Educational - Frost
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[From: U.S. Dept. of Energy's "Ask a scientist" archive]
Frost is not made by freezing dew. It is formed by sublimation - water vapor moving directly to a solid... ice. Frost is formed when the dew point is near or below freezing and the temperature of the air falls to within a few degrees of the dew point.
Generally, the air is warmed or cooled by the surface. The air is largely transparent to radiant energy so heat released by the surface at night is radiated off into space. As the surface cools, the heat from the lowest layer of air is conducted to the ground. Cooling the air from below tends to make the air more stable and keep the cooler air low.