Dear Tom,
On a warm day recently, there was a lake breeze even though it is now mid-autumn. How is this possible?
Phil Westraul
Dear Phil,
The fundamental cause of wind is unequal heating of the air, and this implies that some bodies of air will become warmer than others.
When at the same elevation, a given volume of cool air, because it is more dense, always weighs more than an equal volume of warmer air. The greater weight of cool air causes it to exert greater pressure. Air-pressure differences will in turn cause air to move from a region of higher pressure to an adjacent region of lower pressure. We perceive this movement of air as wind.
If warm and cool air masses happen to sit next to each other–as they do when Chicago heats up on a warm day but air over Lake Michigan remains cool–then a lake breeze develops, regardless of the season.
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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.
