Dear Tom,
Why do storms always move in from west to east?
Garland Wendal
Dear Garland,
It’s true: Weather systems usually approach Chicago from a westerly direction (that is, from the northwest, west or southwest).
The Earth’s atmosphere is divided into several permanent, although undulating, belts of wind that are oriented approximately parallel to the equator. This is called the “primary circulation.” One of those bands of wind extends from about 35 to 55 degrees north latitude, and within that zone the main motion of air is to the north. However, the Coriolis deflection, which acts to the right of the attempted northward motion, causes north-blowing winds to turn to the east. Chicago, at about 42 degrees north, lies within this belt and that explains the general west-to-east motion of weather systems here: the “prevailing westerlies.”
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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 Place, Chicago, IL 60618 or asktomwhy@wgntv.com (Mail volume precludes personal response.)
