated air rise upward and build into afternoon thunderstorms.
On flat terrain, daytime heating is quite uniform because sunlight strikes the ground everywhere at the same angle. But in mountainous terrain, slopes facing the sun experience stronger heating per unit area–and are therefore more susceptible to thunderstorm development.
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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.
