Friday, November 05, 2010

What is the Difference Between a Frost and a Freeze?

Last weekend we had advisories for the possibility of frost, especially for low-lying and rural areas outside of Raleigh, Wilmington and Morehead City. This weekend we face our first freeze of the season.

What is the difference between a frost and a freeze? Frost occurs on a clear, still night, as heat radiates from surfaces (like your car window, a field, etc.) to the sky. The temperature drops below 32 degrees and water vapor freezes on those surfaces (a process called sublimation..or gas being converted to a solid while bypassing the liquid stage). The temperature won't go much below freezing, because energy is released as the water freezes.

Most plants that aren't tropical can handle temperatures a few degrees below freezing with no problem, and if you have plants up against a house (even tender plants) they can usually be spared as heat will be trapped by any overhang.

Clouds or fog will trap heat, usually preventing a frost.

A freeze, however, is caused by cold, Arctic air moving into a region. It is a relatively dry air mass that is much colder than freezing, and you can have moving wind in cases of a freeze.

Much of the damage we see on plants from a freeze is called desiccation -- severe drying of the foliage.

Due to the fact that this involves a chance of air mass, there is less protection for plants by ways of cover or overhangs, so they need to be brought in if at all possible.

Usually a hard freeze marks the end of a growing season. In central North Carolina, we typically see our first freeze of the autumn season anytime from mid-October for the extreme north/northwest Triangle region to after November 1st for the southern Sandhills and Coastal Plain.

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