Snow

From Free net encyclopedia

For other uses, see Snow (disambiguation).

Snow is precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes. Since it is composed of small rough particles it is a granular material. It has an open and therefore soft structure, unless packed by external pressure.

Snow is commonly formed when water vapor undergoes deposition high in the atmosphere at a temperature of less than 0°C, and then falls to the ground.

Occurrence

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See also: List of Countries receiving snowfall

The probability of snowfall varies with season, location, and other geographic factors such as latitude and elevation. In the latitudinal area closer to the equator, there is a rather small chance of snowfall, 35° N and 40°S are often quoted as a rough delimiter. The western coasts of the major continents remain devoid of snow to much higher latitudes.

Permanent snow covering is affected by factors such as the degree of slope on the land, amount of snowfall and the force and nature of the winds. As temperature decreases with altitude, high mountains, even near the Equator, have permanent snow cover on their upper portions, around 5300 m high. Examples include Mount Kilimanjaro in Tanzania and the Tropical Andes in South America; however, the only snow actually to appear on the Equator is at 4690 m altitude of the southern slope of Volcán Cayambe in Ecuador (Google Earth images). Conversely, many regions of the Arctic and Antarctic receive very little precipitation and therefore experience little snowfall despite the bitter cold (below a certain temperature, air essentially loses its ability to retain water vapor). Here, the snow does not melt at sea level. Image:Falling snow Graz 2005 original.jpg

Snow is used as a thermal insulator conserving the heat of the Earth and protecting crops from the freezing weather. While on the other hand, substantial snowfall can, at times, even disrupt the infrastructure and services of a region that is accustomed to such weather. Automotive traffic may be greatly inhibited or may be stifled entirely. Basic infrastructures such as electricity, telephone lines, and gas supply can also be shut down. This can lead to a "snow day", which is a day on which the school or other services are cancelled due to unusually heavy snowfall. In areas that normally have very little snow, this may occur even with light accumulation, something often ridiculed by those people accustomed to colder climates, where streets would remain passable given the same amount of snow.

The highest seasonally cumulative precipitation of snow ever measured was on Mount Baker, Washington, U.S.A during the 1998–1999 season. Mount Baker received a staggering 29 m of snow, thus surpassing the previous record holder, Mount Rainier, Washington, U.S.A which during the 1971–1972 season received 28.5 m of snow.

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In the United States, states which are usually covered with snow in a typical winter include Maine, New Hampshire, Vermont, Massachusetts (except for Cape Cod), Rhode Island, Connecticut, New York (except for Long Island), Pennsylvania, New Jersey, Michigan, northern Ohio, northern Indiana, northern Illinois, Wisconsin, Minnesota, Iowa, North Dakota, South Dakota, Montana, Wyoming, western Colorado and the Front Range, Idaho, northern and eastern Utah, northern Nevada, and high-altitude areas of Washington, Oregon, California, Arizona, New Mexico, Maryland, West Virginia, and Virginia, although the dry area just east of the Rockies in eastern Montana and Wyoming and the western Dakotas (the northern High Plains) is quite often snowless. Neighboring states and high-altitude areas in other states also are quite often covered with snow, but in states farther south or at lower elevation, it may snow for a day but usually melts within a week. Canada is usually completely covered, except for the more temperate area around Vancouver, British Columbia, and occasionally southern Alberta or far southern Ontario. Alaska is likewise covered except for some coastal areas and islands, while snow in Hawaii is only received on the highest peaks (above about 2700 m).

Geometry

Image:SnowflakesWilsonBentley.jpg Image:Tsuzumi snow crystal.jpg A snowflake always has six lines of symmetry, which arises from the hexagonal crystal structure of ordinary ice (known as ice I_h) along its 'basal' plane.

There are, broadly, two possible explanations for the symmetry of snowflakes. Firstly, there could be communication or information transfer between the arms, such that growth in each arm affects the growth in each other arm. Surface tension or phonons are among the ways that such communication could occur. The other explanation, which appears to be the prevalent view, is that the arms of a snowflake grow independently in an environment that is believed to be rapidly varying in temperature, humidity and other atmospherical conditions. This environment is believed to be relatively spatially homogeneous on the scale of a single flake, leading to the arms growing to a high level of visual similarity by responding in identical ways to identical conditions, much in the same way that unrelated trees respond to environmental changes by growing near-identical sets of tree rings. The difference in the environment in scales larger than a snowflake leads to the observed lack of correlation between the shapes of different snowflakes.

However, the concept that no two snowflakes are alike is not necessarily true. Strictly speaking, it is extremely unlikely for any two objects in the universe to contain an identical molecular structure; but, there are, nonetheless, no known scientific laws which prevent it. In a more pragmatic sense, it's more likely, albeit not much more, that a pair of snowflakes are visually identical if their environments were similar enough, either because they grew very near one another, or simply by chance. The American Meteorological Society has reported that matching snow crystals were discovered by Nancy Knight of the National Center for Atmospheric Research. The crystals were not flakes in the usual sense but rather hollow hexagonal prisms.

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