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Earth Sense: Trough caused front to stall, dumping rain on area last week

POSTED: July 27, 2014 12:42 a.m.

The week ending July 20 brought unusual, but not unheard of, weather to Hall County.

It was nice to turn the air conditioning off for a while, even though rain was falling all of Friday and Saturday.

The National Weather Service explanation for those unseasonal 70-degree days was quite technical.

“Impulses moving through an upper-level trough” and a “surface wedge in place” aren’t terms that everyone is familiar with. The “trough” referred to a bounce in the flow of air at high altitude.

This time of the year, it normally runs west to east, in a more or less straight line. But as the South got hot, a wave developed.

It’s similar to placing a garden hose on the ground, and gradually opening the faucet. At some point, pressure in the hose will make it swing back and forth.

What we got last weekend was the southward bounce of the airflow, making a steep curve through the Southeast.

Humidity is found in abundance in Georgia, so the mixing of northern air with southern moisture produced long-lasting rain showers in a wedge-shaped area.

Weather maps showed a line with triangles, pointing southward, and half-circles pointing north. That’s called a stationary front.

Heavy, cool air from the north was forcing lighter, warmer air from the south to lift off the ground. When moist air rises, precipitation is likely.

The front, where most of the action occurred, stayed in place for many hours. A stationary front can produce several days of rain in a row through the uplift along its edge.

One can visualize it like a street sweeper. Even when it stops moving forward, the brushes keep turning. In our case, the “brushes” were a broad line of showers and a few thunderstorms that covered much of the Southeast.

It’s tempting to attribute this unseasonal weather event to climatic change or some other topic of current news interest.

But waves in the upper-air flow, and its fast lane, the jet stream, are normal even in the summer.

In the mid-1980s, we had such a situation that created almost a week of constant rain. As a student landscaper at UGA, I was aggrieved to find my huge zinnia flowerbeds on College Station Road in Athens turning into acres of mildew.

This year’s July rains, though, have helped keep the trees happy and Georgia looking healthy and green.

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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