https://www.scientificamerican.com/article/what-can-we-learn-from-our-sweat/

Yesterday morning, I was riding my bike up a pretty significant hill. To get up that hill, I was pedaling as hard as I could. I had nearly reached the top when I felt that familiar trickle of moisture run down my forehead. Even though it was only 6ºC (42ºF), I was sweating. 

Our human meat-sack bodies work optimally when their internal temperature hovers around 98.6ºF (37ºC). When the body gets warmer than that, the brain doesn't like it, so the hypothalamus (the part that controls temperature) sends a message to your body saying "Let's cool down!" That's when your sweat glands spring into action, and you start perspiring.