Why is humid heat so much more unbearable than dry heat?
In: Biology
The air can only hold so much water. The more full of humidity it is, the harder it is for your body to cool down by sweating. Think about how easy it is to dry off with a dry towel vs a damp towel vs a soaked towel.
Lick your hand, and then blow on it. Your saliva should be body temperature, roughly 98-degrees F. Blowing on it feels cool because when a liquid evaporates, the conversion from liquid to a vapor absorbs a lot of heat.
When your sweat evaporates into dry air, it cools you a bit. You may still feel hot, but you would feel hotter if the air cannot absorb any more moisture because it already is holding as much humidity as it can.
Humid heat doesn’t let water evaporate, so you end up with pockets of warm (because your body and environment) water on you. Dry heat whisks that away and cools you down with the evaporation.
The element that a lot of these explanations about evaporation are missing is that water (and sweat) are insulators. Meaning they are resistant to the transfer of energy and, as a result heat, than the normal air itself.
Sweat is designed to pull heat from your body and take it into the air as it evaporates. Then that heat is distributed.
In dry air, that heat easilly distributes but in wet air (high humidity) the energy resistance of the water in the air makes it difficult for your sweat to transfer that heat (the biproduct of energy) away from you. So, because the air is both “full” of water, you stay wet and insulated.
It is basically the same reason why fans feel cool, despite the fact that they only move room temperature air. What they are doing is moving the air that surrounds your body–tne air filled with energy and heat your body produces–away from you which allows you to vent more more energy and heat.
Because humans have evolved with one of their main methods of cooling being the ability to sweat, which works when sweat is allowed to evaporate. When water evaporates, it takes a lot of heat energy to convert liquid water to water vapor, and that cools people. However, the higher the humidity, the less sweat gets converted to water vapor, so you lose the ability to cool through sweating.
Humid air has a lot more molecules in it because of evaporated water mixing with the atmosphere. Because of this, your body is getting bombarded by more heated up molecules than dry air. Adding to that, your sweat also doesn’t evaporate nearly as well because the air is already soaked, so you don’t cool down as well.
High atmospheric temperature causes precipiration and evaporation furthur causes cooling of body temperature.
If the air is moist this decreases the rate of evaporation thus causing difficulty in cooling.
If the air is dry evaporation is much more favourable thus cooling occurs.
But sometimes both of them can be unbearable😅
FYI, this question is directly related to [this thread](https://www.reddit.com/r/explainlikeimfive/comments/hcqxo2/eli5_what_is_the_humidity_scale_in_reference_to/).
Fun fact from that thread regarding how humid and dry air effect cooling :
> Side note: when you use your car’s defrost function in the winter your AC will also turn on, as the AC will dry the air prior to it being heated which, in turn, warms up your windshield faster and with minimal condensation on the inside of the glass.
The moisture in the air reduces the amount of evaporation of sweat from your body. Evaporation is a cooling process, meaning the highest energy water molecules (which means highest temperature) evaporate first, leaving behind lower energy molecules (lower temperature).
Everyone has answered the question so I will add a little fact.
There is a temperature and humidity combination that causes death even in fit, healthy people.
A continued temperature of 35°C with 100% humidity (wet bulb temperature) causes the body to be unable to shed heat through evaporation. If you can’t find a way of shedding the heat your body is producing you will die.
I experienced this for a few days a couple of years ago and I can say I never want to feel that again.
Latest Answers