Friday Mar. 7, 2008

There are still a few more sets of Expt. #3 materials available for checkout.
The Experiment #4 materials should become available next week.

Note: By the end of next week you should either already have completed an experiment, should be working on an experiment, or should be working on one of the other options (book report, scientific paper report).

Before we start to look at radiant energy balance on the earth we need to learn about filters.  The atmosphere will filter sunlight as it passes through the atmosphere toward the ground.  The atmosphere will also filter IR radiation emitted by the earth as it trys to travel into space.

We will first look at the effects simple blue, green, and red glass filters have on visible light.  This figure wasn't shown in class.

If you try to shine white light (a mixture of all the colors) through a blue filter, only the blue light passes through.  The filter absorption curve shows 100% absorption at all but a narrow range of wavelengths that correspond to blue light.  Similarly the green and red filters only let through green and red light.

The following figure is a simplified easier to remember representation of the filtering effect of the atmosphere on UV, VIS, and IR light (found on p. 69 in the photocopied notes).  The figure below was redrawn after class for improved clarity.


You can use your own eyes to tell you what the filtering effect of the atmosphere is on visible light.  Air is clear, it is transparent.  The atmosphere transmits visible light.

In our simplified representation oxygen and ozone make the atmosphere a pretty good absorber of UV light.

Greenhouse gases make the atmosphere a selective absorber of IR light - it absorbs certain IR wavelengths and transmits others.  It is the atmosphere's ability to absorb (and also emit) certain wavelengths of infrared light that produces the greenhouse effect and warms the surface of the earth.

Note "The atmospheric window" centered at 10 micrometers.  Light emitted by the earth at this wavelength will pass through the atmosphere.  Another transparent region, another window, is found in the visible part of the spectrum.

You'll find a more realistic picture of the atmospheric absorption curve on p. 70 in the photocopied Classnotes, but the simplified version above will work fine for our needs.
 
We will be moving from an outer space vantage point of radiative equilibrium (above) to the earth's surface (below).
Don't let the fact that there are
4 arrows are being absorbed and emitted in the top figure
and only 2 arrows absorbed and emitted in the bottom figure
bother you
We'll be adding a lot more arrows to the bottom figure
It would get to complicated if we had more than 2 arrows of incoming sunlight.
The next step is to add the atmosphere.
We will study a simplified version of radiative equilibrium just so you can identify and understand the various parts of the picture.  Keep an eye out for the greenhouse effect.  We will look at a more realistic version later. 
Here's the figure that we ended up with in class


It would be hard to sort through all of this if you weren't in class (and maybe even if you were) to see how it developed.  So below we will go through it again step by step (which you are free to skip over if you wish).

The figure shows two rays of incoming sunlight that pass through the atmosphere, reach the ground, and are absorbed.  100% of the incoming sunlight is transmitted by the atmosphere (this is not a very realistic assumption). 

The ground is emitting 3 rays of IR radiation.

One of these is emitted by the ground at a wavelength that is NOT absorbed by greenhouse gases in the atmosphere.  This radiation passes through the atmosphere and goes out into space.

The other 2 units of IR radiation emitted by the ground are absorbed by greenhouse gases is the atmosphere.

The atmosphere is absorbing 2 units of radiation.   In order to be in radiative equilibrium,the atmosphere must also emit 2 units of radiation.  1 unit of IR radiation is sent upward into space, 1 unit is sent downward to the ground where it is absorbed.

The greenhouse effect is found in this absorption and emission of IR radiation by the atmosphere.  We tried to put into words what is illustrated above:

Before we go any further we will check to be sure that every part of this picture is in energy balance.

The ground is absorbing 3 units of energy and emitting 3 units of energy

The atmosphere is absorbing 2 units of energy and emitting 2 units of energy

2 units of energy arrive at the earth from outer space, 2 units of energy leave the earth and head back out into space.
The greenhouse effect makes the earth's surface warmer than it would be otherwise.

Energy balance with (right) and without (left) the greenhouse effect.  At left the ground is emitting 2 units of energy, at right the ground is emitting 3 units.  Remember that the amount of energy emitted by something depends on temperature.  Warm ground will emit more energy than colder ground.

Here's another explanation.  At left the ground is getting 2 units of energy.  At right it is getting three, the extra one is coming from the atmosphere.  Doesn't it make sense that ground that absorbs 3 units of energy will be warmer than ground that is only absorbing 2.
Next we will look at how realistic our simplifying assumptions are

In our simplified version of the greenhouse effect we assumed that 100% of the sunlight arriving at the top of the atmosphere passes through the atmosphere and gets absorbed by the ground.  The bottom figure above shows that in reality only about 50% of the incoming sunlight gets absorbed at the ground.

About 20% of the incoming sunlight is absorbed by gases in the atmosphere.  Sunlight is a mixture of UV, VIS, and IR light.  Ozone and oxygen will absorb a lot of the UV (though there isn't much UV in sunlight) and greenhouse gases will absorb some of the IR radiation in sunlight (IR light accounts for about half of the light in sunlight).

The remaining 30% of the incoming sunlight is reflected back into space (by the ground, clouds, even air molecules).
Students working on Experiment #3 will be measuring the energy in the sunlight that actually arrives at the ground.  We watched a couple of short video tapes showing Experiment #3.  The 2nd, and by far the better of the two tapes, was put together several years ago by a student enrolled in NATS 101.

You can find some additional discussion of Experiment #3 here.
Next we will look at our simplified version of radiative equilibrium and a more realistic picture of the earth's energy budget.

The lower part of the figure is pretty complicated.  It would be difficult to start with this figure and find the greenhouse effect in it.  However if you understand the upper figure, you should be able to find and understand the corresponding part in the lower figure.

In the top figure you should recognize the incoming sunlight (green), IR emitted by the ground that passes through the atmosphere (pink), IR radiation emitted by the ground that is absorbed by greenhouse gases in the atmosphere (orange) and IR radiation emitted by the atmosphere (dark blue).  Using the colors you can find each of these parts of the energy budget in the bottom figure.  Notice that conduction, convection, and latent heat energy transport are needed to bring the overall energy budget into balance.  Click here to see a more detailed check to be sure everything is in energy balance.