The Heat Stress Index (HSI) is a new, comprehensive summer index that evaluates daily relative stress for locations in United States based on deviations from the norm. The index is based on daily maximum and minimum apparent temperature, cloud cover, cooling degree hours, and consecutive days of extreme heat. The relative nature of the index suggests that the same meteorological conditions in Boston and Atlanta will yield different HSI values, based on the historical frequency of these conditions. The same conditions at the same place would also yield different HSI values if they occurred in early June and mid July. The main premise of the index is that people respond to weather in a relative, rather than absolute, manner.

The HSI is derived by developing daily statistical distributions for each variable at each locale. Percentile values for each variable are developed daily, based on variation from mean and the standard deviation, and are summed for all the variables, yielding a new distribution of the summed percentile values. The daily HSI value is the percentile associated with the location of the daily summed value under the summation curve. It varies from 0 to 10; a daily value of 9.7 would indicate that only 3 percent of days would be more stressful than this one at this location during this time of the year.

There is interest in using the HSI to aid as guidance in the issuance of excessive heat warnings, as a public index that would be more revealing than the present heat index, and as a research tool for a variety of environmental issues.