The heat index is the perceived outdoor temperature.
It works out real heat plus humidity.
When the air is moist, sweat doesn’t evaporate efficiently.
That makes our bodies think it is hotter than the thermometer says.
In many summers, this happens quickly.
At 38 °C and 70 percent humidity, it can feel as hot as 50 °C.
Governments release alerts when this mix becomes hazardous.
Persistent heat domes or extended periods of humidity often produce these peaks.