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Frost is..
  • A
    a crystalline formation when the OAT is 0°C or less.
  • B
    a crystalline formation on surfaces with a temperature of 0°C or less.
  • C
    a clear formation when the OAT is 0°C or less.
  • D
    a clear formation on surfaces with a temperature of 0°C or less.
ICAO Doc 9640
Glossary of Terms and Abbreviations

Frost. A deposit of small, white ice crystals formed on the ground or other surfaces. Frost is formed by sublimation, that is, when water vapour is deposited upon a surface whose temperature is at or below freezing.

Hoar frost. A greyish-white crystalline deposit of frozen water vapour formed on surfaces in clear, still weather.
 

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