Hey thanks for watching we're going to be talking about whether or not damp air moist air, humid air, whatever you want to call, it is heavier or lighter than dry air. First off the air around us almost always has some amount of moisture content in it. Water vapor in the air and that water vapor, as it increases, also increases our relative humidity and that humidity, of course, is relative to temperatures. So when we say relative humidity, what we're saying is the percentage relative to how much water vapor there could be in the air at the given temperature.

I often use the example of tea or coffee. You know that when your tea or coffee is cold, it doesn't hold as much sugar. You can stir the sugar it just doesn't absorb, but when you heat up that tea or coffee, then the fluid can hold more sugar in it and if you think of air in the same way, it's not exactly the same. But when air is warmer, it can hold more overall water, vapor content, but first, let's address this phrase that heat rises and that isn't exactly true.

What actually happens is hot air is buoyant or hot air floats in colder air and colder air sinks in warmer air. In other words, it has a higher density when it is colder or another way of saying that is, it has a higher pounds per cubic foot, and that's mostly. What we're going to be talking about here is pounds per cubic foot if it weighs more per cubic foot than it is more dense if it weighs less per cubic foot than it is less dense and so warmer air is less dense, and so it floats in Cooler, air - and we know that that happens. We know that warm air rises.

We see this in a lot of different phenomenon, but inside of our homes we see it because, often as you go upstairs in a home, if it only has one ac system, you'll notice that you get that stack effect of the warm air rising and the cool Air simultaneously falling making it cooler downstairs and warmer upstairs just in terms of natural convective forces. So we know that part but is moist. Air is air that has higher relative humidity or higher overall water vapor content in the air. Is it heavier or lighter? Is it more dense if it has water more water vapor in it? So let's look at some numbers quickly.

First, off air is made up of a lot of different things and if it's completely dry, it's mostly made up of nitrogen. That is the largest percentage uh that air is made up of about 78 and then number two way down. The list is oxygen at about 21 20.9 percent and then from there there's a laundry list of different gases that are all under one percent: argon co2 neon methane and a bunch more, but they make up a very small amount of the overall mass of the air. The overall weight of the air most of the weight of the air is nitrogen and oxygen if it's drier, if it has no water vapor in it, if it has water vapor in it, then often that water vapor will take up the third most common constituent of Air, so it would go nitrogen oxygen, water, vapor, argon, co2 neon methane in order of what air is made up of, but in terms of completely dry air completely dry air has a weight of 0.0807 pounds per cubic foot and it's very light of course, because it's A vapor all gases are pretty light, but it's about you know we'll just we'll just round here it's about .08 pounds per cubic foot now compare that to water vapor.

Now again, this is not liquid water. So we're not talking about fog here we're talking about just water vapor when it's truly a gas in the air, it has a weight of 0.0472 again. If we just went around we'll just say you know about 0.5, so water vapor is a lower pounds per cubic foot, lower density than air, and that means that water vapor will float in air when water vapor is a constituent part that makes up the air, then Air that has higher moisture content in terms of water vapor is going to be buoyant, meaning it's going to float in air of the same type at the same temperature, but that has a lower moisture content. So another way of saying that, very simply is that air that has more water vapor in it.

If everything else is the same, is actually lighter than air that does not now. Why do we think that moist air damp air is heavier well? There's several reasons that we do. One is is that when air actually is suspending liquid water particles in the case of fog, then it is heavier in the sense that those liquid particles, those suspended water particles that are in the liquid form, that we call liquid water, those are heavier so liquid water Is significantly heavier than air? That's fairly obvious. I have to you, know, take a glass dump, it upside down and you'll see what happens.

It is heavier than air, but it's the water vapor, that's lighter! So that's one reason why we think, just in our heads that water vapor would be more dense than air is because we're thinking in terms of liquid water. The other reason is, is that air that has higher moisture content or has more humidity in it does feel heavier to us in the sense that it feels more oppressive to us, and the reason is is that air that has more moisture content in it, especially in Terms of relative humidity, our bodies can no longer give off heat via evaporation via sweat, and so when we have higher moisture content in the air, we feel kind of oppressed because we're not able to reject heat off of our bodies. The way our bodies were designed and that's why it feels muggy or it feels uncomfortable on a high relative humidity day, because our bodies can't reject heat in that way, and so that's another reason why. I think often people will think that air that has moisture in it is more dense when in fact it is not more moisture, equals actual less density, and we know this right.

