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Hoar frost crystals. Hoar frost forms in a similar way to dew, in that water is deposited from air that is supersaturated with water vapour. This can occur when moist air cools rapidly. The difference is that the temperature of the deposition surface is below the freezing point of water, and ice crystals form. Hoar frost forms in still, clear and cold weather. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR JEREMY BURGESS)