If I’m outside and I start to sweat very much then the sun on my sweat heats it up and I feel hotter than before so I wipe it off and feel cooler for awhile- doesn’t water, or sweat, act sort of like a “magnifying glass” that heats up the surface of the skin in the same way that if you wash your car in direct sunlight then it burns off/evaporates which leaves water marks on the car?
In: Biology
A droplet of water could act as a magnifying glass, but not in this scenario. To concentrate light on your skin, the droplet would have to be suspended some distance above your skin. Otherwise the optics just don’t work out and no light will get concentrated. Also, this would only make a difference to the average temperature of your skin, if the water droplets were redirecting sunlight that otherwise would not have hit your skin at all. If they merely focused light that was going to hit your skin anyway, then the average effect is zero – it just will concentrate the heat more locally.
When you wash your car in the sun, there’s also no magnifying effect going on. The reason the water droplets evaporate quickly off your car’s surface in that scenario, is because that surface gets hot in the sun, especially any metal parts. Metal is a good conductor of heat so it easily absorbs the sun’s rays and also easily transfers that heat to the water, which therefore evaporates quickly. Water that evaporates will leave marks because anything dissolved in the water won’t evaporate with it, and instead gets left behind. You may get more of these marks when washing your car in the sun simply because the water evaporates before you’ve had a chance to wipe it dry, or before water has run off the car naturally (due to gravity, wind, etc.).
The reason you might feel cooler when you wipe sweat off is that sweat needs to evaporate off your skin to have a cooling effect. Big droplets of sweat don’t evaporate easily, or when they do they evaporate from the outer surface of the droplet, and so the cooling effect is insulated a bit by the water in between. When you wipe sweat off you smear it out into a thin film that evaporates and cools your skin quickly.
Latest Answers