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Definition:
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The condition that exists when, over a widespread area, the surface temperature of the air remains below freezing (0C) for a sufficient time to constitute the characteristic feature of the weather. This is a general term, and the time period necessary is usually considered to be two or more days; only the hardiest herbaceous crops survive. It differs from a dry freeze or black frost, for these terms are usually used to describe purely local freezing due to chilling of the surface air by rapid radiation from a restricted portion of the earth.
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