Snow avalanches

Snow avalanches are masses of snow moving rapidly down a mountain slope of cliff. Snow slides range from small movements on established avalanche tracks to large, sporadic, very rapid movements capable of taking a heavy toll of life. Avalanches are infrequent on slopes of less than 25 degrees and especially numerous on those exceeding 35 degrees, and most commonly start on convex slopes. Both old and new snow may avalanche, but serious avalanches are always possible when 12 inches or more of new snow is present. Movements may be set off by temperature, vibration, shearing, or other slope disturbance. Dry snow avalanches usually occur during, or within several days after, snowfall. They may affect whole slopes, even if wooded, and may exceed 100 mph. Wet snow avalanches are formed during thaws or rainy weather. Their movement is less rapid but may be destructive. Slab avalanches of wind-packed snow are broader and deeper and move rapidly. This type of avalanche is an extreme hazard to life and property.