snow, the solid form ofwater thatcrystallizes in the atmosphere and, falling to theEarth, covers, permanently or temporarily, about 23 percent of the Earth’s surface.

A brief treatment of snow follows. For full treatment,seeclimate: Snow and sleet.

Snow falls atsea level poleward of latitude 35° N and 35° S, though on the west coast of continents it generally falls only at higher latitudes. Close to the equator, snowfall occurs exclusively inmountain regions—at elevations of about 4,900 metres (16,000 feet) or higher.

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Snow cover has a significant effect onclimate and on plant, animal, and human life. By increasing thereflection ofsolar radiation and interfering with theconduction of heat from the ground, it induces a cold climate. The lowheat conduction protects small plants from the effects of the lowestwinter temperatures; on the other hand, the late disappearance of snow in the spring delays the growth of plants. When snow melts in the spring, the resultingrunoff feeds rivers and supplies water for irrigation and other human enterprises. Heavy snowfalls can severely hinder transporation in variable moderate climates but provides a firm surface for travel in remote Arctic, Antarctic, and mountainous areas, using traditional dogsleds or snowshoes or modern snowmobiles.

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Snowflakes are formed by crystals ofice that generally have a hexagonal pattern, often beautifullyintricate. The size and shape of the crystals depend mainly on the temperature and the amount of water vapour available as they develop. At temperatures above about −40°C (−40 °F), ice crystals form around minute particles of dust or chemical substances that float in the air; at lower temperatures, crystals form directly from water vapour. If the air is humid, the crystals tend to grow rapidly, develop branches, and clump together to form snowflakes. In colder and drier air, the particles remain small and compact. Frozenprecipitation has been classified into seven forms of snow crystals and three types of particles—graupel (granular snow pellets, also called soft hail), sleet (partly frozen ice pellets), andhail (hard spheres of ice).

The texture and density of fallen snow undergo constant change. Snow on the ground tends to become increasingly dense, and, where it survives spring andsummermelting for years, it may turn into ice and form aglacier. On hillsides when temperature changes reduce thecoherence of snow particles in the snow cover, gravity and viscosity may overcome friction, causing snow slides and avalanches.