The heat index is how the outside temperature feels.

It calculates real heat combined with humidity.

When the air is wet, sweat fails to cool effectively.

That makes our bodies think it is hotter 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 issue alerts when this mix becomes hazardous.

Heat dome events or extended periods of humidity often trigger these peaks.