system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do heat indexes work, and why? If it's 91 degrees Fahrenheit outside, but "it feels like 98 degrees", why isn't it just 98 degrees then?
gpt
If you held a thermometer up it would read 91.

However, because of the high ambient humidity, your body's heat removal system (which relies heavily on evaporative cooling, that works best in dry air) has to work harder.  Effectively, you're sweating like it's a dry 98, not a dry 91.