Heat Stress Index
Defining a Heat Stress Index
When both temperature and humidity are high, humans can experience considerable heat stress. In the U.S., extreme heat may have greater impact on human health (Kalkstein and Davis 1989), especially among the elderly (Changnon et al. 1996), than any other type of severe weather. The combined effects of temperature and humidity cannot be directly measured but can be assessed by calculation of an "apparent temperature" (A). Ignoring wind effects, one can estimate apparent temperature as A (°C) = -1.3 + 0.92T + 2.2e, where T is ambient air temperature (°C) and e is water vapor pressure (kPa) (Steadman 1984).
This index of "how hot it feels" should not be confused with the Heat Index used by NOAA's National Weather Service. Because the latter index is not defined for temperature below 80°F (27°C) and relative humidity below 40%, it is not suitable for compilation of a climatology. To include values below these limits would be a misuse of the Heat Index, but rejecting those conditions would introduce bias. Therefore we employ the Steadman (1984) apparent temperature.