Heat index calculator
The heat index is the apparent temperature once humidity is accounted for. Above about 27 °C the body sheds heat almost entirely by evaporating sweat, and humid air will not take it, so the same 32 °C reads as 32 at 40 percent humidity, 40 at 70 and 44 at 80. Dry air is the case where the index tells you nothing you did not already know; the gap opens as the humidity climbs.
How to use the heat index calculator
The number comes from the Rothfusz regression, a nine-term polynomial fitted in 1990 to Steadman’s much larger 1979 apparent-temperature table so a forecaster could compute it without the table in front of them. The fit only behaves in hot conditions, so below roughly 27 °C the index is not defined and the air temperature is the honest answer, and two corrections apply at the edges: it is adjusted downward in very dry air under 13 percent humidity and upward above 85 percent, where the raw polynomial drifts.
Steadman’s model assumed a specific person in specific conditions: 1.7 metres, 67 kilograms, walking at 1.4 m/s, in shade, with a light 2.5 m/s breeze, in long trousers and a short-sleeved shirt. That is the whole reason full sun adds as much as 8 °C to the reported figure and still air adds more, and it is why heat warnings are issued against the index rather than the thermometer.
The bands are the part worth carrying around. Above about 32 °C fatigue arrives with prolonged exposure; above 40 heat exhaustion becomes likely with exertion; above 54 heatstroke is imminent regardless of fitness. Canada’s humidex answers the same question from the dew point instead of relative humidity and lands a few degrees away on the same day, so a humidex figure and a heat index figure are not interchangeable even though both are quoted in degrees.
What people use it for
- Deciding whether outdoor work needs a mandated rest schedule
- Moving a training session to a cooler part of the day
- Reading a heat warning issued on the index rather than the thermometer
- Comparing a dry 38 °C against a humid 32 °C
- Checking a workplace heat-stress threshold against today’s conditions
Questions
The apparent temperature with humidity accounted for, computed from the Rothfusz regression. It is how hot it feels rather than what the thermometer reads.
Sweat cannot evaporate into saturated air, and evaporation is the body’s only remaining cooling route once the air is warmer than skin.
No. It assumes shade and a 2.5 m/s breeze. Direct sun can add up to 8 °C to the apparent temperature.
The regression was fitted for hot conditions and misbehaves under it. Below that the apparent temperature is close enough to the air temperature that a correction would be invented rather than measured.
Above about 40 °C heat exhaustion is likely with exertion; above 54 heatstroke is imminent.
No. The humidex is a Canadian measure built from the dew point rather than relative humidity, and lands a few degrees apart on the same day.