Sunday, May 17, 2009

Frosty Night Ahead

A frosty night is in store for the eastern two-thirds of the state tonight. The overnight low temperature is forecast to be down to around 37. Frost is an interesting element of weather. 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. You might think that dew will form and then freeze, but that is incorrect. Frost forms through deposition, the change of water vapor in the air directly into a solid... ice.
Frost forms first on rock, glass, or metal surfaces that lose heat more rapidly through radiative cooling than the surrounding air. This is why car windshields frost over before frost forms on surrounding vegetation. If the surface on which it forms has a temperature below the dew point, frost may even appear when the officially reported air temperature is above freezing. Frost formation may be sporadic across an area, particularly in hilly or mountainous terrain. Since cold air is denser than warm air, it flows like water downhill to pool in low areas or hollows. Areas that are low-lying are thus more susceptible to frost formation. These areas are called frost pockets or frost hollows. They are undesirable places for planting gardens!

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