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Snow is precipitation in the form of flakes of crystalline water ice that falls from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft, white, and fluffy structure, unless subjected to external pressure. Snowflakes come in a variety of sizes and shapes. Types that fall in the form of a ball due to melting and refreezing, rather than a flake, are hail, ice pellets or snow grains. The process of precipitating snow is called snowfall. Snowfall tends to form within regions of upward movement of air around a type of low-pressure system known as an extratropical cyclone. Snow can fall poleward of these systems' associated warm fronts and within their comma head precipitation patterns (called such due to the comma-like shape of the cloud and precipitation pattern around the poleward and west sides of extratropical cyclones). Where relatively warm water bodies are present, for example because of water evaporation from lakes, lake-effect snowfall becomes a concern downwind of the warm lakes within the cold cyclonic flow around the backside of extratropical cyclones. Lake-effect snowfall can be heavy locally. Thundersnow is possible within a cyclone's comma head and within lake effect precipitation bands. In mountainous areas, heavy snow is possible where upslope flow is maximized within windward sides of the terrain at elevation, if the atmosphere is cold enough. Snowfall amount and its related liquid equivalent precipitation amount are measured using a variety of different rain gauges. == Forms == Once on the ground, snow can be categorized as powdery when light and fluffy, fresh when recent but heavier, granular when it begins the cycle of melting and refreezing, and eventually ice once it comes down, after multiple melting and refreezing cycles, into a dense mass called snow pack. When powdery, snow moves with the wind from the location where it originally landed, forming deposits called snowdrifts that may have a depth of several meters. After attaching itself to hillsides, blown snow can evolve into a snow slab—an avalanche hazard on steep slopes. The existence of a snowpack keeps temperatures lower than they would be otherwise, as the whiteness of the snow reflects most sunlight, and any absorbed heat 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 from meltwater that will occur during the following spring. Snow cover can protect crops from extreme cold. If snowfall stays on the ground for a series of years uninterrupted, the snowpack develops into a mass of ice called glacier. Fresh snow absorbs sound, lowering ambient noise over a landscape because the trapped air between snowflakes attenuates vibration. These acoustic qualities quickly minimize and reverse, once a layer of freezing rain falls on top of snow cover. Walking across snowfall produces a squeaking sound at low temperatures. The energy balance of the snowpack itself is dictated by several heat exchange processes. The snowpack absorbs solar shortwave radiation that is partially blocked by cloud cover and reflected by snow surface. A long-wave heat exchange takes place between the snowpack and its surrounding environment that includes overlying air mass, tree cover and clouds. Heat exchange takes place by convection between the snowpack and the overlaying air mass, and it is governed by the temperature gradient and wind speed. Moisture exchange between the snowpack and the overlying air mass is accompanied by latent heat transfer that is influenced by vapor pressure gradient and air wind. Rain on snow can add significant amounts of thermal energy to the snowpack. A generally insignificant heat exchange takes place by conduction between the snowpack and the ground. The small temperature change from before to after a snowfall is a result of the heat transfer between the snowpack and the air. As snow degrades, its surface can develop characteristic ablation textures such as suncups or penitentes. The term ''snow storm'' can describe a heavy snowfall, while a ''blizzard'' involves snow and wind, obscuring visibility. ''Snow shower'' is a term for an intermittent snowfall, while ''flurry'' is used for very light, brief snowfalls. Snow can fall more than a meter at a time during a single storm in flat areas, and meters at a time in rugged terrain, such as mountains. When snow falls in significant quantities, travel by foot, car, airplane and other means becomes severely restricted, but other methods of mobility become possible, such as the use of snowmobiles, snowshoes and skis. When heavy snow occurs early in the fall (or, on rarer occasions, late in the spring), significant damage can occur to trees still in leaf. Areas with significant snow each year can store the winter snow within an ice house, which can be used to cool structures during the following summer. A variation on snow has been observed on Venus, though composed of metallic compounds and occurring at a substantially higher temperature. File:BrockenSnowedTrees.jpg|Snow on trees, Germany. File:Duluth blizzard, December 2007.jpg|Snowdrifts in Duluth, Minnesota, March 2007 File:Falling snowflakes in England.JPG|Falling snowflakes in England File:Fresh snow.JPG|Fresh snow on a thin twig in Poland. File:FluffySnowfallByFence.webm|Fluffy Snowfall in Kelowna. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「snow」の詳細全文を読む スポンサード リンク
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