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Rudi Kiefer: Gardeners have to keep eye on freezing weather
Rudi Kiefer
Freezing weather is coming, as it does every year. The average minimum temperature for Gainesville is 40 F.

But temperature averages are useless for people dealing with plant growth, farmers and gardeners for example. One single nighttime freeze can cause serious damage, no matter what the average daily low might be.

Average first frost is more important. It indicates the days when the mercury is most likely to dip below 32 F for the first time. For Gainesville, that time span already passed between November 1 and Nov. 10, without a report of freezing.

It would be wrong to conclude that we can now expect a warm winter season. Daily weather always shows variability. Topics of climatic change deal with average weather, not daily forecasts.

The term “freezing weather” makes one think of gray sky, some sleet and chilly wind. But the really heavy frost events occur during clear conditions. This is because North Georgia is in the subtropics, and winter-type weather is transient. It arrives in stages.

Our typical cold-weather sequence starts out with relatively mild temperatures in the 40s or 50s. Then a cold front arrives from the northwest. It’s the “front bumper” of a traveling body of air coming from Canada.

Getting hit by a cold front, which on weather maps is marked as a line with teeth pointing in the direction of movement, involves precipitation. The moisture content of the air that’s been sitting here is converted into rain.

Once the cold front has passed, the main body of Canadian air settles into place. It brings very little humidity with it because it’s so much colder. The day after the cold front can bring beautiful sunshine to northern Georgia, but with a noticeable chill.

Experienced gardeners know that now it’s time to cover up sensitive plants, because the nights that follow can be freezing. It doesn’t require snow or sleet for this to happen.

At night, with no clouds in the sky, the ground loses temperature rapidly. Atmospheric moisture, the main “greenhouse gas” that helps keep the air warm, is at a premium. Heat from the soil radiates away into space and is lost.

Some of the effect can be prevented with mulch and drop cloths where plants are in danger. To know whether frost is likely, watch out for sunny but chilly days that follow rain storms.

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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