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?
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It takes heat energy to turn liquid water into vapor.
Humidity is water that has already been turned to vapor.
Misters spray small drops of liquid water, which then evaporates. Since it takes heat to evaporate, this cools stuff down.
Everything everywhere is always trying to reach the same temperature forever, even your body. You generate heat to combat this constantly. A thin layer of water wicks and transfers heat better than dry skin.
If there is a small layer of water on your skin in a cool environment, then it will transfer heat from your body to the cool air quickly.
If there is a small layer of water on your skin in a hot environment, then it will transfer heat from the air to your body quickly.
Think of air like a sponge, at 100 percent humidity, the sponge is filled with water and its hard to soak up more, meaning evaporation occurs much slower so our bodies can’t effectively cool down, meaning it feels hotter than it actually is
The misters work similar to sweat, the water droplets absorb heat from your body and evaporate. The breeze from the fan increases the rate of evaporation. The misters alone aren’t going to be enough to raise the humidity unless it’s a closed space.
The misters aren’t going to work quite as well when it is already humid out compared to a dry heat, though. But they will still help somewhat because the water might be somewhat cooler, and the fan helps increase evaporation. You would have to be at 100% humidity before the mist stopped evaporating.
Think about going into a hot tub. A hot tub at 104 will hear you up very quickly while air of 104 still feels hot but takes much longer to get you to that same point. This may be an easier way to envision what has already been well explained
There are actually two types of misters you’ll encounter in outdoor venues. The most basic type just spray water. These work well in dry climates, because water carries heat with it when it evaporates. So the misters spray water into the air, this water lands on your skin, absorbs heat from your body, then evaporates taking a small amount of heat with it.
This type of mister works similar to sweat, but the water that lands on you is typically cooler than the sweat your body is producing, so the effect is greater than sweating alone. The effectiveness of this type of cooler is limited in humid environments though. Eventually everyone is just drenched in sweat, and the water isn’t evaporating quickly enough to carry heat away.
In more humid climates, a slightly different system is used. The water in high-pressure mister systems is put under a lot of pressure, chilled, then sprayed out of the mister nozzles (under high-pressure). As the water exits, the pressure drops, which causes the water to become even cooler. The mist extracts heat from the air around it, actually reducing the ambient temperature. Water still lands on people near these high-pressure misting systems, and it doesn’t evaporate like it would in a dry climate, but the water is cooler, so it still has some benefit.
When moisture evaporates off your skin, it absorbs a bit of heat to do so, thus cooling you off. If the air is very humid, there is too much moisture in the air for your sweat to evaporate so you don’t get any cooling from evaporation. If it’s hot and DRY, getting mist sprayed on you cools you off because the mist that is sprayed on you then evaporates, thus cooling you. The mist is like fake sweat on your skin.
Misting you when it’s humid doesn’t help unless the mist is cool, then you get the slight cooling of the cool water, but then the moisture jus sits on your skin and makes you feel sticky.
It works by evaporative cooling. Fans, unlike air conditioners, can’t make the air cooler but they can accelerate evaporation because they increase airflow. This can further be enhanced by adding misters, since without them all the fan can evaporate is your own sweat, which means that after it’s gotten rid of most of it you stop feeling a cool breeze. By using misters to spray water on your skin and then evaporate it it the air from the fans always feels like a cool breeze.
Liquid water (mist or sweat) on skin -> take heat away by evaporating into vapor
Humidity = how much water vapor already in the air.
High humidity = less “room” for cooling evaporation to take place = sweat and misters less effective as humidity goes up
In my experience, those mist sprays only work well in dry climates. I don’t know why restaurants use them in humid climates.
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