Why does humidity make temperature feel hot, but restaurants use misters to cool the patrons?


I live in a town with a lot of humidity in summer. I perspire a lot, but restaurants around here have misters outdoors to “cool things down”. How does that work? How can adding more humidity make the heat more tolerable?

In: 340

If the air has to much moisture in it the water on your skin has a hard time evaporating. Yhe process of evaporation is what makes sweat/misters work on cooling you down.

Sweat cools you down because when any substance undergoes a phase change (e.g. changing from liquid to gas) it takes a bit of extra energy to execute the change. This energy typically comes from the surroundings, and typically in the form of absorbing heat. So when you sweat, the sweat on your body evaporates, using some of the heat on your skin during the process. Using this heat is what cools you down.

The misters mimic that process. The light mist settles on your skin, evaporates, and cools you down much like sweat would.

How efficient this process is is affected by many factors, one of which is how much water is already evaporated and hanging out in the air. If the air is very dry, sweating is more effective because it is easier (takes less heat) for the water to evaporate; if the air is very wet, the process is harder (takes more heat). So on very humid days, your sweat isn’t going to cool you as effectively, making you feel warmer.

So there’s two effects working simultaneously here.

Mainly, air has a certain capacity for how much water vapour it can hold. So humid air makes sweating less efficient, as the sweat finds it more difficult to evaporate into air already full of water, and evaporation is how sweat cooling works. It’s also how evaporative coolers work.

The other fact is that water has a far larger heat capacity than dry air for the same temperature. So cool water will drain far more heat from you than cool air, while warm water will make you far hotter than warm air.

So whether water in the air is good or bad depends on if it’s warm or cool. Warm moisture in the air prevents you from cooling down by sweating. And if it’s hot and humid enough, it will dump massive amounts of heat into your skin, while preventing you from sweating. That’s the “wet bulb temperature”, and if it’s above your body temperature, it’s lethal.

But cool moisture added to relatively dry air will make you cooler, like sweating but without the part where dried sweat is sticky. Artificial, clean sweat if you will.

On the other hand, humid winters also feel much colder than dry winters, for this reason. The water in the air will happily infiltrate all your clothes and sap the heat from you. And if you try to wrap yourself against it, you’re trapping your own sweat. It’s nasty and almost as bad as hot, with the one difference that it’s easier to insulate and heat yourself than cool down. You can wrap against cold, you can’t tear your skin of against heat.

Some great answers below.

What’s going on with the misters is also how ‘swamp coolers’ work. These are the units that you fill with water and plug in. The water is drawn up into an absorbent mat and air is blown over it by a fan. Heat is removed from the warm air by the evaporating water and cool – but more humid – air comes out the front.

These are not air conditioners (which also control humidity), but there are a lot of misleading online adverts for devices that claim to have transformed or ‘destroyed’ the air conditioning marker and only use a tiny fraction of the power. These are almost all cons. Swamp coolers can work, but they suck in very humid conditions.

Misters are generally used in places with low humidity, that way the water can hit you and evaporate quickly. Not to mention the temp of the mister is usually cooler than the temp of the humidity