What is an avalanche?

An avalanche is a huge mass of ice and snow that breaks away from the side of a mountain and slides downward at great speed. Most avalanches result from weather conditions, such as heavy winds and earth tremors, that cause snow on a mountain slope to become unstable.

A large avalanche in North America might release 300,000 cubic yards of snow—the equivalent of 20 football fields filled 10 feet (3.3 meters) deep with snow. Wintertime, particularly from December to April, is when most avalanches occur.