Dear Tom,
On August 21, we noticed the full moon low on the eastern horizon glowing orange. It was beautiful, and as it rose it became more yellow till finally it was white. What caused this?
Joanne Prusik, Chicago
Dear Joanne,
The spectacle of the moon’s gradual color change as it rose from the horizon was a visual demonstration of the effect our atmosphere has on light that passes through it.
Moonlight (actually, reflected sunlight that is white when pure) is massively scattered when the moon is on the horizon and its light must pass through a great distance of dense air. Only the longer wavelength components of light, shades of red and orange, arrive at our eyes. As the moon climbs, its light makes a shorter passage through dense air and less scattering occurs. Shorter wavelength colors, like yellow, emerge, then finally the full white spectrum.
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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.
