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Avalanches : 

Destructive power of snow

While our motherland is gradually recovering from the damage caused by the recent tsunami, our neighbouring country Pakistan is experiencing devastation caused by snow storms.

There is no doubt that we all love snow, not only kids but even adults too love snow. But can these beautiful white flakes cause massive devastation, killing people and damaging valuable properties? We all know what snow is; crystals which have formed around tiny bits of dirt that have been carried up into the atmosphere by the wind.

A single snowflake is a unique, beautiful crystal. But when millions of flakes are compressed together on a steep mountainside and gravity has its way, snow can become a destructive force, in the form of an avalanche.

An avalanche or snowslide is the rapid movement of snow down a mountainside. Most avalanches occur on slopes of 25 to 50 degrees - the same slopes favoured by many skiers, snowboarders and snowmobilers.

A fall of pellet-like snow (graupel) is more prone to lead to an 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.

The most dangerous type is called a dry slab avalanche. When a weak layer in the snow cover can no longer support new layers of snow above, the crystalline structure collapses, sending the top layers down the slope. Hard slab avalanches involve large blocks of snow and debris sliding down a slope. In soft slab avalanches, the snow breaks up in smaller blocks as it falls.

On average, dry slab avalanches travel from about 100 to 130 kilometres per hour - too fast to outrun. The risks of an avalanche increase during major snowstorms and periods of melting. According to experts, wind can gather up snow and deposit it 10 times faster than snow falling from storms. This makes wind the most common weather-related cause of avalanches.

Many factors influence the production of an avalanche, including slope angle, snow factors such as the size of grains, slab thickness and density, terrain, such as the presence of trees, wind speed and direction and the sun aspect.

Although avalanches can occur on any slope, given the right conditions, in countries like the United States, certain times of the year and certain locations are naturally more dangerous than others. Winter, particularly from December to April, is when most avalanches will "run" (slide down a slope). However, avalanche fatalities have been recorded every month of the year.

The highest number of fatalities occur in January, February and March, when the snowfall is highest in most mountain areas. A significant number of deaths occur in May and June, demonstrating the hidden danger behind spring snows and the melting season that catches many recreationists off-guard. During the summer months, it is often climbers who are caught in avalanches.

All that is necessary for an avalanche is a mass of snow and a slope for it to slide down.

While the temperature is cold, the snow sticks to the surface and doesn't slide off. When temperatures go up a little however, the snow will 'sluff' or slide down, often in small slabs. This is an avalanche on a miniature scale.

Of course, mountain avalanches are much larger and the conditions that cause them are more complex. A large avalanche in North America might release 300,000 cubic yards of snow. That's the equivalent of 20 football fields filled 10 feet deep with snow. However, such large avalanches are often naturally released. Skiers and recreationists are usually caught in smaller, but often more deadly, avalanches.

The three parts of an avalanche path are, starting zone, track, and runout zone. The starting zone is the most volatile area of a slope, where unstable snow can fracture from the surrounding snowcover and begin to slide. Typical starting zones are higher up on slopes, including the areas beneath cornices and "bowls" on mountainsides. However, given the right conditions, snow can fracture at any point on the slope.

The avalanche track is the path or channel that an avalanche follows as it goes downhill. When crossing terrain, those in avalanche-prone areas should be aware of any slopes that look like avalanche 'chutes'. Large vertical swaths of trees missing from a slope or chute-like clearing are often signs that large avalanches run frequently there, creating their own tracks.

There may also be a large pile-up of snow and debris at the bottom of the slope, indicating that avalanches have run.

The runout zone is where the snow and debris finally come to a stop. Similarly, this is also the location of the deposition zone, where the snow and debris piles are at their highest. Although underlying terrain variations, such as gullies or small boulders, can create conditions that will bury a person further up the slope during an avalanche, the deposition zone is where a victim will most likely be buried.

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