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Dew, Hoar Frost, Rime & Glaze

Dew and Hoar Frost

On clear, chilly nights grass, twigs and other surfaces (e.g. car windscreens) 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 brisk 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 0degC, this water will remain liquid as drops of dew. [ NB: the products of dew formation should not be confused with the guttation process, where water is exuded from the tips of plants, usually under conditions of warm, moist soil; these droplets are usually quite large and concentrated at the extremities of growing plants, and careful examination will avoid misattribution.]

If the temperature is much below 0degC, the droplets of dew will begin to freeze since, unlike droplets condensing in the free atmosphere, they will contain some freezing nuclei from the hard surfaces. As soon as a speck of ice has formed, further ice crystals will grow directly onto the ice nucleus without the intermediate formation of water droplets (sublimation, or direct deposition from vapour to solid). Thus, on a cold morning, the grass, vehicle surfaces etc., is covered with a feathery structure of loose ice crystals, or hoar frost, which reflect light from all their surfaces and so appear white.

Glazed Frost (or Freezing Rain)

Where rain has formed in relatively warm air at the cloud level and falls through a layer of very cold air near the ground, the droplets may become 'super-cooled'. When the droplet strikes the cold ground, aircraft in flight etc., it begins to freeze; however, the heat released on freezing warms the droplet up to around 0degC, and allows the water to remain liquid and spread out: only when more heat is removed from it (to the object upon which it has impacted), will the entire droplet freeze, forming a layer of clear ice. In this way, a rapid build of clear ice (or glaze) is achieved, with dramatic consequences for transport, power/communication lines, safety of the public etc. (It is clear ice because the delayed freezing of the droplet allows air to be expelled - if the freezing were to be sudden, the trapped air makes a frozen small droplet look cloudy - see Rime Ice.)


Cloud and fog droplets are very much smaller than rain droplets and when, on a very cold, foggy night super-cooled droplets impact upon objects which have been cooled below 0degC, they immediately begin to freeze in the same way as for Glaze. However, because of the small size, the heat released on freezing can be quickly conducted away either to the object or to the air, so that the whole droplet will freeze very rapidly and will not spread out as in the case of Freezing Rain. As more and more cold fog droplets strike the windward side of the object they will build up a mass of porous ice or rime, with many air spaces between the solidified droplets. It is because rime contains these air spaces that it appears white in contrast to glazed frost which is clear, solid ice with little or no trapped air.