Snow is a type of precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. The process of this precipitation is called snowfall. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by external pressure. Snowflakes come in a variety of sizes and shapes. Types which fall in the form of a ball due to melting and refreezing, rather than a flake, are known as graupel, with sleet and snow grains as examples of graupel. A blizzard and snow storm indicate heavy snowfalls, while flurries are used for the lightest snowfall. Snow can fall as much as one meter at a time during a single storm in flat areas, and meters at a time in rugged topography, such as mountains. Lake-effect snow can lead to more localized high amounts downwind of unfrozen bodies of water. When snow falls in significant quantities, travel by foot and car becomes highly restricted, and mobility is decreased to the use of snowmobiles and skis. When heavy snow occurs early in the fall, significant tree damage occurs to trees still in leaf.

Once on the ground, snow can be categorized as powdery when fluffy, granular when it begins the cycle of melting and refreezing, and crud or eventually ice once it packs down into a dense drift after multiple melting and refreezing cycles. When powdering, snow drifts with the wind, sometimes to the depth of several meters. After attaching to hillsides, blown snow can evolve into a snow slab, which is an avalanche hazard on steep slopes. The existence of a snowpack keeps temperatures colder than they would be otherwise, as the whiteness of the snow reflects all sunlight, and the heat it absorbs goes into melting the snow rather than increasing its temperature. The water equivalent of snowfall is measured to monitor how much liquid is available to flood rivers during the upcoming spring. Snow cover can protect crops. If snowfall stays on the ground for a series of years uninterrupted, the snowpack develops into a glacier.

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