Mon., Mar. 10 notes

Monday Mar. 10, 2008

Quiz #2 is Wednesday this week. The Quiz #2 Study Guide is still available online.

The 3rd Optional Assignment was collected in class today. A sheet with answers to the questions was distributed in class.
You'll find answers to previous "in-class" and "hidden" optional assignments online (go back to the class home page).

The Expt. #2 reports have been graded (after a superhuman effort) and were returned in class. You are allowed to revise your reports. The revised reports are due on or before Wed., Mar. 26 (the Wednesday after Spring Break).

The Experiment #4 materials were also handed out in class. There are still several sets of Expt. #4 materials (and some Expt. #3 materials) available. They'll be back in class on Wednesday. You should already have done an experiment or be working on an experiment (or book report). If you haven't you should check out the necessary materials this week.

Don't forget that the 1S1P Assignment #2 reports (and optional worksheets) are due this week. If you don't turn in your report in class you will need to bring it by my office (PAS 588) by the end of the day on Friday.

We'll wrap up the section on radiative equilibrium and the atmospheric greenhouse effect and start some new material today.

You can use the simplified picture of radiative equilibrium to understand the effects of clouds on nighttime low and daytime high temperatures. You'll find this discussed on pps 72a and 72b in the Classnotes.

Here's the simplified picture of radiative equilibrium (something you're probably getting pretty tired of seeing). By now you should be able to identify each of the colored arrows in the figure above and explain what they represent.

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, 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. 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.

Typical daytime highs and nighttime lows in Tucson for this time of year. Note how the clouds reduce the daily range of temperature.

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.

Many people know that sunlight contains UV light and that the ozone absorbs much of the 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.

Next up in NATS 101 - Causes of the seasons
This new material won't be covered on Quiz #2

First some very basic information (that every college graduate should know)

Many people would have missed the 3rd question. Many people think the moon orbits the earth in about a day. This is because they see it in about the same position in the sky on successive nights. We can see what actually happens in the next figure (not shown in class)

On the first night in Fig. A the person looks up and sees the moon. One day later on night B, the earth has completed one rotation on its axis and the person is looking up at the same point in space. The person doesn't see the moon in the same position as the night before; the moon has moved a little bit in its orbit. In Fig. C, a little more than 24 hours after Fig. A, the person again sees the moon overhead. If you were to make a note of the time the moon rises you would notice it rises a little later each successive night.

Many people know that the earth's orbit around the sun is not circular and that the distance between the earth and sun changes during the year. Many people think this is the main cause of the seasons. The earth is closest to the sun on the perihelion, furthest on the apehelion.

The earth is closer to the sun in January than in July. If this were the main cause of the seasons, summer in Tucson would be in January and winter would be in July. Summer and winter would both occur at the same times in both hemispheres. Neither of these is true. The changing distance between the earth and the sun has an effect but is not the main cause of seasonal changes.

The main cause of the seasons is the fact that the earth is tilted with respect to its orbit around the sun. This is shown in the next figure.

This figure shows the tilted earth at four locations in its orbit around the sun. You should be able to start with a blank sheet of paper and draw a picture like this. Note how the N. Pole tilts away from the sun on Dec. 21st, the winter solstice. The N. Pole is tilted toward the sun on June 21. Those are good places for you to start your sketch. You should also be able to name and attach a date to each of the four locations.

Before going on, try to imagine what this picture would like if instead of standing at Point A you moved to the other side of the scene and looked back toward the sun from Point B. This possibility wasn't covered in class. Click here for a sketch.

Seasons on the earth are caused by the changing orientation of the earth relative to the sun. The figure above doesn't really explain why this is true.

In the summer when the sun reaches a high elevation angle above the horizon, an incoming beam of sunlight will shine on a small area of ground. The ground will get hot. The two people sharing the shaft of summer sunlight will get a sunburn.

In the winter the sun is lower in the sky. The same beam of sunlight gets spread out over a larger area. The energy is being used to try heat a larger amount of ground. The result is the the ground won't get as hot. 4 people are able to share the winter sunlight and won't get burned as quickly.

As sunlight passes through the atmosphere it can be absorbed or reflected. On average (over the globe) only about 50% of the sunlight arriving at the top of the atmosphere actually makes it to the ground. A beam of sunlight that travels through the atmosphere at a low angle (right picture above) is less intense than beam that passes through the atmosphere more directly (left picture).

The sun shines for more time in the summer than in the winter. In Tucson the days (daylight hours) are around 14 hours long near the time of the summer solstice. In the winter the sun only shines for 10 hours on the winter solstice. Days are 12 hours long on the equinoxes.