Dear Tom,
You have written that “Clean air is transparent to sunlight, letting sunlight pass through it without being warmed by it.” So, does the air then warm by absorbing the heat radiated from those objects the sunlight does warm?
Jim Stewart, Deerfield, Ill.
Dear Jim,
That is correct, and, in addition, air is warmed by direct contact with objects heated by sunlight.
The result is that the atmosphere is warmed at the bottom, and this is an inherently destabilizing process. Like a pan of water being heated on a kitchen stove, warm air lying beneath cooler air will eventually overturn if it becomes warm enough (and the air aloft is cool enough) during the course of daytime heating.
This fundamental atmospheric process drives the formation of summertime cumulus clouds and, if the warming is great enough, leads 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.
