[MUSIC]
So we constructed, at the very beginning of the class,
a very simple model of the climate of the Earth.
And since then we've been improving it and talking about ways in which
it should be more realistic in order to approximate the real world.
And so here we started with just original bare rock model of
energy coming into and out of a planet and that determines the planet's temperature.
Then we added the greenhouse effect with this
plate of glass that absorbs all the infrared light coming from the ground.
And then we talked about how the atmosphere,
the greenhouse gases in the atmosphere don't absorb all of the light.
They absorb only certain parts of the light energy.
And those other frequencies such as the atmospheric window here which
greenhouse gases don't absorb at all.
And then in the last unit we talked about another mechanism of carrying heat around
in the atmosphere in addition to the heat fluxes from light which is convection.
And in particular, we found out that moist convection is what
drives the thermal structure of our atmosphere, so you have here,
rising and it cools and condenses, and that's where it rains.
But it dumps latent heat up here and also carries heat around.
And so that's sort of where we are.
But there's one more thing that the real world has,
that our conceptual models do not have yet.
And that is, that the whole world is not a single place.
It's not all the same temperature and the sunlight intensity is not always in
balance with the energy going out by infrared locally.
[SOUND] So, the real geometry is that you have sunlight
all coming from one direction, from the sun, and
it's coming in most strongly at the equator.
Infrared light is leaving everywhere around the Earth and
the rate at which it leaves depends on the temperature of the ground
according to epsilon sigma T to the fourth.
And it turns out that if you look at a given
location on the Earth, the incoming and outgoing energy budgets do not balance.
They only balance for the Earth as a whole on a long enough time average.
So, here is plot of the sunlight intensity in watts
per square meter as a function of latitude and it's very strong at the equator.
Here is a plot of the infrared energy leaving the earth as a function of
latitude, and it's also stronger near the equator because it is warmer at
the equator than it is at the poles, but there is a deficit.
It's not hot enough at the equator to drive the energy out
as quickly as the sun is bringing it in.
And it's not cold enough at the equator at the high latitudes for
the infrared to be as low as the sunlight coming in.
And the difference is what we call heat transport.
It's heat being carried by the atmosphere and
by the oceans from the low latitudes to the high latitudes.
[MUSIC]
David ArcherProfessor
