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Rudi Kiefer: Dew point is when precipitation happens
Rudi Kiefer
It’s hard to stand in the pounding rain and still think of moisture as an energy source. And yet, humidity is one of the driving forces that make storms possible.

Earlier this month, my students were working on an exercise about tornado formation. Many had trouble with the dew point concept. The dew point is one of several ways to measure humidity.

When air cools, its ability to hold water decreases. The water is in the air as a gas, so we can’t see it. But rising air, getting increasingly colder as it climbs, finally has to shed some of that moisture. This happens at what’s called the dew point temperature.

Warm springtime weather is usually brought to North Georgia by air coming from the Gulf of Mexico. It carries a great deal of moisture with it. While we’re enjoying conditions that the forecast may call “unseasonably warm,” a cold front arrives from Canada.

Our “unseasonable” warm air is forced out of the way, up into the sky. It reaches its dew point quickly as it cools up there, and storm clouds develop from the water vapor. The more water vapor was in the air to start with, the more quickly the dew point is reached. Air with a high dew point produces bad weather more easily than air that has to cool a great deal before condensation starts.

This is what makes summer thunderstorms pop up so fast. Sweltering temperature, combined with shirt-soaking humidity, produces a high dew point. The air has to rise only a short distance before it’s cooled to its dew point. Clouds start just a couple of thousand feet above the ground and build to enormous tower shape, thanks to all that moisture condensing out.

There are a few things absent, and that’s why — thankfully — we don’t get a tornado during summer thunderstorms. One is a low “lifted index.” Even those ominous dark summer clouds lack the brutal punch that a mass of cold air delivers to North Georgia in the springtime.

The other factor, rare during summer but common in spring, is helicity. That’s the tendency to rotate.

Bodies of air with very different temperature and moisture begin to dance around each other when helicity is high. The motion becomes increasingly violent, until a twister is born. Its main ingredient is a high dew point.

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

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