Air frost

Air frost

Air frost 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.

There is a rather subjective scale to show several degrees of air frost severity:

  • slight air frost: 0 °C to -3.5 °C
  • moderate air frost: -3.6 °C to -6.5 °C
  • severe air frost: -6.6 °C to -11.5 °C
  • very severe air frost: below -11.6 °C

Air 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.

Air frost, frost and hoar frost

In English language, these terms are sometimes used in a confusing way. For example, "frost" is used sometimes to indicate ice which forms on ground during cold nights but other times to indicate air temperature below freezing point which should be called "air frost". "Hoar frost" should indicate big ice crystals forming on very cold surfaces, but it is sometimes used for every frosted surfaces.

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