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
Sweat works because when water evaporates, it needs to absorb energy (latent heat of vaporization)
When sweat is stuck on your skin, it’s not evaporating, so you aren’t cooling as much as you can. When you wipe it off, you increase th e surface area of the sweat that remains on your skin so that sweat can evaporate faster, giving you that cooling feeling.
Sweat works because water can evaporate if there’s a bit of wind, or if the air is not humid (the air can accept more water vapor). So when you “force” water to evaporate by aiming some wind at a wet surface, the process of evaporation still requires heat energy, so the water SUCKS heat out of your body, cooling it.
Now if you sit in the sun, there’s so much heat coming from the sun that a little bit of sweat evaporation cooling may not do much for you.
Evaporation is an Endothermic process which means that a liquid needs to absorb energy from its surroundings to evaporate. When the sweat evaporates from your skin it pulls energy (heat) from your skin and evaporates. When you have heat pulled from you, it feels cooler to you so you get “cooled off”.
When you wipe the sweat off of your forehead you are thinning out the layer of sweat, allowing the sweat to evaporate quicker (as evaporation is largely based on surface area, and a thinner layer of sweat has more surface area) this quicker evaporation allows for a much cooler sensation and cooler overall body heat quicker.
When you sweat your body puts warm water on your skin. If the air temperature is high enough and the relative humidity is low enough that water will turn in to water vapor and be blown away from you body by anything stronger than a light breeze. This process of turning into vapor pulls heat energy from the surrounding area like your skin and feels cool.
If where you are has high humidity or no breeze then your sweat will have a hard time turning into vapor and will not give you the cooling sensation. It just sits on you warming up and if it is in direct sunlight it will quickly get warmer than your body temperature so when you wipe it away that will feel cool.
I’ve felt cooler standing on the dunes in Saudi Arabia as the shamals blow down even though the air temperature was 42 degrees C (130 degrees F) than when it hit 32 degrees C (90 degrees F) and 100% humidity in Tokyo.
In the first I had to drink alot of water or risk health consequences. In the 2nd it just sucked
sunlight may add a little to the heat of the sweat, but most of that heat is coming from your body. When it evaporates, the physical change of state is fueled mostly by your body heat. This cools you down.
If the relative humidity is high, the rate at which your sweat evaporates lowers and it is not as effective at cooling you down. And you remain gross and soggy vs. gross and dry.
Already some good answers concerning evaporative cooling as the key when it comes sweat. However, I also think it is important to address your other major item:
>doesn’t water, or sweat, act sort of like a “magnifying glass” that heats up the surface of the skin … then it burns off/evaporates which leaves water marks on the car?
The idea of droplets heating garden plants (or your car) is widely believed myth. Here is one of many sources that verify that this is false: [https://aces.nmsu.edu/ces/yard/1999/062899.html](https://aces.nmsu.edu/ces/yard/1999/062899.html)
The focal length of a magnifying object is outside of the object. The droplets would need to be suspended above the leaves/car to do any damage. Sure you can burn the ground with a magnifying glass, but try laying that magnifying glass directly on the ground and see if anything happens.
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.
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