Avalanche, sudden flow of a large mass of snow or ice down a slope or cliff, sometimes at speeds exceeding 160 km/hr (100 mph). Such flows can be destructive of life and property. Avalanches are most common on slopes exceeding 30°, frequently when a deep snow falls suddenly and does not have a chance to cohere, or when a thaw undercuts a blanket of older snow. Pellet-like snow (graupnel) is also more prone to avalanche than a fall of ordinary snowflakes. Flows of wind-packed slabs of snow can be especially hazardous.
Avalanches are set off by a combination of factors, including temperature, shearing of creeping snow masses, and sudden vibrations, including loud noises. Snow patrols in mountain areas reduce the hazard by detonating strategically placed explosives that cause smaller, less destructive flows. A landslide is a similar massive movement of rock and soil.
Snow Crystal
Snow is actually transparent, although reflection from the many sides of its crystals makes it appear to be white. A closeup of a snow crystal reveals its beautiful symmetry and hexagonal design. The crystals form when supercooled water vapour condenses or ice particles collect around a piece of dust. Partially melted crystals stick to each other to form snowflakes. All snow crystals have six sides, but each individual snowflake has a unique pattern.
Snow, transparent ice crystals formed around dust or other small particulates in the atmosphere when water vapour condenses at temperatures below freezing point. 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.
Structurally, elemental crystals of snow occur in any of various hexagonal forms, depending upon exact atmospheric temperatures during formation. Among these 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, and it is the large number of reflecting surfaces of the crystal that make snow appear white. 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.
Snowfall measurement is usually stated as depth in centimetres, 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.
Antarctic Iceberg
Icebergs are free-floating chunks of glaciers, particularly common in the polar regions. These often spectacular ice formations pose a twofold problem for navigators: 90 per cent of their bulk is hidden below the surface, and it is impossible to map their position because they are constantly moving.
Ice, water in the solid state. (Frozen forms of other substances, such as carbon dioxide, are also known as ices.) Ice is colourless and transparent; it crystallizes in the hexagonal crystal system. Its melting point is 0° C (32° F); pure water also freezes at 0° C, but ice will only form at 0° C if the water is disturbed or contaminated with dust or other objects.
Properties
One important property of ice is that it expands upon freezing. At 0° C it has relative density 0.9168 as compared to relative density 0.9998 of water at the same temperature. As a result, ice floats in water. Because water expands when it freezes, an increase of pressure tends to change ice into water and therefore lowers the melting point of ice. This effect is not very marked for ordinary increases of pressure. For instance, at a pressure 100 times the normal atmospheric pressure, the melting point of ice is only about 1° C (1.8° F) less than at normal pressure. At higher pressures, however, several allotropic modifications, or allotropes (different forms of the same element that exist in the same physical state) of ice are formed. These are designated Ice II, Ice III, Ice V, Ice VI, and Ice VII. Ordinary ice is Ice I. These allotropes are denser than water and their melting points rise with increased pressure. At about 6,000 atmospheres the melting point is again 0° C and at a pressure of 20,000 atmospheres the melting point rises above 80° C (176° F).
The expansion of water when it freezes has important geological effects. Water that enters minute cracks in rocks on the surface of the Earth creates an enormous amount of pressure when it freezes, and splits or breaks the rocks. This action of ice plays a great part in erosion.
These properties of freezing water explain the way in which open bodies of water freeze. When the temperature of the surface of an open body of water is reduced towards the freezing point, the surface water becomes denser as it cools, and therefore sinks. It is replaced at the surface by warmer water from beneath. Eventually the entire body of water reaches a uniform temperature of 4.0° C (39.2° F), the point at which water has its maximum density. If the water is cooled further, its density decreases and finally ice is formed on the surface. Bodies of water freeze from the top down rather than from the bottom up because of these density differences.