Humidity is more dangerous than one would initially think.
The body cools itself by sweating, and then the act of the air evaporating the sweat cools you down. Unfortunately in humid air, since the air is so full of moisture already, sweat evaporates more slowly, and if the humidity is high enough, not at all.
So on a really humid day, you could be perfectly hydrated and still dangerously overheat because sweating just doesn't work right in high humidity. So, be careful out there, and dial it down when it's high.