Sweating is less effective at cooling the body on humid days because the high moisture content in the air inhibits sweat evaporation.

“Whereas on a humid day, the reason you see people in New York and Florida on a humid summer day and they're like moving their shirts off themselves and you see people with, you know big sweat stains and back sweat stains and all this kind of stuff is because they're sweating as they normally would, but it's humid and so there is the humidity, the air doesn't allow transfer of that sweat into the atmosphere as readily and so you're hot, okay?”