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. Next we will 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).
The object of Experiment #3 is to measure 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, you should expect to get an answer that is close to 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.
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).
The object of Experiment #3 is to measure 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, you should expect to get an answer that is close to 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. Just as we did earlied, you can check to see that each part of the lower figure is in energy balance: 147 units absorbed and lost by the ground, 160 units absorbed and emitted by the atmosphere, 70 units of incoming sunlight absorbed by the ground or atmosphere and 70 units emitted by the earth back into space by the ground or the atmosphere.
There are a few other 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 gets lots of energy back from the atmosphere.
(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 than near the top of the atmosphere.
(iv) Note that energy transport by conduction and convection (7 units) and by latent heat (23 units) from the earth's surface to the atmosphere are needed to bring this figure into energy balance.
Next we will use our simplified representation of the greenhouse effect to understand the effects of clouds on daytime high and nighttime low temperatures.
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.
The next two figures compare clear and cloudy days.
Clouds are good reflectors of visible light (that is why clouds appear white). 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 Tucson daytime high and nighttime low temperature values on clear and cloudy days for the fall and spring semesters when this course is normally taught.
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.
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. Just as we did earlied, you can check to see that each part of the lower figure is in energy balance: 147 units absorbed and lost by the ground, 160 units absorbed and emitted by the atmosphere, 70 units of incoming sunlight absorbed by the ground or atmosphere and 70 units emitted by the earth back into space by the ground or the atmosphere.
There are a few other 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 gets lots of energy back from the atmosphere.
(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 than near the top of the atmosphere.
(iv) Note that energy transport by conduction and convection (7 units) and by latent heat (23 units) from the earth's surface to the atmosphere are needed to bring this figure into energy balance.
Next we will use 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.
The next two figures compare clear and cloudy days.
Clouds are good reflectors of visible light (that is why clouds appear white). 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 Tucson daytime high and nighttime low temperature values on clear and cloudy days for the fall and spring semesters when this course is normally taught.
We'll use
our simplified representation of radiative equilibrium to understand
enhancement of the greenhouse effect and global warming.
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.
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.
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.