An avalanche (or snow slide) occurs when the force of gravity trying to pull snow downhill exceeds the strength of the snow cover (from bonds between snow grains). The four ingredients of an avalanche are a steep slope, snow cover, a weak layer in the snow cover and a 'trigger.'

Avalanche danger increases with major snowstorms and periods of thaw. The large majority of avalanches (in the range of 80%) fall during or just after large snowstorms. The most avalanche-prone months in the Northern hemisphere are, in order, February, March, and January. Avalanches caused by thaw occur most often in April.

About 90% of all avalanches start on slopes of 30-45 degrees; and about 98% of all avalanches occur on slopes of 25-50  degrees.