Snow

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 (32°F), and then falls to the ground.

Occurrence

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 lesser 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.

Snow is used as a thermal insulator conserving the heat of the Earth and protecting crops from the freezing weather. As temperature decreases with altitude, high mountains, even near the Equator, have permanent snow cover on their upper portions, around 16,000 feet (5,300 meters) 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 4,690 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 snow 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.

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 28.96 meters (1,140 in) of snow, thus surpassing the previous record holder, Mount Rainier, Washington, U.S.A which during the 1971– 1972 season received 28.5 meters (1,122 in) of snow.

One particular place that snow should be noted is Lake Tahoe. Tahoe gets more snow on average anually than anywhere in the United States and 99 percent of the world. In fact, on a regular basis Tahoe gets storms and blizzards which would make world news in other places. It is not uncommon for it to snow up to a week straight in Tahoe. From a single storm, dozens of feet can acumulate. This is why Tahoe is known for its skiing.

In the United States, states and parts of states which are usually covered with snow in a typical winter include the New England states, New York, Pennsylvania, New Jersey, West Virginia, Michigan, Wisconsin, Minnesota, the Dakotas, Montana, Wyoming, western Colorado, Idaho, and high-altitude areas of Washington, Oregon, and Utah, although the dry area just east of the Rockies in eastern Montana and Wyoming and the western Dakotas 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. Alaska is likewise covered except for some coastal areas and islands.

Geometry

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.

Electron Microscope Gallery

Recreation

Forms of recreation dependent on snow:

• Many winter sports, such as skiing, snowmobiling, snowshoeing and snowboarding
• Playing with a sled or riding in a sleigh
• Building a snowman (or 'snowwoman') or snow fort
• Throwing snowballs mutually in a snowball fight or at others to tease them. ( Humans seem to be the only being that throw their snowballs. Pygmy chimpanzees have been known to carry snowballs around, but never to throw them.)
• Making a snow angel

Where snow is scarce but the temperature is low enough, snow cannons may be used to produce an adequate amount for such sports.

The world´s biggest snowcastle is built in Kemi, Finland, every winter.

Density

The water equivalent of a snow pack is the amount of water that it contains, regardless of its depth. For example, if the snow covering a given area has a water equivalent of 20 inches, then it will melt into a pool of water 20 inches deep covering the same area. This is a much more useful measurement to hydrologists than snow depth, as the density of even freshly fallen snow widely varies. New snow often has a density of around 12% of water, and even under cold conditions, the same snow will settle under its own weight until it is approximately 33% water. More snow on top of this will compress it even further. By late spring, snow densities often exceed 50% of water [1].

Water equivalent is of great interest to water managers wishing to predict spring runoff and the water supply of cities downstream. Measurements are made manually at marked locations known as snow courses, and remotely using special scales called snow pillows.