To my understanding we cool off via sweat by heating up the sweat and letting it evaporate off. That system should only function based on my understanding if the air is cooler than our skin. If that is the only system in play our body temp minimum should always be the temp of the outside air, making 110F weather fatal. What am I missing here?
In: Biology
Two things.
1. Humidity. If the air is very dry, you can still use evaporation, because the air sucks up moisture from your skin. This still results in cooling, because the molecules gain energy when they become vapor. Of course, at hot enough temperatures, it doesn’t matter because the evaporation is not enough to compensate, but it can still work. This is why we speak of “dry heat” being more bearable than “muggy” heat.
2. It takes time for the body to reach critical heat overload, aka heatstroke, or just partial overload, aka heat exhaustion. This means you can survive *short-term* exposure to such weather, so long as you follow it up with seeking shelter in comfortable conditions. With the right protective clothing, access to cool water and shade, and enough breaks, you can spend most of a day in very hot weather and never suffer medical problems.
It is *possible* for humans to briefly endure even temperatures over 120°F. It’s just extremely dangerous to try if you have any other option. My city had a 114°F heatwave a couple years ago, and I survived despite not having any AC (it’s usually not needed where I live.) I just drank a lot of water, stayed inside, and kept fans and coolers pointed at me for the day.
When water dissolves in air it requires an amount of energy, in the case of your sweat, the energy is taken from your body as heat. You don’t need the air to be cold for this system to work better but relative humidity has to be low, in this case the air will dissolve more water.
Your body cools to the dew point, not temperature; a dew point higher than body temperature is fatal.
On a side note from what was explained to you regarding cooling via evaporation.
Tuaregs in the sahara desert keep from the heat by covering themselves completely. This is not only to avoid exposing to the sun UV ray, but also because the body temperature is lower than the external temperature so you want to keep the “cool” (or colder) air in the clothes.
Sweat is only a part of cooling. Most heat we lose is radiated as infrared light. Converting liquid water to vapor requires energy to be put into the process, not just having sufficient temperature, so it actually cools whatever it is evaporating from.
Air itself is very poor at moving heat around, which helps us endure higher temperature while our body is cooler. It’s ultimately when the body can’t remove heat faster than what is gained that you start to overheat, and that point depends on many factors beyond just air temperature.
you just sweat more when it’s really hot to account for the loss of efficiency. This is what makes very hot weather dangerous. You can lose multiple pounds worth of water an hour without even doing anything, which also leads to lower levels of other things like sodium in your body.
You can cool off when the air temperature is above body temperature because the evaporation of sweat from your body takes off enough heat to keep your body at the right temperature. However, given a high enough humidity combined with the heat, you can get into a situation where your body can no longer cool to a safe temperature. This is called the “wet bulb” temperature. You wrap a wet rag around a thermometer and see what the temperature reading is. If the temperature of the wet thermometer (the wet bulb temperature) goes above human body temperature, that’s when things get very dangerous. Even if you’re sitting still in front of a fan, you can die from heat exhaustion because your body can’t get to a safe temperature from the evaporation of your sweat.
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