The concept of apparent temperature (AT) was initially put forward by Robert Steadman in 1971 and was intended as an assessment of what exposed body surfaces feel like in cold, windy conditions. Subsequently he (Steadman, 1979) extended his work on the thermal resistance of an unclothed body to warm, humid conditions. A further paper (Steadman, 1984) combined, and bridged the gap between the temperature ranges into a universal scale.
Regression equations of this universal scale are formulated for indoors, outdoors in shade but exposed to wind, and outdoors exposed to wind and solar radiation. Of these, outdoors in shade but exposed to wind, has been chosen as most informative:
AT = -2.7 + 1.04*T + 2.0*e -0.65*vwhere AT and T (air temperature) are °C,
e is vapor pressure in kPa,
and v is 10m wind speed in m/sec.
Of course there can be no accurate formula for what a person feels like. There are many variables that cannot be put in to the equation because they are subjective, such as area of skin exposed, metabolism and sweat rate. In fact, there are numerous other formulations of "heat index", for example Rothfusz (1990), and Masterton and Richardson (1979), but they are all non-linear and can give spurious results if used with temperatures or humidities outside of their designated regression limits. They are, therefore, unsuitable for building a comprehensive database, albeit for summer months only, which would make the calculation of mean and threshold values possible. Steadman's universal AT is a linear regression that is not restricted, and is more appropriate to outside conditions because it includes wind.
Excessive heat can be a killer, in fact, it is the number one weather-related killer in the United States, resulting in hundreds of fatalities each year (National Weather Service). The National Weather Service NWS) is charged with issuing forecasts, advisories and warnings. They use the complex, non-linear formula given in Rothfusz (1990) for this purpose, so values may be slightly different from those given by the AT equation above especially as wind is not included by NWS).
Hourly station data are obtained from the National Centers for Environmental Information's Integrated Surface Data (ISD) dataset (ftp://ftp.ncdc.noaa.gov/pub/data/noaa/isd-lite/). In this dataset observation times are UTC, but maximum, minimum and average apparent temperatures are calculated for local standard time days.
The mean base period is 1981-2010, and the "feel hot" threshold is defined as the 85th percentile of daily July and August values in this same period.