The computation of the heat index is a refinement of a result
obtained by multiple regression analysis carried out by Lans P. Rothfusz
and described in a 1990 National Weather Service (NWS) Technical Attachment
(SR 90-23). The regression equation of Rothfusz is
**T** is temperature in degrees F and **RH** is relative humidity
in percent. **HI** is the heat index expressed as an apparent
temperature in degrees F. If the **RH** is less than 13% and the
temperature is between 80 and 112 degrees F, then the following adjustment
is subtracted from **HI**:
**ABS** and **SQRT** are the absolute value and square root
functions, respectively. On the other hand, if the **RH **is greater
than 85% and the temperature is between 80 and 87 degrees F, then the following
adjustment is added to **HI**:

whereHI = -42.379 + 2.04901523*T + 10.14333127*RH - .22475541*T*RH - .00683783*T*T - .05481717*RH*RH + .00122874*T*T*RH + .00085282*T*RH*RH - .00000199*T*T*RH*RH

whereADJUSTMENT = [(13-RH)/4]*SQRT{[17-ABS(T-95.)]/17}

The Rothfusz regression is not appropriate when conditions of temperature and humidity warrant a heat index value below about 80 degrees F. In those cases, a simpler formula is applied to calculate values consistent with Steadman's results:ADJUSTMENT = [(RH-85)/10] * [(87-T)/5]

In practice, the simple formula is computed first and the result averaged with the temperature. If this heat index value is 80 degrees F or higher, the full regression equation along with any adjustment as described above is applied.HI = 0.5 * {T + 61.0 + [(T-68.0)*1.2] + (RH*0.094)}

The Rothfusz regression is not valid for extreme temperature and relative humidity conditions beyond the range of data considered by Steadman.