Heat index
Temperature & humidity HI
The heat index combines air temperature and relative humidity into a feels-like value for warm to hot weather. It explains why humid heat can be especially demanding.
Sweat cools when it evaporates
Evaporating water uses heat from the skin. When the surrounding air is very humid, this cooling works less effectively. The same 30 °C can therefore feel more demanding in humid air than in dry air.
What the value assumes
The usual heat index is calculated from temperature and relative humidity and expressed in Celsius or Fahrenheit. It assumes shade and light wind. Direct sunshine adds radiant heat, which a value based on shaded conditions does not fully describe.
Do not confuse different heat measures
- Air temperature: a measurement independent of a model of human thermal sensation.
- Heat index: a comparison value based on temperature and humidity.
- Wet-bulb temperature: a different physical measure describing cooling by evaporation.
What else matters
Physical effort, clothing, duration and personal circumstances all affect heat strain. A moderate index is not a general all-clear for exercise in strong sunshine. Planning also requires attention to shade, wind and official heat warnings. The index does not predict an individual person’s body temperature.