All related- Why does lower temperatures and higher humidy feel so hot? Is overheating when hiking as dangerous or more likely in no humidity or high? How does sweating help cool you down?
In: Biology
It’s pretty simple. Your body sweats to reduce heat. That sweat is evaporated into the air normally, which cools your body (exothermic reaction).
When the air is already saturated, your body sweats, but not very much of that sweat can be evaporated into the air, as it is already saturated with water (high humidity). So you sweat, but you aren’t cooled off.
When water evaporates, the bonds between its molecules need broken. Much like pulling magnets apart, this takes energy. For water, especially saltwater, it’s a significant amount of energy. This process is so powerful that even spraying hot water onto your body will still cool you down.
So we sweat, the water evaporates into the air, and we are cooled down a lot in the process.
Humidity measures how much water is in the air, or, more relevant here, how difficult it is for more water to evaporate into the air. If the air is already full, your sweat will just stick to you. The only way your body can cool down is by warming up the air around you, but if the air around you is already almost as hot as you are then this is going to be ineffective. You’ll overheat.
When water evaporates, heat energy is required to break the bonds between water molecules and separate the molecules from their closely packed liquid form to their distance-separated gas (vapor) form.
Now if you blow air at a wet surface, the air has some capacity to pick up water molecules, and it does so depending on wind speed and on how much water is already in the air (humidity). But the molecules of water that are “peeled off” the surface still need that heat energy to separate, so they literally suck heat out of their immediate vicinity.
Therefore, wind-evaporation sucks heat out of the water (sweat) and your body. This is called the wind chill effect, and is why we sweat. We sweat so the wind can evaporate that water off our skin and suck heat out of our bodies in the process.
Not all animals can sweat.
And how much water is picked up by the air depends on how much water may be already in the air. When it’s humid outside, the air is pretty full of water already, and won’t pick up more.
Therefore, humid hot feels much hotter to you than dry-air hot, especially if the dry-air has a wind to it. Overheating is more dangerous if the air is humid, because your body can’t use sweat to cool down.
Water carries heat. A lot of it. It really, really, REALLY loves to soak up heat. When this happens, the water turns into vapor. It’s like when you put a hot pan under a faucet. The heat from the metal moves into the water, and the water vaporizes and bubbles and floats away.
That’s what is happening when you sweat. Your body excretes water. It takes energy to evaporate water off of your skin, and that energy is heat. As your excess body heat is used to convert beads of water (sweat) into vapor, you start to cool down.
Okay, cool… So why does humidity suck so bad?
The problem with humidity is that the air is already full of warm water vapor. At very high humidity, your sweat can’t evaporate because the air has already reached its maximum water capacity. And because it can’t evaporate, it can’t carry your excess body heat away. Therefore, overheating is MUCH more likely to happen in high humidity.
Two reasons that humid air makes you hot:
1) water transfers heat much faster than air (thermal conductivity). So moist hot air makes you hotter faster than dry hot air (same for moist cold air making you cold faster)
2) when you sweat, it’s the evaporation of the sweat that cools you down. When the air is humid, it is already holding a lot of water (humidity) so sweat takes longer to evaporate because the air is slower to absorb more water.
You don’t feel hot or cold, you feel the rate at which you are losing heat.
You are constantly sweating, but most of that sweat is immediately evaporating off your skin, and cooling you down by pulling away the “latent heat of vaporization” that is the energy that it takes to evaporate water.
When the relative humidity it high, the air is holding almost all the water it can, so sweat can’t evaporate off your skin as easily, and it builds up on your skin while not taking away any heat.
When the air is cold it can’t hold as much water, so a high relative humidity in cold air doesn’t result in as much sweating because you still lose enough heat through radiation and conduction directly into the air, even though you can’t sweat as easily, you don’t need to.
At any temperature, water wants to be at an equilibrium with its environment. If you take a closed container of water, it will evaporate until the amount of water in the air above it can’t absorb any more. But getting water to evaporate takes energy, which also cools the water slightly. This process is called “evaporative cooling.”
So what happens when you sweat is that the sweat on your body evaporates due to your body heat, and this evaporating water removes the heat from your body, cooling you. But what happens if there is already a lot of water vapor in the air (high humidity)? In this situation, your sweat doesn’t evaporate as quickly since the air already has too much water vapor in it. If the humidity is high enough, this can make it impossible to lose heat be evaporative cooling. This is why a “dry” heat feels less harsh than a muggy heat. In a dry heat evaporative cooling is very effective.
Sweating is one of the human body’s mechanisms for cooling itself.
The sweat evaporates off your skin and you feel cooler.
When humidity is very high, your sweat won’t evaporate as readily. So your cooling mechanism doesn’t work as efficiently.
It’s like running the air conditioner with the window open… you get some relief, but not that much.
Latest Answers