Hungry for Fog
Frank Watson, Meteorologist
October Weather Feature 2004

If it hasn't happen already, frost is just right around the corner and this is a good time to take a look at the phenomenon before it strikes, most likely in this month.

Two kinds of frost exist. One, black frost, comes with wind and often with clouds while the other, white frost, comes with clear skies and calm air.

Black frost is so-called because the sight of what was green vegetation is replaced by the black color of freeze-killed material. It occurs when an outbreak of very cold air is borne on a cold northerly wind. We can expect that the first frost will be a black frost about one year of ten.

With black frost, the temperature drop is caused by advection.

White frost is co-called because the sight of frozen water on the vegetation gives a white color. It occurs when the temperature as sunset is above freezing but then falls below 32 degrees at some time during the night.

With white frost, the temperature drop is caused by radiational cooling. Here plant surfaces lose heat by a process called radiation - the same thing that makes the reading a on a thermometer drop below the true room temperature if you hold the thermometer a few inches away from a block of ice. To a plant, the night sky is mighty cold, like the block of ice.

Over most of the northlands between the Appalachians and Rockies, the first frost occurs in October, through the Dakotas and parts of Minnesota, Wisconsin and Upper Peninsula often see frost in September. The same area normally sees its last frost in April; through May often offer the last date.

Frank Watson is a White Bear Lake, Minnesota meteorologist and can be found on the web at WeathermanWatson.com