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 ssnowflakes. 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 is actually transparent, although reflection from the many sides of its crystals makes it appear to bbe 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 fform 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 iit 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 wwere melted in place; 25-30 cm (10-12 in) of snow melts to 2.5 cm (1 in) of water.
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.
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 mmarked 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, tthe 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.
In rivers, however, ice is sometimes formed beneath the surface. On cold winter nights the surface of a swiftly flowing stream may become cooled well below 0° C because of its contact with the air. Such “undercooled” water, mixing with the warmer layers beneath, produces a spongy mass of ice crystals known as frazil, which floats downstream. Sometimes masses of frazil lodging under surface ice in quieter water may dam a stream and cause floods. Another form of below-surface ice is anchor ice, which is formed around rocks on streambeds. During cold nights enough heat may be radiated from the rocks so that they become cool enough to freeze the water flowing around them. When the rocks are warmed
by the sun in the daytime, masses of the anchor ice may detach and rise to the surface of the stream. See Snow.
Whenever glaciers or ice sheets reach the sea, the movement of the ice eventually pushes the end of the sheet into water which is deeper than the thickness of the glacier ice. Portions of the end of the glacier break off and form floating masses known as icebergs or bergs. Icebergs are often of enormous size and mmay reach a height of 90 to 150 m (about 300 to 500 ft) above the surface of the sea. Yet about 90 per cent of the mass of an iceberg is beneath the surface. Icebergs are common in both the Arctic and Antarctic regions and are often carried into lower latitudes by sea currents, particularly in the North Atlantic Ocean. North Atlantic icebergs all come from the Great Greenland ice sheet and have been observed as far as 3,200 kkm (about 2,000 mi) from their origin. After the Titanic disaster, 16 nations instituted an iceberg patrol of the North Atlantic. Now known as the International Ice Patrol, it tracks icebergs and reports their location to ships.
In Alaska mmany alpine, or valley, glaciers flow down mountainous valleys to the sea. Here, for example, the Hubbard Glacier enters Glacier Bay near Yakutat, Alaska. Icebergs form when pieces break off the snout of the glacier and float around in the sea. This is called calving and is the primary process by which glacial ice is recycled into liquid form.
Glacier, large, usually moving mass of ice formed in high mountains or in high latitudes where the rate of snowfall is greater than the melting rate of snow. Glaciers can be divided into four well-defined types—alpine, piedmont, ice cap, and continental—according to the topography and climate of the region in which the glacier was formed.
The snow that falls on tthe walls and floors of valleys in high mountain regions tends to accumulate to a great depth, because the rate of melting, particularly in wintertime, is far lower than the rate at which the snow falls. As a result, the ...