Heat index calculator

In hot weather, humidity stops sweat from cooling the body. Type the temperature and the relative humidity: you get the heat index and the caution level.

Result

37.1 °CHeat index

In the other unit: 98.7 °F; Extreme caution (90 to 103 °F): cramps and heat exhaustion possible with long activity. Take breaks in the shade.

Inputs

Air temperature (in the shade): 32
Unit: °C
Relative humidity: 60

How it is calculated

NWS method: a simple formula (Steadman) comes first. If it gives 80 °F (≈ 27 °C) or more, the Rothfusz regression (1990) is used.

Result

Heat index

37.1 °C

In the other unit: 98.7 °F

How it is calculated

NWS method: a simple formula (Steadman) comes first. If it gives 80 °F (≈ 27 °C) or more, the Rothfusz regression (1990) is used.
Rothfusz: HI = −42.379 + 2.04901523 T + 10.14333127 RH − 0.22475541 T RH − … (T in °F, RH in %).
NWS adjustments: subtract when RH < 13 % between 80 and 112 °F, add when RH > 85 % between 80 and 87 °F.

Example

32 °C with 60 % humidity: heat index of 37.1 °C (98.7 °F), “extreme caution” level.

Good to know

The index assumes shade and a light wind: in full sun it can be up to 15 °F higher.
This is not medical advice. Babies, older people and sick people are more at risk: follow official alerts.
Temperature accepted up to 60 °C.

Sources

NWS (NOAA), The Heat Index Equation (Rothfusz 1990, with adjustments)
Steadman, R. G. (1979), The Assessment of Sultriness

Frequently asked questions

What is this tool used for?

In hot weather, humidity stops sweat from cooling the body. Type the temperature and the relative humidity: you get the heat index and the caution level.

How is the result calculated?

NWS method: a simple formula (Steadman) comes first. If it gives 80 °F (≈ 27 °C) or more, the Rothfusz regression (1990) is used.

What should I check before using the result?

The index assumes shade and a light wind: in full sun it can be up to 15 °F higher.