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Earth Sense: North Georgia's summer pattern can vary
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North Georgia’s summer climate is often called “hot and humid.” But that’s not really accurate. There are times when it’s warm and humid, and other times when it’s hot and dry.

Which one we get is largely controlled by the Bermuda High. You see it on most weather maps now as an “H” to the east of the state. A high is a large area where air is forced to descend from great altitude. As it sinks, its temperature rises and its relative humidity decreases.

When the high settles squarely over Georgia, humidity is low, especially in the early afternoon if the sky is clear and the sun warms the ground with full force. That’s the situation where the thermometer climbs into the 90s and even 100s.

It’s also a drought condition because descending air will suppress precipitation. The common phrase “it’s not the heat, it’s the humidity” doesn’t apply in this case because when temperatures hover around 100 degrees, it’s the heat that causes discomfort, not the humidity.

But many times during summer, we’re on the fringe of the Bermuda high. In addition to dropping air downward onto the Atlantic Ocean, that system also rotates slowly clockwise, like a giant sprocket wheel. At its fringe, air pushes into Georgia at ground level. This is the “warm and humid” situation, bringing us loads of humidity from the ocean.

As the air becomes heated further by the ground, it gets a tendency to rise again. Bubbles of air called convection cells, covering 1 or 2 square miles, climb upward and shed their moisture in the process. Popping up like mushrooms on a wet lawn, summer thunderstorms form and rain is received in a patchy pattern.

This has a cooling effect, but after a thunderstorm humidity tends to be extremely high. That’s when most people are uncomfortable because the body tries to get cooler by turning on its perspiration system. In high humidity, it doesn’t work well, since the humidity suppresses evaporation. Clothes get soaked with perspiration mostly on those “warm and humid” days, and less during the “hot and dry” periods because dry conditions allow cooling through perspiration.

In both cases, though, there’s a serious risk of dehydration. Even when one is “sweating it right out” again, body fluids need to be replenished by drinking plenty of water in both types of North Georgia summer weather.

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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