The heat index is the perceived outdoor temperature.
It calculates real heat plus humidity.
When the air is humid, sweat won’t work well.
That makes our bodies think it is more scorching than the thermometer says.
In most 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 dangerous.
Heat dome events or extended periods of humidity often produce these peaks.