Numerous indices have been developed that give a measure of how comfortable a person feels based on the interaction of several weather variables. However, virtually all of these indices are based on absolute conditions and do not consider the importance of relative stress and adaptation based on location and time of season. A summer relative comfort index, known as the Heat Stress Index, has been developed, which improves upon the limitations of the current widely-used indices.

The Heat Stress Index evaluates daily relative stress values for 229 first-order weather stations in the United States and Canada based on deviations from the norm using 30 years of data (1971-2000). The index is based on an apparent temperature algorithm recently developed by Robert Steadman and other derived meteorological variables including cloud cover, cooling degree days, and the number of consecutive days of extreme heat. Statistical distributions are fit to 10-day variable frequencies so that daily values for each variable can be determined based on their location under the distribution curves; these are represented as a percentile. The daily percentile values for each variable are then summed, and a statistical distribution is fit to the summed frequencies. The daily Heat Stress Index value is the percentile associated with the location of the daily summed value under the summation curve. Therefore, a 97 percent Heat Stress Index value indicates that only three percent of days on that date are expected to experience more stressful conditions than the day under review.

The Heat Stress Index can be implemented within various health initiatives such as heat/health watch-warning systems, where it is necessary to determine threshold conditions that lead to declines in human health. However, the index is not limited to human application, because it has been uniquely created to measure climate abnormalities that have universal biological implications. The Heat Stress Index can also be easily calculated up to 48 hours in advance using model forecast information. This will allow advisories, watches, and warnings to be issued when necessary which gives the public sufficient time to prepare for a potentially stressful situation.