+1/2

How much you sweat is related to the body's internal temp. Sweat evaporates faster in dry air, so you don't notice it as much. However it's not the sweating itself, but it's evaporation that cools the body, so sweating isn't as effective as a cooling mechanism in high humidity. Since you're not cooling well, you're body tries harder by sweating more.

It's not an illusion, you do sweat more in high humidity. Wind is also a factor, and regardless of humidity, breezes cool you, and you sweat less.