The Heat Index

For many years the United States National Weather Service has been forecasting and diagnosing heat discomfort in terms of the Heat Index (HI) developed by Steadman (1979a). The HI only employs temperature (T, in ° C) and relative humidity (RH, in %). In the same year Steadman (1979b) improved his empirical heat index equation to include the effects of wind, radiation, and even pressure, on the apparent temperature, but the NWS never incorporated these refinements.

The Heat Index (° C) is given by a multiple regression equation of T and RH on the apparent temperature. A web-based Heat Index calculator can be found here.

Daytime hourly mean and maximum temperatures for July and August during 1980-89 at Atlanta (Georgia) were tabulated by Dixon (3), along with corresponding values of the Heat Index , relative H=humidity (RH) and dewpoint (Td). These indicate the risk of hot humid conditions affecting athletic performance. The data also show how invariant is the dewpoint, changing only between 19.4° C) and 20.6° C, ie 1.2 K, during the hours 6 am to 9 pm in July. In August the range was from 17.8° C to 20.6° C, ie 2.8 K.On the other hand, the RH varied in July from 86% at 6 am to 52% at 3 pm and 69% at 9 pm; August figures were similar.

(1) Steadman, R.G. 1979a. The assessment of sultriness, Part I: a temperature-humidity index based on human physiology and clothing science. J. Appl. Meteor., 18, 861-873.

(2) Steadman, R.G. 1979b. The assessment of sultriness, Part II: effects of wind, extra radiation, and barometric pressure on apparent temperature. J. Appl. Meteor., 18, 861-873.

(3) Dixon, R. 1996. Climatology and multinational sporting events, Atlanta 1996. Bull. Amer. Meteor. Soc., 77, 771-2.