Snow

I

Introduction

Snow, ice crystals with six sides that form around dust or other small particulates in the atmosphere when water vapor condenses at temperatures below the freezing point. Snow forms in clouds and falls as precipitation (see Weather). Partly melted crystals usually cling together to form snowflakes, which may in rare cases grow in size up to 7 to 10 cm (3 to 4 in) in diameter.

Snow typically occurs during cold seasons or at high altitudes. It plays an important role in storing fresh water in snowpack and glaciers (see Water Cycle). In parts of the world that experience very cold winters, snow is common. Heavy snowfall may interfere with transportation and other human activities. Severe snowstorms with high winds are called blizzards.

II

Structure of Snow

Other forms of precipitation formed from ice, including hail, graupel, sleet, and freezing rain, lack the special crystal structure of snow. Elemental crystals of snow occur in various hexagonal (six-sided) forms, depending upon exact atmospheric temperatures during formation. Among the possible six-sided, basically symmetrical shapes are needle, columnar or stud, platelike, and star-shaped crystalline types. Because of the infinite variability of weather conditions, every snow crystal is unique in its precise configuration. The longer rays that constitute the arms of the six-rayed stars are generally hollow tubes; they are evidently built up by additions to the edge of an original crystal.

Although the ice crystals in snow are transparent, the large number of reflecting surfaces of the crystal make snow appear white. Many animals that are active during the winter or live in Arctic regions develop a white coat or plumage for camouflage against a background of snow. White snow cover also reflects sunlight back into space and has a cooling effect on the climate. Loss of snow cover could contribute to global warming.

---

---

III

Snowfall and Snow Accumulation

Snowfall measurement is usually stated as depth in centimeters (or other unit) of newly fallen snow; it is also measured in terms of the depth of the layer of water that would result if the snow were melted in place; 25-30 cm (10-12 in) of snow melts to 2.5 cm (1 in) of water.

The accumulation of snow in the mountains during the winter season feeds streams and rivers as the snow gradually melts in warm seasons. This mountain snowpack is crucial to the water supply for people in many parts of the world. However, heavy layers of snow on sloped mountain surfaces can also lead to destructive avalanches.

When yearly snowfalls accumulate over long periods of time, they can compact and form bodies of moving ice called glaciers. In Greenland and Antarctica, accumulated snow has formed into gigantic ice sheets that cover thousands of square kilometers. Together these ice sheets hold about 80 percent of all the fresh water on Earth. During the Ice Ages, vast ice sheets created from snow also covered the northern parts of North America, Europe, and Asia. When glaciers and ice sheets reach the edge of the ocean, they can break off and form icebergs.

When snows falls and accumulates in layers, it traps air. The trapped air gives snow insulating properties. When snow covers the ground, it can protect the soil beneath from extreme low temperatures. When snow melts, the water can penetrate the soil without quickly evaporating. The insulating properties of snow were known to the Inuit people of the Arctic, who learned to build shelters called igloos from blocks of snow. Stranded climbers in the mountains also build snow caves for protection. Scientists can also study the air trapped by snow in ice cores dug into glaciers and ice sheets. The trapped air preserves the gases, dust, and pollutants in Earth’s atmosphere in regular layers that can date back thousands of years.

IV

People and Snow

Humans in cold regions have learned to live with snow and take advantage of its special properties. The Inuit used snow to build shelters (igloos) and developed dog sleds for transportation. Native Americans in North America devised snowshoes to allow them to walk on snow without sinking. Northern Europeans invented skis thousands of years ago for rapid movement over snow and constructed animal-drawn sleds and sleighs. Modern travel over snow includes motorized snowmobiles and special tractors with wide treads. Airplanes equipped with skis can land and take off on snow-covered areas.

Snow sports are an important form of recreation in the winter and in alpine regions. Skiing, snowboarding, bobsledding, and tobogganing exploit the smooth surface of fallen snow for fast movement.

Natural disasters caused by snow include severe blizzards, which have killed both humans and livestock. Avalanches have also killed many people, including hikers, skiers, and soldiers, and destroyed buildings and other structures, sometimes burying the people inside. Heavy snowfalls can cause the collapse of buildings and cripple transportation. Windblown snow at ground level may create dangerous whiteout conditions that reduce visibility to near zero. Ultraviolet radiation in sunlight reflected off snow can cause a medical condition called snow blindness.

The importance of snow for commercial recreation and other uses has led to the development of artificial snowmaking machines for ski resorts. Using pressurized air, a mixture of tiny water droplets and ice crystals is projected into open air that is below freezing. Snow crystals form and fall to the ground. Cloud seeding from airplanes has also been used to build up the snowpack in mountains.

	More from Encarta
	•  Important Notice: MSN Encarta to be discontinued. Learn more. •  Presidential Myths Quiz •  Coffee break: Recharge your brain