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When air is cooled the amount of water vapour that it can hold decreases. At the dew point temperature, air becomes saturated. A further fall in temperature will result in condensation of excess water vapour in the form of water droplets. If the dew point temperature of air close to the ground is below freezing, condensation will form not as dew but as hoarfrost.

Hoarfrost is made up of white crystals. Usually, air is too moist for hoarfrost to form directly. More usually dew forms first, which if ground temperature falls below 0C will freeze. Both frost and hoarfrost can be particularly damaging to outdoor crops and plants. Since cooling air will always drain downhill if possible, valleys and hollows suffer the greatest risk from frost. Coastal areas benefit from the moderating influence of the warmer sea. If the air is particularly dry, its dew point may be well below freezing. Whilst an air frost may occur, hoarfrost will be absent if the air temperature does not fall below the dew point.

Hoar frost