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Hoar frost   [ Meteorology ]

Dictionary of botanic terminology - index of names

  The white deposits of ice having a crystalline appearance that forms on grass or leaves in the morning when the dew freezes  
Hoar frost is dew turned to frost and refers to the ice crystals generally assuming the forms of scales, needles, feathers or fans deposited on plants or exposed objects, that form when the air is moist (saturated) and touches a very cold surface and freezes on it instantly. Hoar frost is often seen on cold, clear autumn nights. Hoar frost can occur at higher temperatures than rime frost usually when the air temperature is around 0 C. However, the ground or exposed surfaces are usually much colder.

Pediocactus despainii winter frost
Pediocactus despainii

Deposition of hoar frost on the  spines.

Dew and hoarfrost accumulate on objects during clear, chilly nights only when there is more humidity in the air than the air can carry. Grass, twigs and other surfaces are strongly cooled with a net loss of heat driven by strong, uninterrupted outward radiation. (If there were clouds present, or fog were to form, then these would absorb, then "re"-radiate heat back to the surface, slowing or cancelling the fall of temperature. A breeze would also stir the lower atmosphere, mixing warmer air above the surface with that on the surface - this too would offset surface heat loss.)
Provided that this 'loss' of heat is not made up from elsewhere (e.g. upward flux from the soil, emission of heat from adjacent buildings etc.), the temperature of such objects will fall below that of the adjacent air. As soon as the surface is cooled below the saturation temperature (dew point), water vapour will begin to condense onto these surfaces (either vapour from moist soil surfaces diffusing upwards to the chilled object, or downward mixing of vapour from the over-lying atmosphere): if the air temperature remains above 0 C, this water will remain liquid as drops of dew.
Once hoar-frost crystals form, they can remain as long as conditions for their existence are favourable. Hence in late winter we see the sun's warming rays removing hoar-frost from the south sides of objects. Hoar frost is white because the crystals contain air.

The formation of frost is an example of meteorological  deposition. The opposite of deposition. is sublimation.

Hoar frost on Opuntia hystricina (Photo courteously provided by Pal Vajda - Hungary)





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