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Earth Sense: Colorful fall skies determined by moisture

POSTED: October 27, 2012 11:59 p.m.

Fall is the season for enjoying colors in North Georgia. But it’s not just the colors of tree leaves that are changing. The sky shows a much greater variety of hues than we get at other times of the year.

Deep, postcard-grade blue tones come out best right about now. It’s a function of the moisture content of the air. There are three elements to the spectrum of visible light: blue, green and red. Blue is the shortest of the wavelengths and becomes dominant in dry air when the sun is high. Great amounts of moisture change the color to a white haze, indicating that the other wavelengths get scattered along with the blue.

When the sun drops to the horizon, its rays have to penetrate a thicker layer of the atmosphere, and red becomes represented more strongly. A red sky in the evening indicates very dry air. Since sunset is in the southwest, we’re looking in the general direction that brings most of our weather, so it’s a good indicator that the next day is likely to be clear as well.

In late fall, you often notice a dark gray layer near the horizon. That’s not air pollution; it’s called an inversion layer. The ground cools strongly in the evening, drawing heat out of the atmosphere which lacks the moisture to keep it warm. At high humidity air stays warm, explaining our muggy summer nights.

Now, with much less water vapor to produce the greenhouse effect, the lowermost parts of the atmosphere cool fastest and a chilly layer of air settles near the ground, condensing what moisture there is and producing the dark haze. A few hundreds of feet up, there’s a temperature increase. So the normal sequence of cold, colder, coldest with increasing height is upside down, hence the term inversion.

When there is an actual source of pollution present, for instance smoke, it won’t rise beyond the upper boundary of the inversion. Instead it gets trapped near the ground, and everybody gets to partake in the neighbor’s burning leaf pile or smoky chimney.  Exquisite colors are provided by so-called sun dogs. They are cousins of the rainbow, produced by ice crystals high in the atmosphere. When those are present in the form of thin, feathery cirrus clouds, there are beautiful rainbow-like stretches that glow in all the colors of the spectrum.

Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University. His column appears Sundays and at gainesvilletimes.com.


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