There's a term called the Heat Index that is tossed around to describe how hot the weather feels. For those not familiar with the heat index, it's a bit like the Wind Chill index used to describe how cold the weather feels under certain conditions.
With the heat index (HI) it's the air temperature and the relative humidity that are combined in an attempt to indicated the "human perceived" equivalent temperature — how hot it feels, not just how hot the thermometer says it is.
A high heat index becomes a survival issue for humans because it thwarts our natural ability to cool ourselves. Normally, humans cool themselves through the evaporation of perspiration. According to the laws of physics, evaporation carries heat away from the body.
But when the heat index is high, that indicates a high relative humidity in addition to the high air temperature. Humidity in the air drastically reduces the evaporation rate, leaving humans unable to cool themselves by that method. You'll work up a sweat, but that moisture on your skin will not evaporate and carry away your excessive body heat. You'll just be hot and wet.
If you don't take measures to counteract the high heat index, eventually, you can fall victim to heat cramps, heat exhaustion and perhaps even deadly heat stroke.
When the heat index is high:
- Stay inside an air conditioned room
- Reduce work load
- Increase water intake
- Take cooling showers or baths