Snow is a type of precipitation that falls as ice crystals. The ice crystals form in clouds when water vapor cools to below zero degrees Celsius, if the atmosphere is sufficiently cold the ice crystals remain frozen as they fall to earth as snow. Snow is made up of individual snowflakes which are composed of ice crystals in various configurations. Snowing can occur in varying degrees of strength ranging from light to heavy snowfall, if sufficiently high winds (greater than or equal to 56 km/h) occur for a sustained period of time the snowfall is called a blizzard. Layers of snow build up on the ground over time, if this occurs on a slope then a large body of snow may become unstable and slide down the slope as a rapidly moving mass called an avalanche. Snow occurs on the earth in the temperate and polar regions and at high elevations such as on mountain tops where the ambient temperature is below freezing so that the falling ice crystals do not melt before they reach the ground.