Measure of apparent temperature devised by the US National Weather Service. It factors relative humidity into the air temperature in order to produce a figure representing the comfort and hazard level in bipedal mammalians.

Above a certain point, the body is reduced in its capacity to rid itself of excess heat by means of evaporation if the air is too saturated to let the moisture adequately evaporate and at the same time the air is too warm to allow the body to lose heat by means of radiation. It simply feels hot. Like Orlando in August. The heat index does not factor in wind chill.

Between 85°F/30°C and 95°F/35°C: Discomfort and possibility of fatigue due to physical activity
Between 95°F/35°C and 105°F/40°C: Heat exhaustion likely with prolonged exposure
Above 105°F/40°C: Heat exhaustion likely; Heat stroke possible
Exposure to direct sunshine should be calculated as up to 15°F above the heat index

The ridiculously complicated formula (there's another variant that looks even worse):

HI = -42.379 + (2.04901523 x T + 10.14333127 x RH) - (0.22475541 x T x RH) - (.00683783 x T²) - (.05481717 x RH²) + (.00122874 x T² x RH) + (.00085282 x T x RH²) - (.00000199 x T2 x RH²)

HI = heat index
RH = relative humidity
T = temperature in degrees Fahrenheit

US Army

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