Heat Index Calculator

Air temperature, °C

Relative humidity, %

Result

Feels like, °C37.1
Hotter by, degrees5.1
Dangercaution: cramps and exhaustion are possible
Air temperature, °C32
Humidity, %60
Index in Fahrenheit98.7

Sweat only cools you if it can evaporate

Calculates the heat index from air temperature and humidity and shows where the risk of heat exhaustion and heat stroke begins.

Cooling by sweating depends on evaporation, and evaporation depends on how much water the air can still take. At ninety percent humidity the sweat simply sits there, and the body loses its main way of shedding heat.

The heat index turns that into a temperature: how hot dry air would have to be to load the body the same way. Thirty-two degrees at sixty percent humidity feels like thirty-seven.

What this calculator shows

The apparent temperature and how far above the air temperature it sits
The danger band it falls into
The same index in Fahrenheit, the units the formula was built in

What to keep in mind

The Rothfusz regression is used, defined from about 26 °C upward, with the standard corrections at very low and very high humidity.
It assumes shade and a light wind. In direct sun the effective load can be up to eight degrees higher.

FAQs

Why is humid heat so much harder?

Because sweat that does not evaporate does not cool. It only takes heat away as it turns to vapour, and saturated air will not accept the vapour.

At what point does heat become dangerous?

Above 32 °C of heat index cramps and exhaustion become possible, above 41 heat stroke is a real risk, and above 54 it is likely with any exertion.

Does the sun change the figure?

Yes. The index is defined for shade; direct sunlight adds up to eight degrees to the effective load.

What is the wet-bulb temperature?

The lowest temperature evaporation can reach at that humidity. Above about 35 °C wet bulb the body cannot shed heat at all, whatever it does — the physical limit the heat index only hints at.

Worked example

Thirty-two degrees and humid

Input: 32 °C, 60 percent humidity

Output: feels like 37.1 °C

Note: The same 32 °C at 20 percent humidity feels close to 31 — cooler than the thermometer reads, because dry air lets sweat work.

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Heat Index Calculator

Result

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