How frost forms when it is above freezing - WVVA TV Bluefield Beckley WV News, Weather and Sports

How frost forms when it is above freezing

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(WVVA) Frost is likely Tuesday morning with lows expected to mainly be in the 30s.  Some thermometers will not dip below 32 degrees, yet there will probably be frost around.  How is this possible?

Cold air is more dense than warm air. On a clear calm night, heat radiates from objects (like your car and the ground) into outer space.  The dense cold air sinks and stays close to the surface and objects.  Thermometers are generally a few feet above the ground and can have readings that are several degrees warmer than the air very close to the surface.  This is one way frost can form when your thermometer shows a temperature above freezing.

As the temperature of the ground or object reaches the dew point, water vapor in the air begins to form crystals of ice on objects. If it is colder than 32 degrees this process known as deposition.  Deposition is when water goes from a gas to a solid, bypassing the liquid phase. 

When the air isn't cold enough, dew is formed as water vapor changes into liquid.  Sometimes the air can cool enough for the dew to freeze solid, but frozen dew is not frost.  Frost forms through a different physical change, making it somewhat fluffy while frozen dew is often hard and very icy and difficult to scrape from your windshield.

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