[MUSIC] So we're in the process of trying to figure out why the temperature up high in the atmosphere is different from the temperature at the ground. And the reason why we're doing that is because that temperature difference is what drives the greenhouse effect, because we're absorbing the light from the warm ground and we're replacing with light from the cold upper atmosphere. So we started the class with a very simple model of the temperature in the atmosphere as a function of height with the layer model, where we don't actually have a vertical coordinate exactly, we just have this pane of glass sitting there and it has a different temperature than the ground because of the way the light is carrying the energy around.

So if you were to take the atmosphere and freeze it like some kind of aerogel that would be like air but it couldn't move and the only thing that was carrying heat around was light. You would have a temperature difference between the ground and high altitude, that is actually much stronger than the temperature difference that you actually observe. It would get colder about 16 degrees for every kilometer that you go up in the atmosphere. Well, it turned out that that much of a temperature difference is unstable to convection because you have buoyant air underneath dense air and so it's unstable. It wants to turn over. And so, convection tends to mix things in a column of fluid. You heat it from below or you cool it from atop and it stirs and mixes. Well, it's complicated in a gas because when the air rises, it expands and therefore cools, and so the product of convection doesn't make the temperature of the whole column the same. They way it would in water which doesn't expand and cool in the same way because it's not compressible. But this is the temperature profile that you would get if you just had expansion of gas as we've talked about. And it would give you temperature difference as you go up in the atmosphere of about ten degrees C per kilometer. So that's better. That's in the right direction. But actually the observed temperature gradient in the atmosphere is about six degrees C for kilometer. So added to this cooling off from expansion, we have to add some heating. And the heating is coming from water vapor and latent heat. [NOISE] When you boil water, you've got H2O in liquid and H2O in the gas. So it seems like a pretty boring chemical reaction. But you also have to add a significant amount of heat to the molecules to turn them into vapor from the liquid. This is called latent heat. So if you turn the burner on underneath the pan of water and boil some water. The temperature in the pan of water will rise until it reaches the boiling point, and then it stays at that boiling point and doesn't get any hotter because the water can't get hotter than that without boiling. So you're putting heat into this all the time. The flame is still burning, but the temperature doesn't go up. Where does that heat go? It goes into making the vapor. And then you've got steam coming off the top like out of a tea pot you can see steam coming off. Actually what you see there is not the water vapor. Water vapor itself, real legitimate gas H2O is invisible, we can't see it. There's water vapor in between you and me right now, and we can't see it because it's invisible. What you see coming out of the teapot is little droplets of water that interact with the light. But you gotta pay attention to those droplets of water because if you stick your hand in there when steam comes out, you will burn yourself severely. And it's not because the steam is all that hot. 212 degrees Fahrenheit would be sort of a bread warming oven. You could stick your hand into a 200 degree oven and the air 200 degrees would not burn you, but the steam would condense on your skin and release all of it's heat. And so you get a very strong burn from that, because of this latent heat.

The way that water vapor works in the air is the amount of water vapor that you want to have, the pressure of vapor, of gas, of water gas is a very strong function of the temperature. And it makes sense because if you make it all warmer, the water molecules they wanna break free of the constraints of the liquid and they wanna go into the vapor phase. So as the temperature rises, the amount of water vapor that you find goes up very strongly. Now this curve represents what's called the equilibrium amount of water vapor. In other words, that's the state at some temperature, if you have water available, liquid, and you let it sit for a long time, it will evaporate until it reaches that line. Or, if you have too much, it will tend to condense until it reaches that line. So this line is called by chemists they would call it the equilibrium. A meteorologist or somebody who just reads a weather report calls this 100% relative humidity. So if you have temperature and amount of water vapor that's 50% of this halfway between zero and that line. You would call that 50% relative humidity. Or if you're higher than that, it could be, say right here it might be 200% relative humidity. It doesn't stay relative. 200% relative humidity for long. It condenses out very quickly because this line is where the system wants to go. So let's imagine we start at the ground and we're going to raise this air up and it's going to expand and it's going to cool. We learned that in the last lesson. So, that's this first part of the trajectory. Let's say that that air started out at 50% relative humidity, meaning that it's halfway between 0 an 100% relative humidity here. Now, the air rising it's going to cool but initially it's not going to change its amount of water vapor. So, it's going to move along in a horizontal line like this. We're changing the temperature but holding the water vapor constant. And so what happens is at some altitude, you reach what's called the dew line, which is where water starts to condense.

When you start to cross this 100% relative humidity line, the water starts to wanna condense out. So let's say we just keep on doing that for a while, we rise up and keep going and we keep the water vapor the same, we're venturing now into more than 100% relative humidity territory here. And then at some point, what happens if you get too far above this line is that you condense and make rain. So you make raindrops or ice crystals in clouds, and that would tend to decrease the water vapor pressure. And I suppose it would actually also tend to increase the temperature. So this arrow here should probably be drawn more like that, because we're trading water vapor for liquid water plus the heat. [MUSIC]

Water Vapor and Latent Heat

Global Warming I: The Science and Modeling of Climate Change

Meet the Instructors