Why does a person start to sweat because of the sun?

When solar radiation heats up the skin, your body has to react to it to prevent the body temperature from rising. Our body temperature is kept constant at 36-37°C because our metabolism and life functions work best at that temperature. We use sweating to cool down. The sweat drops that are secreted by the sweat glands in the skin will evaporate on the skin surface. And with evaporation, heat is extracted from that surface – that’s a physical principle. You notice this best when you apply a volatile substance topically to your skin, such as ether for example. That liquid will evaporate quickly (faster than water or sweat) and your skin surface will feel locally cold.