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ice formed by the condensation of atmospheric water vapor on a surface when the temperature of the surface is below 32°F; (0°C;). In the formation of frost, a gas (water vapor) is changed directly to a solid (see dew).
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ice crystals that form on surfaces on cold, clear, still nights. The particles of hoarfrost are similar in shape to snowflakes but are less regular. Like dew, hoarfrost forms because of the cooling of the surface of the earth as a result of thermal radiation, which causes a drop in the temperature of the adjacent layers of air and the sublimation of the water vapor on the surface, which is cooled below 0°C.