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We're in the process of trying to
figure out why the temperature

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up high in the atmosphere is different
from the temperature at the ground.

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And the reason why we're doing that is
because

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that temperature difference is what drives
the green house effect,

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because we're absorbing the light from the
warm ground

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and replacing it with light from the cold
upper atmosphere.

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We started the class with a very
simple model of

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the temperature in the atmosphere as a
function of height with the layer model.

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Where we don't actually have a vertical
coordinate exactly,

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we just have this pane of glass sitting
there and

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it has a different temperature than the
ground 

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because of the way the light is carrying the
energy around.

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so if you were to take the atmosphere and freeze it like some kind of

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aerogel that would be like air, but it
couldn't move, and

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the only thing that was carrying heat
around was light, you would

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have a temperature difference between the
ground and high altitude that

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is actually much stronger than the

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temperature difference that you actually
observe.

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It would get colder about 16 degrees for

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every kilometer that you go up in the
atmosphere.

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It turnes out that

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that much of a temperature difference is
unstable to convection,

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because you have buoyant air 

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underneath dense air, and so it's
unstable, it wants to turn over.

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Convection tends to mix things in a
column of fluid.

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You heat it from below or you cool it from
the top and it, and it stirs and mixes

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It's complicated in a gas because
when

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the air rises it expands and therefore
cools.

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And so, the product of convection doesn't
make

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the temperature of the whole column the
same

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the way it would in water, which doesn't
expand and

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cool in the same way, because it's not
compressible.

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00:02:13,650 --> 00:02:18,300
But this is the temperature profile that
you would get if you just had expansion

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of gas as we've talked about.

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And it would give you temperature
difference as you go

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up in the atmosphere of about ten degrees
C per kilometer.

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So that's better, that's in the right
direction.

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But actually, the observed temperature
gradient in the

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atmosphere is about six degrees C for
kilometer.

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So added to this cooling aloft from
expansion we

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have to add some heating, and the heating

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is coming from water vapor and latent
heat.

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When you boil water you've got H2O in
liquid and H2O

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in the gas, so it seems like a pretty
boring chemical reaction.

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But you also have to add a significant
amount of heat to the

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molecules to turn them into vapor from the
liquid. This is called latent heat.

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So if you turn the burner on

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underneath a pan of water and boil some
water,

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the temperature in the pan of water will
rise

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until it reaches the boiling point, and
then 

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it stays at that boiling point and doesn't get any
hotter

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because the water can't get hotter than
that without boiling.

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So you're putting heat into this all the
time, 

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the flame is still burning, but the
temperature doesn't go up.

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Where does that heat go?

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It goes into making the vapor.
And then you've got steam coming off

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the top, like out of a teapot, you can see
steam coming off.

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Actually, what you see there, is not the
water vapor, water vapor itself, 

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real legitimate gas H2O, is invisible, we
can't see it.

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There's water vapor in between you and me
right

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now and, and we can't see it because it's
invisible.

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What you see coming out of the teapot

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is little droplets of water that
that interact

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with the light.

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But, you must pay attention to those
droplets

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of water, because if you stick your hand
in there

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when steam comes out you will burn
yourself severely,

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and it's not because the steam is all that
hot.

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212 degrees Fahrenheit (100 deg C) would be 

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like a bread warming oven, you could stick your
hand

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into a, a 200 degree oven (~95C), and the air
would not burn you.

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But the steam would condense on your skin

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and release all of its heat,

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you get a very strong burn from that,
because of this latent heat.

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The way that water vapor works in the air
is,

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the amount of water vapor that you want to
have, the pressure of vapor,

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of gas, of water gas, is a very strong
function of the temperature.

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And it makes sense, because if you make it
all warmer, the water

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00:05:13,660 --> 00:05:18,150
molecules, they want to break free of 
the constraints of the liquid, and

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they want to go into the vapor phase.
So as the temperature rises, the amount of

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water vapor that you find goes up very
strongly.

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Now this curve, represents what's called
the equilibrium amount of water vapor.

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In other words, that's the state at some
temperature,

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00:05:39,270 --> 00:05:43,240
if you have water available liquid, and
you let it

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sit for a long time, it'll evaporate until
it reaches that line.

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Or if you have too much, it'll tend to
condense until it reaches that line.

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So this line is called by chemists the equilibrium.

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A meteorologist or somebody who just reads
a weather report, calls this 100% Relative Humidity.  

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00:06:03,232 --> 00:06:08,446
If you have a temperature and

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00:06:08,446 --> 00:06:13,804
amount of water vapor that's 50% of this,
halfway between zero

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00:06:13,804 --> 00:06:18,890
and that line, you would call that 50%
relative humidity.

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00:06:18,890 --> 00:06:20,880
Or, if you're higher than that it could

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00:06:20,880 --> 00:06:25,180
be, say right here might be 200% relative
humidity.

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00:06:25,180 --> 00:06:29,130
It doesn't stay 200% relative
humidity for long, it condenses out

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00:06:29,130 --> 00:06:33,850
very quickly, because this line is where
the system wants to go.

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00:06:33,850 --> 00:06:41,670
Let's imagine we start at the ground
and we're going to raise this air up,

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00:06:41,670 --> 00:06:48,530
and it's going to expand and it's going to
cool, we learned that in the last lesson.

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00:06:48,530 --> 00:06:51,980
So that's this first part of the
trajectory.

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00:06:51,980 --> 00:06:55,680
Let's say that that air started out at
50% relative humidity, 

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meaning that it's
halfway between zero and
100% relative humidity here.

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00:06:59,245 --> 00:07:05,980
Now the air rising, it's going to cool, 

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00:07:05,980 --> 00:07:10,320
but initially it's not going to
change its amount of water vapor.

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00:07:10,320 --> 00:07:14,180
It's going to move along in a
horizontal line

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00:07:14,180 --> 00:07:19,530
like this, we're changing the temperature, but
holding the water vapor constant.

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00:07:19,530 --> 00:07:24,960
And so what happens is, at some altitude,
you reach what's called the dew line,

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00:07:24,960 --> 00:07:27,380
which is where water starts to condense.

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00:07:28,700 --> 00:07:31,550
When you start to cross this 100% relative
humidity

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00:07:31,550 --> 00:07:34,520
line, the water starts to want to condense
out.

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00:07:34,520 --> 00:07:40,000
So let's say we just keep on doing that
for a while, we rise up and keep going and

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00:07:40,000 --> 00:07:42,580
we keep the water vapor the same, 


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00:07:42,580 --> 00:07:49,380
we're venturing now into more than 100% relative
humidity territory here.

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00:07:49,380 --> 00:07:54,910
And then
at some point, what happens if you get too
far above this line is that

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00:07:54,910 --> 00:07:59,622
you condense and make rain.
So you make rain drops or ice crystals 

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00:07:59,622 --> 00:08:06,255
in clouds, and that would tend to decrease the
water vapor pressure.

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00:08:06,255 --> 00:08:11,771
And I suppose it would actually also tend
to increase the temperature.

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00:08:11,771 --> 00:08:15,267
So this arrow here, should probably be
drawn

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00:08:15,267 --> 00:08:18,855
more like that, because we're trading
water

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00:08:18,855 --> 00:08:21,906
vapor for liquid water plus the heat.