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An avalanche is a large mass of snow that moves quickly down a mountain. It breaks loose from the area around it and collects more and more snow as it pushes downward. Avalanches occur in mountainous regions, for example the Alps, Himalayas or the Rocky Mountains. They can reach speeds of over 100 km an hour and destroy forests and villages that are in their way. They can block roads and train tracks and make areas unreachable from the outside. Avalanches kill over a hundred people in Europe and North America every year, burying them below tons of snow. Although the danger of avalanches is high throughout the winter months there are weather and snow conditions in which avalanches occur more often. The composition of snow is an important factor. Once the snow is on the ground temperature changes its structure so that different layers of snow can form on the slope. Avalanches occur on slopes that have an angle of 30 to 60 degrees. On slopes that are not so steep there is not enough gravity to let the snow break off from its surroundings. On steeper slopes the snow usually breaks off more quickly. Avalanches can be triggered by many factors. In some cases an increasing amount of new snow can start an avalanche. Strong winds can transport snow to the leeward side of the mountain. There, the snow stays soft and does not bind with other layers. In other cases, skiers, snowboarders or other alpine tourists can trigger an avalanche. As snow falls it builds up layer upon layer. After a certain time the layers harden and bond with one another. If they don't the snow profile becomes unstable and soft layers can slide on top of harder ones. The starting point is where snow breaks off and begins to slide. It is mostly in higher regions of a slope. The track of an avalanche is the path that it follows downhill, which can be hundreds of metres long. As the avalanche moves towards the valley it gathers more and more snow. The runout zone is where the avalanche slows down and finally comes to a stop. Powder snow avalanches occur when more and more powder snow accumulates and starts moving like a cloud in the sky at speeds of up to 400 km an hour. Wet snow avalanches are triggered when snow starts becoming warmer and wetter. These avalanches spread sideways as they move downhill. Snow hardens so that buried victims hardly have a chance of breathing. Slab avalanches are the most common. They occur when layers of snow don't cling together so that skiers or snowboarders trigger the top layer which slides down on the other harder ones. In areas where avalanches are likely to occur, people have taken action to protect themselves and their surroundings. Avalanches can be set off on purpose through explosions. In some places rakes are set up on slopes to slow down and split up the moving snow. Trees, fences, rocks or concrete barriers can also to slow down moving avalanches. Most alpine countries have avalanche warning systems that tell tourists when and where it is dangerous to move around in the mountains. In most countries the danger of avalanches is displayed on a risk scale ranging from one to five. One is a low risk situation with stable snow conditions while five displays a very high risk of avalanches with unstable snow conditions.