An avalanche is a mixture of snow, ice, soil, rock, and boulder that moves downslope at terrifying speed, annihilating everything in its path. It can generate a strong accompanying wind that tears trees from their roots. On a steep slope, the underlying material supporting the massive snow cover can give way because it has been soaked by spring rains or destabilized by alternating periods of precipitation and warm, dry winds (föhn or foehn) flowing down the leeward side of a slope. Sometimes a slope is so unstable that thunder or even a loud shout is sufficient to trigger sliding of the overloaded snow (which is why skiers are asked to observe complete silence when crossing potentially unstable areas). An avalanche can travel at speeds ranging from 40 to 300 km/h (25 to 190 mph), depending on the angle of the slope, the density of the snow mixture, and the length of its path.