As the winter storm pummels the Sierra Nevada, here’s how avalanches can happen and how often they occur across California: An avalanche is an increased amount of snow sliding or tumbling down a slope. Avalanches can reach speeds of up to 100 mph and can vary in their power to kill and destroy, according to the American Avalanche Association.

The Colorado Avalanche Information Center is advising backcountry travelers to think about avalanche safety.

Backcountry ski touring means traveling on much more gradual slopes on trails that serve as easy hikes in the summer.