For many of us, it doesn't even need to be hot before we start building up a sweat the second as we step outside. But unless you're drenched, you're better off not wiping the sweat off your head, since it can cool you down faster.
Photo by Carine06
Brian Palmer over at Slate explains the science:
Don't wipe unless you're drenched. Sweat releases heat by evaporative cooling. As each gram of sweat transitions from liquid to gas phase, it absorbs 2,427 joules of energy from the body and dissipates the heat into the environment. But if you wipe away the perspiration before it evaporates, that process will get cut short, and you'll need to sweat more just to achieve the same degree of cooling. On the other hand, any sweat that drips to the ground before it can evaporate won't do you any good, so if you're really soaked you may as well reach for the towel.
So the next time you go to mop your brow, think twice — if you're really serious about cooling off, you may want to leave those sweat beads to evaporate on their own. Of course, you can always speed up the process by knowing your body's quick cooling spots too.
Let Them See You Sweat [Slate]