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frost FROST
On clear, cold nights in
the spring and autumn, the ground cools as its heat radiates away into space.
The air right at the ground can chill five degrees colder than air just five
feet above the ground. When temperatures dip to below freezing, frost forms
as water vapor in the air changes directly to ice and giving a white coating
to grass, windshields and porches. Cold air being denser than warm air, allows
it to drain downhill; that's why places in valleys, even small ones, are usually
colder and frostier than higher elevations. Frost usually forms when a large
dome of high pressure is parked overhead, keeping skies cloud-free and winds
light.
Cold, not frost kills the
plants. The frost is just a sign that it's cold. Ice is a good insulator. Spraying
trees with water under near freezing temperatures, gives them a coating of ice
which protects them from temperatures just a few degrees colder. But as it gets
even colder and persists through the night, this will no longer work as the
tree or plant begins to freeze. Some area growers also use fans to stir up the
air, mixing the cold air near the ground with the warmer air higher up. This
keeps the air near the ground warmer and works best when temperatures are only
a little below freezing.
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www.weather.gov 
