The heat index is how the outside temperature feels.
It calculates real heat combined with humidity.
When the air is moist, sweat doesn’t evaporate efficiently.
That makes our bodies think it is hotter than the thermometer says.
In most summers, this happens fast.
At 38 °C and 70 percent humidity, it can feel like 50 °C.
Governments put out alerts when this mix becomes risky.
Heat dome events or extended periods of humidity often produce these peaks.