The heat index is how the outside temperature feels.
It works out real heat plus humidity.
When the air is wet, 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 like 50 °C.
Governments put out alerts when this mix becomes hazardous.
Heat domes or long humid stretches often bring these peaks.