Friday Mar. 7, 2014

Music before class from Tesoro ( a local group).  You heard "Malaguena" and the first minute or so of "Motivation" (I couldn't find either online but here is a good substitute).  You can find music from Tesoro on their "Live in Studio 2A" and "Live at Hotel Congress" CDs (downloads available from CDBaby.com).

The Experiment #3 materials were available for pickup for the first time today.  Remaining sets of materials will be brought to class on Monday.  Take advantage of any clear sunny day to collect your data (it only take 30-40 muinutes).  You never know what the weather will be like if you wait until the last minute.  Try to make the measurements around midday, on a day without clouds and little or no wind.

A list of students that don't seem to have turned in any 1S1P reports at all is now online.  If you on are the list you may turn in one report (on either the Causes of the Seasons or Ultraviolet Light topics by next Wednesday without a penalty).  It is important that you start earning some 1S1P pts.  The number of remaining opportunities will eventually not be enough for you to reach the 45 max. no. of pts allowed goal.

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.  This is the only number in the figure you should try to remember.

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 most of the UV (UV makes up only 7% of sunlight).  Roughly half (49%) of sunlight is IR light  and greenhouse gases will absorb some of that.

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

Now that we know only about half of the incoming sunlight makes it to the ground we can modify and make more realistic our simplified picture of energy balance.



In this case we'll assume that 1 of the 2 incoming arrows of sunlight is absorbed in the atmosphere instead of passing through the atmosphere and being absorbed at the ground.  The ground is still emitting 3 arrows of IR light.  Have a careful look at this picture and see if you can (i) determine how many units or arrows of IR light must be emitted by the atmosphere in order for it to be in energy balance.  Once you've done that see if you can (ii) figure out how that light should be split up.  I.e.  how many arrows should go up and into space, how many should go downward and be absorbed by the ground.  You'll find the answer to this question at the end of today's notes.

A more realistic picture of energy balance on the earth is shown below (the bottom figure below).  The simplified version is also shown for comparison (top figure and also the solution to the situation in the figure above).



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 (violet), IR radiation emitted by the ground that is absorbed by greenhouse gases in the atmosphere (orange) and IR radiation emitted by the atmosphere (blue). 

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 simplified version.  Once you understand the upper figure, you should be able to find and understand the corresponding parts in the lower figure (especially since I've tried to use the same colors for each of the corresponding parts).

Some of the incoming sunlight (51 units in green) reaches the ground and is absorbed.  19 units of sunlight are absorbed by gases in the atmosphere.  The 30 units of reflected sunlight weren't included in the figure.

The ground emits a total of 117 units of IR light.  Only 6 shine through the atmosphere and go into space.  The remaining 111 units are absorbed by greenhouse gases. 

There were a few things I wanted you to notice in the bottom figure.

1.  Note that the ground is emitting 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).  That's thanks to the greenhouse effect.  If you're really paying attention you would notice that 117 units emitted doesn't balance 96 + 51 = 147 units absorbed.  The surface is emitting 117 units but an additional 30 units are being carried from the ground to the atmosphere by conduction, convection, and latent heat (at the far left of the figure).  That brings everything into balance (117 + 30 = 147).

2.  The ground is actually receiving more energy from the atmosphere (96 units) than it gets from the sun (51 units)!  Doesn't that seem odd?  I think the main reason for this is that the sun just shines for part of the day.  We receive energy from the atmosphere all the time, 24 hours per day.

3.  The atmosphere is emitting energy upward into space (64 units) and downward toward the ground (96 units).  Why are the amounts different? 


Anytime you get a question about the amount of radiant energy emitted by something (the something being the atmosphere) you should think of the Stefan Boltzmann law


So one reason might be that the lower atmosphere is warmer than the upper atmosphere (warm objects emit more energy than cold objects). 

But I think a bigger part of the explanation is probably that there is more air in the bottom of the atmosphere (the air is denser) than near the top of the atmosphere.  It is the air in the atmosphere that is emitting radiation.  More air =  more emission.

4. Back to the energy transported by conduction, convection, and latent heat.  Notice how small they are compared to EM radiation. 

We can use our simplified representation of the greenhouse effect to understand the effects of clouds on daytime and nighttime temperatures.  Does it get colder or stay warmer on a cloudy night compared to a clear night.  Does it get hotter or stay cooler on a cloudy day compared to a clear day.  The following can be found on pps. 72a & 72b in the ClassNotes (I've rearranged things slightly to try to make it clearer)



Here's the simplified picture of radiative equilibrium again (that you're probably getting pretty tired of seeing).  You should be able to say something about every arrow in the picture.  How would you modify the picture to make it a nighttime picture?




Note first of all that neither picture is in radiative equilibrium.  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 far infrared radiation (10 μm wavelength).  They reflect near IR light (1 μm wavelength) just like they do visible light.    If we could see 10 μm far IR light, clouds would appear black, very different from what we are used to (because clouds also emit IR light, the clouds might also glow).  That would pretty cool.  Because of the clouds 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: we'll draw 1 going upward into space, the other 2 downward to the ground. 

There is still a net loss of energy at the ground on the cloudy night but it's smaller, 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 somewhat warmer 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 (we see visible light and 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 daytime high and nighttime low temperature values on clear and cloudy days for this time of the year.



clear day
cloudy day
daytime high
temperature
 75 F
 65 to 70 F
nighttime low
temperature
 45 F
 50 to 55 F

The difference between the daytime high and the nighttime low, the daily or diurnal range of temperature is 30 F on the clear day and only 10 to 20 F on the cloudy day.

One last topic before we wrap up this whole section on energy, energy transport, and the greenhouse effect: a very common misconception regarding 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 (only about 7% of sunlight is UV) and just a portion of that would reach the ground.  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 (after long periods of exposure to the UV light) and those kinds of problems but not global warming.

If all 7% of the UV light in sunlight were to reach the ground it probably would cause some warming.  But it probably wouldn't matter because some of the shortest wavelength and most energetic forms of UV light would probably kill us and most other forms of life on earth.

Here's the cause of global warming and the reason for concern


Here's a whole progression of what we've been doing for the last class or two.

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, only 0 F.

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 downward 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.  This is what I refer to as the beneficial greenhouse effect.  It makes the earth more habitable (average surface temperature of 60 F versus the 0 F without a greenhouse effect).

In the right figure the concentration of greenhouse gases has increased even more (due to human activities).  The earth might 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.

This is the concern about global warming.  Increasing the concentrations of greenhouse gases in the atmosphere will strengthen the greenhouse effect which will warm the earth.

Here's the answer to the question found near the start of today's notes



The atmosphere is absorbing three units of energy: one comes from the sun, two are IR light coming from the ground. 

To see how those 3 units should be deployed we need to look at the ground.  The ground is absorbing 1 unit of sunlight and emitting 3 units of IR light.  It needs 2 arrows of the IR emitted by the atmosphere.  The 3rd unit of atmospheric IR goes up and into space.