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Earth Sense: Temperature inversions create bubbles that affect sound, smoke
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Atmospheric conditions often influence the course of history.

In late May 1862, the Civil War had Union troops approaching Richmond, Va., Confederate Gen. Joseph E. Johnston had planned to bring large units together in a three-pronged approach at Seven Pines crossroads. At the time, most commanders made tactical decisions “by ear,” judging from the noise of nearby battles where reinforcements were needed the most.

On Friday, May 30, a very heavy thunderstorm changed the situation. Saturday had been chosen for the battle, and Johnston’s units were 2 miles from Seven Pines while the rest were approaching from the opposite direction.

Waiting for battle noise, Johnston heard none. Finally, he moved his troops at about 4 p.m., only catching the tail end of the fight and getting badly wounded. Fighting had actually started that morning, so loud that people 10 miles away could hear it, but no one in Johnston’s camp did.

By eyewitness accounts, the weather that Saturday was likely to have been a temperature inversion. There was cool air near the ground, trapped beneath a cloud cover with layers of warmer air above.

Inversions tend to bend sound waves downward. The situation can be imagined like a huge arc made by the sound, originating on the battlefield and traveling upward, skipping large areas like Johnston’s camp 2 miles away, then coming down to ground level again at a greater distance. Without this acoustic shadow delaying Johnston’s actions, the outcome at Seven Pines might have been quicker and more decisive.

Temperature inversions are common during the fall season now, when the ground cools quickly during a clear night. If you live near railroad tracks, you may have noticed you can hear train horns more clearly at this time of the year. The part of the sound that normally travels upward is deflected down again by the warmer air, reinforcing what arrives normally.

Apart from carrying sound differently, temperature inversions also have the nasty habit of trapping smoke and other pollutants near the ground. Contrary to the opinion of some, the best time to burn fall leaves isn’t a clear evening. Smoke travels upward, then gets stopped by warm air layers, and is forced to spread all over the neighborhood.

Instead of burning the leaves, it makes more sense to compost them, or till them straight into the soil, which provides free fertilizer.

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

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