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What causes frost?
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| Frost forms in much the same way as dew. Frost forms when a surface cools
(through loss of infrared radiation) to a temperature
which is colder than the dewpoint of the air next to the surface. Frost is frozen water that has
condensed from some of the water vapor contained in the air.
If a significant layer of air next to the ground were actually cooling to the dewpoint, then a visible shallow
layer of fog would form. Instead, the air next to the surface (say grass) must be so thin that
the only evidence that condensation has occurred is the frozen water on the grass.
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| Interesting facts:
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HOAR FROST: Huge ice crystals of frost can develop if a cold surface (typically
well below 32 degree f) is exposed for a long period of time to humid air. This can happen on the underside
of ice covering if the river level has fallen since the ice was formed, leaving a humid layer of air trapped
between the surface of the river and the underside of the ice. Hoar frost is also found in some layered snowpacks.
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