Avalanches are most common on slopes between about twenty and forty degrees. Any gentler than this, and snow tends not to move. Any steeper, and the snow does not build up in the first place.

Avalanches are most common in spring, when there is sometimes a warm day that melts the surface of the ice (which later refreezes during the night) and cold days when snow falls.

The layers of ice and new snow are very unstable, like a pack of cards on a steep slope. The point is, you never know when the avalanche will begin, and it can be triggered by something as simple as a loud shout.