The steepness or incline of a piece of terrain

Slope angle is a key factor for dictating whether or not the terrain can produce an avalanche. Most slab avalanches initiate on slopes where the steepest slope angle is between 30° and 50°  (about the steepness of a black or double black diamond run at a ski hill) and subsequently run into lower angled terrain. Avalanches initiating on slopes less than 30 degrees are rare because the incline is so flat. Slopes steeper than 50 degrees are so steep that small loose avalanches run frequently so they only rarely develop the structure for larger slab avalanches. 

These slope angles were measured on site after the avalanche. You can see that the avalanche released on slopes steeper than 30° and overran lower slope angles below. Credit: Utah Avalanche Center

Data from a Swiss dataset of over 1,000 avalanches shows 96% of avalanches released on slope angles between 30 and 50 degrees, and most released on slopes between 34 and 45 degrees. Credit: Utah Avalanche Center