The heat index is what the outside temperature feels like.
It works out real heat combined with humidity.
When the air is humid, sweat doesn’t evaporate efficiently.
That makes our bodies think it is more scorching than the thermometer says.
In many summers, this happens fast.
At 38 °C and 70 percent humidity, it can feel like 50 °C.
Governments release alerts when this mix becomes dangerous.
Heat dome events or long humid stretches often trigger these peaks.