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Microclimates of frost

The next couple of nights are going to be quite chilly in northern parts of Alabama and Georgia and frost is likely in higher elevations.  Some extension agents have told me that in some areas frost damage to fruit trees is a concern because they are blooming so early this year due to the warm weather we have been having.  The general frost forecasts have quite a large area being affected on Sunday night into Monday morning by temperatures of 32-36 F, but some areas may experience below 32 F temperatures. They could be even lower on Tuesday morning if the wind dies down enough.  The temperature you see at a location is due not only to the large scale weather pattern but also to highly local factors that make up the microclimate of a particular area.

Frost in a local area can be affected by the presence of water, pavement or other urbanization, including being in a city, the exposure to wind and open air, and the topographic pattern of the land.  Water bodies like lakes tend to hold heat and can prevent frost in areas just downwind when the lakes are warm compared to the air.  Pavement and urban conditions do the same thing, so most often television forecasters will note frost in suburban areas when the downtown cities are still frost-free.  Windy conditions tend to make frost less likely since warmer air aloft is mixed down to the surface by the wind and help keep it from getting really cold.  A tree or roof overhead can prevent heat from radiating to space, also keeping the area warmer.   On the other hand, if your garden is in a low-lying area, cold air that forms is denser than surrounding air and is likely to flow downhill into the lowest area, making local frost pockets in areas that might otherwise be frost-free.  When I was in Wisconsin, there were areas that received frost each month of the year due to their low elevation.

Here are a couple of informative publications on microclimates that you might find useful.

http://www.gardening.cornell.edu/weather/microcli.html

Microclimate

Source: Commons Wikimedia

Source: Commons Wikimedia