Dear Tom,
I always thought that cold air moves in from the north and warm air moves in from the south, but earlier this winter we got colder with winds blowing from the southwest. What gives?
Sandhya Srinath
Dear Sandhya,
Your observation that cold air arrives in Chicago from a northerly direction (north, northwest or northeast) and warm air from a southerly direction is essentially correct–but the atmosphere does not always adhere to established conventions.
Low pressure systems, the hundreds-of-miles-in-diameter storm systems that routinely sweep across the world’s middle latitudes, feature counterclockwise wind circulation–southerly (warm) winds as the system approaches and northerly (cold) winds as it departs. But when a low moves slowly or stalls, cold air sometimes wraps around the system and blows in from the southwest and south.
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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.
