This index evaluates the impact of heat stress on the individual taking into account the combined effect of temperature and humidity.

The formula used by the SA Weather Service to calculate discomfort index is: 
Discomfort Index = (2 x T) + ( Rh/100 x T) + 24
T is the dry-bulb or air temperature in degrees Celsuis
RH is the percentage relative humidity

This index gives the following degrees of discomfort:
90-100 - very uncomfortable
100-110 - extremely uncomfortable
110+ - hazardous to health

Since the relative humidity of the air can be calculated from the dry-bulb and wet-bulb temperatures, the formula can also be adapted to use the wet-bulb temperature instead of the relative humidity.