Calculates the index used to judge the risk of heat illness, as a weighted sum of the wet-bulb, globe and dry-bulb temperatures. The wet bulb carries a weight of 0.7, so humidity counts for far more than the air temperature.
This index measures the risk of heat illness, folding humidity and radiant heat into a single figure alongside the air temperature.
is the wet-bulb temperature, the globe temperature and the dry-bulb temperature, meaning the ordinary air temperature. Indoors there is no sun, so the dry bulb drops out and its weight passes to the globe.
The wet bulb takes the largest weight of the three. It is what a thermometer wrapped in a wet cloth reads, and it stands for the cooling the body gets from sweat evaporating. In high humidity the sweat does not evaporate and the wet-bulb reading stays high. That is why the same air temperature is far more dangerous on a humid day.
Outdoors with a wet bulb of 28°C, a globe of 38°C and air at 33°C, the index is . Of that, 19.6 comes from the wet bulb, 7.6 from the globe and 3.3 from the dry bulb, so the wet bulb alone accounts for nearly two thirds.
The index is quoted in degrees Celsius but is not an air temperature. Air at 33°C gives an index of 30.5.
Widely used guidance treats 28 and above as requiring serious caution, and 31 and above as dangerous, with exercise called off.