National Geographic Avalanches 101
Speed of snow sliding down a slope reaches around 90 Km/hr during a rapid Avalanche.
An avalanche happens when a layer of snow becomes loose and slides downhill. Thousands of avalanches occur every winter in the mountains and are usually triggered by people. Avalanches are caused by these four factors: a steep slope, a snow cover, a weak layer in the snow cover and a trigger such as a snowboarders or skiers.
Look for this button (above) on the READ page of the 'Find Information' tab, then search for these eBooks: