It’s about the combination humidity and temperature. For instance, I’ve been to Egypt in the desert where it’s super hot, but also bone dry. Your sweat evaporates which causes cooling (evaporation costs energy). I’ve also been to Thailand in the hottest months, which is cooler than Egypt, but super moist. My sweat would not evaporate which made me feel so much more hot.

When the temp is above bodytemp and the humidity is high enough you will overheat.