Heat Index Calculator
What the heat actually feels like once humidity blocks your sweat.
Why humidity makes heat dangerous
Your main cooling system is evaporating sweat, and humid air takes sweat slowly. The heat index (the Rothfusz regression used by the US National Weather Service) folds that into one feels-like number: 32 °C at 70% humidity feels like about 41 °C. The official figure is defined for shade and a light breeze; full sun adds up to another 8 °C (15 °F).
The categories
Below a heat index of 27 °C (80 °F) there's no meaningful adjustment. From there the NWS bands run caution, extreme caution (32 °C+), danger (39 °C+) and extreme danger (52 °C+). Most heat casualties happen to people exerting themselves in the danger band, not just sitting in it; scale activity down before the number says you must.