The Heat Stress Index (HSI) is a relative index designed to evaluate daily meteorological stress on humans and other organisms for locations throughout the United States, based on deviations from normal conditions for a particular time of year. The HSI is designed to be operational from mid-May through late-Septmeber. The index can vary from 0 to 10, with 10 representing the most stressful conditions, 0 the coldest conditions, and 5 normal conditions for that time of year. A value of 9.9 for a given day indicates that 99 percent of days at the particular location for the particular time of year would be expected to have less meteorologically stressful conditions. The HSI thus adjusts from place to place; a sunny day with an apparent temperature of 95F with clear skies in June would be significantly more stressful in Philadelphia (HSI above 9) than in Phoenix (HSI below 5), where it is much more common. Similarly, the same type of weather becomes less stressful as summer progresses, with the HSI in Philadelphia under similar conditions in mid-July around 8.

The index is based on a combination of factors, including apparent temperature, cloud cover, cooling degree hours, and the number of consecutive days of extreme heat. Each of these has a significant impact upon human health and well-being. For example, minimal cloud cover increases the heat load on structures and the human body. Cooling degree hours measure the sum of hourly apparent temperature values above 65F over the 24 hour day. The consecutive day variable is necessary as the capacity of humans to deal with excessive heat decreases as the heat wave continues.

The HSI is the only existing index that considers many of these factors, and along with its relative nature, the index is a unique and accurate way to determine the meteorological stress impacts upon humans and other organisms.

The daily HSI value ranges from 0 to 10.

9.6 - 10.0 EXTREME

9.0 - 9.5 HIGH

7.0 - 8.9 MODERATE

4.0 - 6.9 LOW

0.0 - 3.9 NONE

The Heat Stress Index was developed by Jill Watts and Larry Kalkstein at the University of Delaware. This website was co-developed by Jill Watts and Scott Sheridan, and is maintained by Scott Sheridan at Kent State University (ssherid1@kent.edu). Copyright 2007.