What conditions cause avalanches?

Dear Tom,
Avalanches have occurred in recent weeks in the western states and in Europe. What conditions cause avalanches?
Jeremy Atchlorr, Peoria, Illinois
Dear Jeremy,
Avalanches occur tens of thousands of times annually across the world. A majority of them are unobserved and most occur on terrains sloping from 30 to 45 degrees. They are characterized as “loose snow” or “slab” avalanches.
Slab avalanches are the most frequent and deadly, and sometimes are a mile or more across. Forward speeds can reach 120 mph, and about 85% occur during or after snowstorms during which temperatures rise, snow is falling heavily, or has recently fallen, and it is windy (15 mph or greater). Snow falling off trees or overhanging cliffs, even skiers, can initiate avalanches.

Notice: you are using an outdated browser. Microsoft does not recommend using IE as your default browser. Some features on this website, like video and images, might not work properly. For the best experience, please upgrade your browser.