Heat index calculator

The heat index is how hot it feels once humidity is accounted for. High humidity stops sweat evaporating, and evaporation is the only way the body sheds heat once the air is warmer than skin.

That mechanism explains why the index rises so steeply: at 32°C and 40 percent humidity it feels like 33, and at 32°C and 80 percent it feels like 46. Nothing about the air temperature changed — what changed is whether sweating still works. The index assumes shade and a light breeze, so full sun adds up to about 8°C more, which is why heat warnings are issued on the index rather than the thermometer. The risk bands are worth knowing: above about 40°C heat exhaustion becomes likely with exertion, and above 54 heatstroke is imminent regardless of fitness.