A Comparison of the NWS Heat Index and the Steadman Apparent Temperature Formulas over Southeast Texas

A Comparison of the NWS Heat Index and the Steadman Apparent Temperature Formulas over Southeast Texas

Mark J Keehn and Lance Wood

National Weather Service Houston/Galveston, Texas

Excessive heat was the biggest contributor to weather related deaths in the United States during the period between 2006 and 2015.  Because of the impact that heat has on the population, the National Weather Service (NWS) provides heat related decision support services to emergency managers.  The current U.S. NWS Heat Index (HI) equation was developed in 1990 (Rothfusz, 1990) to fit a multiple regression analysis of the Apparent Temperature (AT) model developed by Robert Steadman. (Steadman, Part I, 1979)  The benefit of the HI equation was that it simplified the complex formulas that comprised the AT model and made it possible to calculate the HI using only temperature (T) and relative humidity (RH) as input variables.

In his research, Steadman describes AT as a combined measure of all the climatic variables that affect human comfort and performance.  However, wind and solar radiation are treated as constants in the simplified AT model that was used as the basis for the HI formula.  (Wind velocity ~ 2.5m s^-1. Solar Radiation ~ 0 W m^-2.)   In subsequent studies, (Steadman, Part II, 1979) and (Steadman, 1984 & 1993), the author revised the simplified AT model to better accommodate wind and radiation.

The HI formula has been modified slightly since its inception to adapt to extremely high and extremely low RH conditions; however, there is no correction for the elevated winds that are common along the western Gulf Coast in the summer.  The formula also assumes no additional radiation, effectively calculating the HI in the shade.  Steadman’s research indicates that surface winds along the Western Gulf Coast may lower the AT by as much as 2° C in the summer, while solar radiation can raise the AT by up to 6° C.  The NWS gridded forecast database contains the necessary parameters to calculate the AT using, T, RH, wind and solar radiation as input. This study examines if accounting for the additional effects of wind and radiation can improve the quality of the heat related guidance that the NWS provides.