Fri., Oct. 8 notes

Energy balance on the earth with (right) and without (left) the greenhouse effect. The earth's surface has to be warmer in the picture at right in order to be able to emit 3 arrows of IR radiation.

Here's another way of thinking about the situation.

The ground is absorbing 2 arrows of energy in the picture at left, 3 arrows in the picture at right. Doesn't it seem reasonable that ground that is absorbing 3 arrows instead of 2 will be warmer?

In our simplified explanation of the greenhouse effect we assumed that 100% of the sunlight arriving at the earth passed through the atmosphere and got absorbed at the ground. We will now look at how realistic that assumption is.

The bottom figure above shows that on average (over the year and over the globe) only about 50% of the incoming sunlight makes it through the atmosphere and 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 (Roughly half of sunlight is IR light).

The remaining 30% of the incoming sunlight is reflected or scattered back into space (by the ground, clouds, even air molecules).

Student performing Experiment #3 will be measuring the amount of sunlight energy arriving at the ground. About 2 calories pass through a square centimeter per minute at the top of the atmosphere. Since about half of this arrives at the ground on average, students should expect to get an answer of about 1 calorie/cm2 min.

Next we will look at our simplified version of radiative equilibrium and a more realistic picture of the earth's energy budget.

In the top figure (the simplified representation of energy balance)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. The amount of energy transported by conduction, convection, and latent heat is small compared to what is transported in the form of EM radiation.

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. That's why we used a simplied version. Once you understand the upper figure, you should be able to find and understand the corresponding parts in the lower figure.

Two or three things to note in the bottom figure
(i) First the ground receives more energy from the atmosphere (96 units) than it gets from the sun (51 units). Part of the reason for this is that the sun just shines for part of the day. We receive energy from the atmosphere 24 hours per day.

(ii) The ground emits more energy (117 units) than it gets from the sun (51 units). It is able to achieve energy balance because it also gets energy from the atmosphere (96 units).

(iii) The atmosphere emits 64 units upward and 96 units downward. This might be explained by the lower atmosphere being warmer than higher up in the atmosphere. Part of the explanation is also that there is more air in the bottom of the atmosphere (the air is denser) than near the top of the atmosphere.

(iv) Note also the roles played by conduction & convection (7 units), and latent heat (23 units) energy transport at the left side of the bottom figure. Though much less than the amount of energy transported by EM radiation, this is necessary to bring the figure into energy balance.

Next a couple of the questions from the In-class Optional Assignment.

This is Question #5. In the original figure the ground is absorbing 1 unit of sunlight and emitting 3 units of IR radiation. To be in energy balance the ground must get 2 units of energy from the atmosphere. The atmosphere is absorbing 3 units of energy and must emit three units to be in energy balance. The ground needs two of these so we'll draw them pointing downward (arrows f and g). The remaining arrow (arrow h) goes upward and into space.

Question #7 asked whether UV-A, UV-B, or UV-C was the most dangerous form of EM radiation.

UV light is more dangerous than visible or IR light because it is a more energetic form of radiation. UV is a more energetic because it has shorter wavelength than either visible or IR light. Following the same line of reasoning, UV-C is more dangerous than UV-A or UV-B because it has shorter wavelength and more energy.

Finally we used our simplified representation of the greenhouse effect to understand the effects of clouds on daytime high and nighttime low temperatures.

Here's the simplified picture of radiative equilibrium (something you're probably getting pretty tired of seeing). The two pictures below show what happens at night when you remove the two green rays of incoming sunlight.

The picture on the left shows a clear night. The ground is losing 3 arrows of energy and getting one back from the atmosphere. That's a net loss of 2 arrows. The ground cools rapidly and gets cold during the night.

A cloudy night is shown at right. Notice the effect of the clouds. Clouds are good absorbers of infrared radiation. If we could see IR light, clouds would appear black, very different from what we are used to (because clouds also emit IR light, if we could see IR light the clouds might also glow). Now none of the IR radiation emitted by the ground passes through the atmosphere into space. It is all absorbed either by greenhouse gases or by the clouds. Because the clouds and atmosphere are now absorbing 3 units of radiation they must emit 3 units: 1 goes upward into space, the other 2 downward to the ground. There is now a net loss at the ground of only 1 arrow.

The ground won't cool as quickly and won't get as cold on a cloudy night as it does on a clear night. That makes for nice early morning bicycle rides this time of the year.

The next two figures compare clear and cloudy days.

Clouds are good reflectors of visible light. The effect of this is to reduce the amount of sunlight energy reaching the ground in the right picture. With less sunlight being absorbed at the ground, the ground doesn't need to get as warm to be in energy balance.

It is generally cooler during the day on a cloudy day than on a clear day.
Clouds raise the nighttime minimum temperature and lower the daytime maximum temperature. Here are some typical daytime high and nighttime low temperature values on clear and cloudy days for this time of the year.

We'll use our simplified representation of radiative equilibrium to understand enhancement of the greenhouse effect and global warming. This material wasn't covered in class.

The figure (p. 72c in the photocopied Class Notes) on the left shows energy balance on the earth without an atmosphere (or with an atmosphere that doesn't contain greenhouse gases). The ground achieves energy balance by emitting only 2 units of energy to balance out what it is getting from the sun. The ground wouldn't need to be very warm to do this.

If you add an atmosphere and greenhouse gases, the atmosphere will begin to absorb some of the outgoing IR radiation. The atmosphere will also begin to emit IR radiation, upward into space and downard toward the ground. After a period of adjustment you end up with a new energy balance. The ground is warmer and is now emitting 3 units of energy even though it is only getting 2 units from the sun. It can do this because it gets a unit of energy from the atmosphere.

In the right figure the concentration of greenhouse gases has increased even more (due to human activities). The earth would find a new energy balance. In this case the ground would be warmer and would be emitting 4 units of energy, but still only getting 2 units from the sun. With more greenhouse gases, the atmosphere is now able to absorb 3 units of the IR emitted by the ground. The atmosphere sends 2 back to the ground and 1 up into space.

The next figure shows a common misconception about the cause of global warming.

Many people know that sunlight contains UV light and that the ozone absorbs much of this dangerous type of high energy radiation. People also know that release of chemicals such as CFCs are destroying stratospheric ozone and letting some of this UV light reach the ground. That is all correct.

They then conclude that it is this additional UV energy reaching the ground that is causing the globe to warm. This is not correct. There isn't much UV light in sunlight in the first place and the small amount of additional UV light reaching the ground won't be enough to cause global warming. It will cause cataracts and skin cancer and those kinds of problems but not global warming.