Winter Severity Index

The Winter Severity Index (WSI) is an algorithm used by the Wisconsin Department of Natural Resources to predict the impact of winter weather on deer survival. Wildlife biologists calculate the WSI between December 1 and April 30 by adding the number of days with 18 inches or more snow depth to the number of days when the minimum temperatures were below 0°F.

Snow depths greater than 18 inches reach above the chest of an adult deer and make walking more difficult. Temperatures below zero require substantially more calories to maintain body temperature. The most severe winter mortality occurs in winters with at least nine weeks of 18 or more inches of snow on the ground. The fawn-to-doe ratios can be 20% lower after severe winters than after mild ones.