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Frost vs. freeze

Although the terms frost and freeze are often interchanged, they describe two distinct phenomena. An advective freeze is windborne & occurs when a cold air mass moves into an area bringing freezing temperatures. Radiation frost occurs when a clear sky and calm winds (less than 5 mph) allow an inversion to develop, and temperatures near the surface drop below freezing.

Feature Radiation Frost Advective Freeze
Wind <5 mph >5 mph
Sky clear clouds may exist
Cold air mass depth 30–200 ft 500–5000 ft
Inversion yes no
Cold air drainage yes no
Protection likely successful limited success

Types of radiation frost

Hoar frost, or white frost, occurs when atmospheric moisture freezes onto solid surfaces as small crystals without first condensing as dew.

Black frost, where few or no ice crystals form, occurs when the air temperature is below freezing but above the dew point (the temperature to which air must be cooled to cause atmospheric moisture to condense). The drier the air, the lower the dew point.

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