‘It’s not the heat, it’s the humidity’


Why does the amount of humidity in the air affect comfort moreso than the temperature?

In: 43

A big part of how the body regulates temperature is by sweating. This is because when water evaporates, it takes some of the heat away with it.

When there’s more humidity in the air, there’s less opportunity for water to evaporate, so that leads to you sweating, and it taking longer to evaporate & cool you down. So on top of being just uncomfortably humid & “wet”, you’re also not losing as much heat from sweat so it feels warmer.

When relative humidity is high – meaning that the air is holding a lot of the moisture that it can hold at a given temperature – it becomes more difficult to cool yourself down. We cool ourselves down by sweating: we sweat, the sweat evaporates into the air, and (in ELI5 terms) takes some energy with it, cooling you slightly.

When the air is already holding a lot of moisture, your sweat is less likely to evaporate because it has nowhere to go – the air is full. That means you get much less of this cooling effect.

Humans remove extra heat from their bodies by sweating. Sweat is moisture expelled from the skin glands. When this moisture dries up (evaporates) it lowers the skin’s temperature, because evaporating requires heat energy which is thus removed from the body. In humid conditions sweat evaporation is slow because the air is already loaded with water vapor and does not take up moisture as easily as dry air does. Therefore sweating does not help very much in keeping the body temperature in the normal range, and we feel too hot, compared to the same outside temperature in dry conditions.

Humid air means that it’s harder for us to sweat. We cool down by sweating, and if the air is already saturated with water, the water in our sweat has no where to go.

If your sweat evaporates quickly and fully your temperature gets reduced by that a lot, maybe as much as 20 degrees Celsius in most extreme examples. So you can just drink more water to replenish it, but not going to overheat.

But if it’s high humidity sweating doesn’t cool you nearly as well – if humidity is at 100% sweat just doesn’t evaporate and doesn’t provide any cooling, only makes you wet.

Beyond the sweating effects that others have mentioned, high humidity also means there’s a lot of water in the air which means it holds more energy.

In low humidity, it can be 100F during the day, but cool down to 50F at night.

In high humidity, it can be 100F during the day, but only cool to 90F at night.

As a finn I can say when you are in sauna and temperature is +80°C it is nice. Then you throw some water on those hot rocks and you feel how hot it is. The truth is temperature didn’t rise but humidity did. Hope this helps.

Check out “Wet Bulb” temperature.

Basically, you wrap the bulb of a thermometer in a water-soaked cloth and put a fan in front of it.

If the thermometer is in a low humidity environment, the moisture from the cloth has the ability to transfer to the air around it via evaporation and therefore will be cooler than the environment… So you’ll get a reading on the thermometer that’s lower than the temperature.

Put that thermometer in a high humidity (100%) environment, where there’s excessive moisture in the air and the moisture from the cloth can’t be transferred to the air via evaporation and so the temperature of the environment will be the temperature of the wet cloth.

So, the higher the humidity, the less capable of cooling via evaporation.

I spent one summer overseas in a place that hit 46C (115 F) everyday, and one week it was as high as 49C (120 F) but this area had extremely low humidity. It was hot but it was hot in a way that feels like you’re burning, like when the sun is hitting you directly through a window.

I left and went home and the first week that I was home, it went up to 29C (85F) but it was at like 50%+ humidity and I wanted to die.

The humidity makes it harder for your sweat to evaporate so it just kinda builds up on you and you’re sweaty ask the time. When you get out of the shower, it takes longer for your body to get dry so you just feel like you can’t ever get away from feeling sweaty and sticky. The air also becomes harder to breathe in because you’re breathing more water vapor at that point

When it’s hot, Humans sweat. The sweat transfers heat from inside the body, to the outside of said body, and that heat is then transferred to the air when the sweat evaporates. The water component of sweat can also be cooled by say, a breeze. The cool sweat can then absorb more heat from the upper layers of your skin, free of charge.

So heat is removed from you internally, by way of sweat. And then the wind cools the sweat making your outer layers of skin colder, thus making room for more heat to move into your outer layers of skin. It’s a 2 for one deal.

This is what happens in a “Dry Heat”.

But in a humid climate, that sweat doesn’t evaporate. And in all probability, doesn’t get cooler. So you’re in a situation where you’re covered in sweat that won’t go away, and the sweat itself may even absorb the warmness of the air around it, making the sweat warmer than it was when it left your body. So now, you feel gross, covered in warm sweat that does nothing but keep the warmth in and around you. This bodily function has failed you and left you much worse off.

And what about feeling way colder when it’s not so cold but really humid? Like feeling cold for longer even after you got in a warm environment… Does the cold humid air attaches to your body for longer orrrr?

You know that feeling when you pull a big, fuzzy blanket out of the dryer and it feels dry, but it’s just really hot? Wrap that all around your body all day, and it NEVER cools off or dries out.

High humidity makes heat worse.

Go to Michigan during the summer, and you’ll feel like death