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 green house effect,
because we're absorbing the light from the warm ground
and replacing it with light from the cold upper atmosphere.
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.
It turnes 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.
Convection tends to mix things in a column of fluid.
You heat it from below or you cool it from the top and it, and it stirs and mixes
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
the 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 aloft from expansion we
have to add some heating, and the heating
is coming from water vapor and latent heat.
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 a 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 teapot, 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, and we can't see it because it's invisible.
What you see coming out of the teapot
is little droplets of water that that interact
with the light.
But, you must 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 (100 deg C) would be
like a bread warming oven, you could stick your hand
into a, a 200 degree oven (~95C), and the air would not burn you.
But the steam would condense on your skin
and release all of its heat,
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 want to break free of the constraints of the liquid, and
they want to 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'll evaporate until it reaches that line.
Or if you have too much, it'll tend to condense until it reaches that line.
So this line is called by chemists the equilibrium.
A meteorologist or somebody who just reads a weather report, calls this 100% Relative Humidity.
If you have a 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 might be 200% relative humidity.
It doesn't stay 200% relative humidity for long, it condenses out
very quickly, because this line is where the system wants to go.
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 zero and 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.
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 want to 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 rain drops 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.