One of the most important factors in identifying avalanche terrain is slope angle. Most slab avalanches are triggered on slopes between 30° and 45°, with 38° being the the most common angle. This range of angles is similar to an advanced Black Diamond run on a ski field.
Slab avalanches are the main killers so pay special attention to these angles.
Areas where the slope angle is less than 25° are generally safe, unless there is a threat from a large steep slope above. When slopes get steeper than 55°, they struggle to accumulate enough snow to form a slab. They are more likely to empty themselves regularly with small sloughing avalanches.
Judging slope angles is tricky and often your perspective is skewed depending if you are looking up from below, or down from the summit of a slope. It can also be misleading depending if you are up close or faraway from the slope you are estimating. The simple solution is to not guess, but measure the slope angle.
Here at the NZ Avalanche Centre we have developed a durable plastic card which is a great tool for anyone heading into the backcountry.
The Avalanche Assessor contains a simple checklist to help you make evidence-based decisions in avalanche terrain, including a mechanism to help measure slope angles.
Click image below for PDF - Assessor Card User Guide
Here is Utah Avalanche Specialist Bruce Tremper giving some great tips on measuring slope angles