Why do we sweat?
Short answer
Sweat helps keep your body from getting too hot. You sweat more in hot weather or during exercise, but nervousness can also make you sweat.
How it works
When your body warms up, sweat comes onto your skin. As the water in sweat evaporates (turns into invisible water vapor), it takes heat from your skin and carries that heat into the air. This helps cool your body.
High humidity means more water vapor in the air, so sweat evaporates more slowly. Less heat can escape this way, which is why you may still feel hot on a muggy day even while sweating.