A coating of ice crystals is called frost. Frost forms when the temperature is low enough for water vapour in the air to condense as ice instead of water droplets. Frost is nothing but small crystals of ice, which clings to any surface such as twigs, blades of grass or windows. The air around us always contains some water vapour. At the end of a warm day the air cools down. If the air contains moist, then some of the water vapour will condense as mist or dew. If the temperature falls below freezing point, ice crystals are formed instead of water droplets. Frost is likely to appear on a clear calm night. Because cold air tends to sink, frost may occur in hollows and valley bottoms and not on the hill slopes above. This how frost is formed on a surface.