Avalanche Risk Calculator

Slopes between 30° and 45° are the classic zone for slab avalanches — steep enough for gravity to overcome the bond between snow layers, but not so steep that snow simply can't accumulate into a cohesive slab. That range adds the largest contribution to the risk score (+1.5), while slopes over 55° add only +0.5 because very steep terrain tends to shed loose snow continuously through small sloughs rather than build up and release as one large slab.

The calculator adds +1.0 to the risk score for a warming trend and subtracts -0.5 for a cooling trend, because rising temperatures weaken the bonds between snow layers and can trigger wet-snow avalanches, while cooling temperatures tend to stabilize and strengthen the existing snowpack structure. A stable temperature trend contributes nothing either way.

Danger rating (1-5, Low to Extreme) uses the same category names as the official North American Public Avalanche Danger Scale published by the American Avalanche Association and USDA Forest Service National Avalanche Center, and reflects how likely an avalanche is to be triggered, built from the summed risk score across stability, slope, snowfall, wind, and temperature. Destructive size (D1-D5) is calculated separately and estimates how large an avalanche would be if one occurred, driven mainly by heavy snowfall, heavy wind loading, and the combination of steep slope with a large recent snowfall. A slope can have a high trigger likelihood but a moderate destructive size, or vice versa.

No — it's a fixed lookup tied directly to the danger rating (5% at Low, 20% at Moderate, 50% at Considerable, 75% at High, 95% at Extreme), meant to communicate relative risk in a more intuitive way than the 1-5 scale alone. It is not derived from historical avalanche-triggering data for your specific slope, so treat it as an illustrative figure alongside the danger rating rather than a measured probability.