Dear Tom,
Why is the color of the sky in Arizona a much deeper and darker blue than it is in central Illinois?
Robert Drury Springfield, Ill.
Dear Robert,
The sky appears blue because the very small molecules of our Earth’s atmospheric gases, primarily oxygen and nitrogen, selectively scatter the shorter blue wavelengths of incoming sunlight, allowing them to reach our eyes. When sunlight is scattered by larger particles in the atmosphere such as haze, dust or other condensation nuclei, some of the longer red and green wavelengths are selectively scattered, with these colors mixing with the blue making the sky appear paler. This is the reason that the sky seldom takes on that deep, dark blue Arizona appearance you describe in hazy and humid industrial areas like the Midwest, except for short periods after the arrival of an exceptionally pristine air mass.
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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.
