I'm in hot muggy country, even worked up a good sweat taking the dog for a walk last night. Essentially your body is trying real hard to cool itself, and since your head and face are highly vascular it shows a lot.

I find a damp wet washcloth helps (post walking), and while riding, squirting water throught the holes of my helmet. Actually soaking my head after any outdoor activity works pretty well. Cool off those blood vessels.