The heat index is the perceived outdoor temperature.
It determines real heat combined with humidity.
When the air is wet, sweat won’t work well.
That makes our bodies perceive it is more scorching than the thermometer says.
In some summers, this occurs fast.
At 38 °C and 70 percent humidity, it can feel like 50 °C.
Governments release alerts when this mix becomes hazardous.
Heat domes or extended periods of humidity often bring these peaks.