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Frost or freezing occurs when the temperature of air falls below the freezing point of water (0 °C, 32 °F, 273.15 K). This is usually measured at the height of 1.2 m above the ground surface.
There is a rather subjective scale to show several degrees of frost severity:
Frost is not necessary to get ground frost or hoar frost; they can form even if air temperature is above freezing point if the surfaces have been chilled by heat emission during a cold night.
Frost and hoar frost
In the English language, these terms are sometimes used in a confusing way. For example, frost is used sometimes to indicate ice which forms on the ground during cold nights, but other times to indicate air temperature below freezing point. Hoar frost should be used for any frosted surface but it is sometimes reserved to indicate big ice crystals forming on very cold surfaces. The WMO uses hoar frost in every case.