The heat index is how the outside temperature feels.
It determines real heat plus humidity.
When the air is moist, sweat fails to cool effectively.
That makes our bodies perceive it is more scorching than the thermometer says.
In most summers, this occurs quickly.
At 38 °C and 70 percent humidity, it can feel as hot as 50 °C.
Governments put out alerts when this mix becomes hazardous.
Heat domes or prolonged moist spells often bring these peaks.