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Fall brings quirky weather to North Ga.
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Sunshine. Then more than 6 inches of rain, setting an all-time record for Gainesville. Then sunshine again.

The season for quirky weather changes is just beginning as fall settles in. There are days when we need air conditioning during the day and heat at night. Three months from now, we may see snow showers, followed by days of mild weather.

What makes North Georgia weather so quirky? Quite simply put, it’s the jet stream. Like an invisible river of air, the jet stream is a zone of rapid windflow, high up in the atmosphere, that determines the track which storms take. It extends all the way around the globe.

In the summer season, the warm and humid air mass situated over the Southern states is quite powerful, and keeps the jet stream traveling in an almost straight line across the Midwest and through New England.

But now, as the sun doesn’t come as high over the horizon, the continent is cooling. Dry, cold air is settling heavily over Manitoba, Saskatchewan, Yukon and the other northern portions of the landmass. It presents a thick barrier to the jet stream, which is forced to flow around it and curve toward the South. This happens in an uneven way, with the cold air launching one push after another toward our area.

An early one sent the storm track right through North Georgia two weeks ago. The continental storms typically consist of a warm front, a cold front and a low pressure cell where most precipitation is received. Because the jet stream was this far south, we were visited by all three. After the warm front brought moderate rain showers, the low and the cold front followed up with tremendous downpours.

High pressure usually dominates after such storms have moved through, bringing clear skies and warmer temperatures because winter isn’t here yet. We will now see more and more of these bouncy changes. They are usually slow in October.

But from now on, each push of cold air will produce rain, followed by a clearing and gradual warming. Another frontal system will arrive, repeating the pattern. The sequence speeds up going into winter.

When it’s most rapid and two cold fronts follow each other closely, we commonly get snow. Starting in March, warmer air from the Gulf provides for longer-lasting periods of mild weather. Until then, things are likely to remain bouncy.

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