We know that when water leaves a lake or leaves the ocean, it travels upward that water makes it up and it creates clouds, and then those clouds rain, the water back down. If water vapor were heavier than air, then our clouds would be on the ground. We know that water vapor is lighter because our clouds end up in the sky, so pretty obvious stuff right, but there's another factor for technicians when you start to get kind of deep into the weeds of enthalpy, so enthalpy, split or delta h is something that's sort Of an advanced concept in terms of looking at things like total, delivered capacity of a system or even when you're doing something like a delta t calculator where you have to use wet, bulb temperature of the return air. In order to calculate what your target delta t is, what you find is is that air that has moisture in it higher moisture in it, higher relative humidity or higher total moisture content to be more exact, has more enthalpy, and so why would that be if air That has moisture in it is lighter, it's less dense.

Why would it have more enthalpy and the secret? There is that air that has water vapor in it has latent heat in it. That shows up when that air travels over the evaporator coil and the evaporator coil is lower than dew point, which means that there's actually heat being removed from that water vapor that's doing more than just changing its temperature. It's actually changing its state. So those of you who understand the basic refrigerant circuit, you know that far more heat energy is transferred in order to change the state of something than it is to just change its temperature once it has changed state.

So from a very practical standpoint, if you run a 70 degree stream of return air over an evaporator coil that has 10 percent relative humidity in it and you run a 70 degree airstream that has 80 relative humidity in it in terms of density, the one with The higher relative humidity or therefore the higher moisture content, actually is lighter. It is less dense, but what happens is is that when it goes over that evaporator coil, some of that energy goes to turning that water vapor into liquid water. So long as that evaporator coil is below dew point and that in effect holds up the surface temperature of that evaporator coil, because now as that water is changing state, its temperature can't drop. It's just like the kind of in reverse.

When you have the boiling pot of water, you, you slowly increase the temperature of that water until you hit 212 degrees fahrenheit. Once you hit that temperature. Now you can keep adding heat, but the temperature doesn't change, and the same thing is true when you're taking heat out of air when it hits that temperature that dew point temperature and you're changing the state of that water vapor to liquid water. It artificially holds up the temperature of that evaporator coil, therefore putting more load on it and in turn you don't get as much of a temperature split from a very practical standpoint when you have drier air running over an evaporator coil everything else being the same, the The temperature of the refrigerant moving through all of that, your delta t is going to be higher when it's drier air than when that air has more moisture content in it.

Even though, like the point of this video said, even though that air is less dense, meaning it actually weighs less per pound, and so that all comes down to that latent heat transfer, you actually are transferring latent heat out of that water and changing it from water. Vapor to liquid water, but again main point of this video. The thing you can take away is, if you're ever doing any calculations - and you run into this - you know finding that water vapor is lighter than air. There's nothing wrong with your calculations.

You will find that actually, it's significantly lighter than air again water, vapor 0.0472 pounds per cubic foot versus air, which is 0.0807 pounds per cubic foot. Now again, these are sort of standard equations, there's a lot of factors when we say air we're giving you a standard air equation. The density of air changes based on many many factors, barometric pressure and temperature being some of the bigger ones. In addition to total moisture content, so that's a moving target.

Some people may ask why water vapor is lighter than air and just quickly. If you look at the weight of hydrogen, we know that water is made up of hydrogen and oxygen h2o right. The weight of hydrogen is so light, it's 0.0051, so it's about 16 times lighter than air, and because hydrogen in its gaseous form is one of the constituent parts of water vapor. That is the reason why it's significantly lighter than air.

It's that hydrogen, because the other two parts of water vapor are oxygen and the weight of oxygen is actually very similar to air. It's very close, it's actually slightly more heavy, but it's very, very close. It's that hydrogen atom. That's part of that molecule that makes that water vapor significantly lighter than air all right.

So there you go. Hopefully you found that helpful. It might been a little nerdy for you, but it's something that you can wow you're friends with at parties. Actually, don't do that because you won't have any friends left all right, we'll catch you on the next video.