Wed., Feb. 28 notes

Wed., Feb. 28, 2007

Optional Assignment #3 was collected today together with the short, one question, optional assignment question given in class on Monday. Here is the answer to that question.

Today was the first of two 1S1P Assignment #2 due dates. You can also turn in a report (no more than one report) on Friday this week. You can only turn in a total of 2 reports as part of this assignment. If you turned in two reports on Wednesday you can't turn in any additional reports on Friday.

The Experiment #3 materials were distributed for the first time in class today. The Experiment #4 materials should become available next week.

A preliminary version of the Quiz #2 Study Guide is now available online.

We need to finish up a couple more energy balance type situations.

The top figure shows energy balance on the earth with an atmosphere containing greenhouse gases.

We modify the figure somewhat in the bottom picture. The incoming sunlight has been removed; something that happens at night. The ground is emitting 3 units of energy and getting 1 back from the atmosphere. That is a net loss of 2 units. The ground will cool fairly rapidly during the night.

In the top figure it is still night but a layer of clouds has been added. The clouds are good absorbers of IR radiation even at wavelengths that would otherwise pass through the atmosphere. If people were able to see IR light instead of visible light, clouds would appear black - it is surprising how the appearance of something can change depending on the kind of light you are able to see.

Clouds reduce the net loss of energy at the ground. The ground still cools but cools more slowly and doesn't get as cold during the night.

The bottom figure is a daytime figure (the green arrows of sunlight are back in the picture ). The clouds will reflect some of the incoming sunlight (this is what gives clouds their white appearance) and reduce the daytime high temperatures (you can tell it is cooler because the ground is only emitting 2 arrows of IR energy).

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

One last thing before we leave energy transport, energy balance, and the greenhouse effect.

The figure on the left shows energy balance on the earth without an atmosphere or with an atmosphere that doesn't contain greenhouse gases (so that IR energy emitted by the ground isn't absorbed by gases in the atmosphere). The ground only needs to emit 2 units of energy to reach energy balance. The ground wouldn't need to be very warm to do this.

In the middle picture the natural greenhouse effect is in operation. The ground is warmer and emitting 3 units of energy. It can do this even though it is only getting 2 units of energy from the sun because greenhouse gases absorb some of the emitted energy and emit some of it back to the ground.

In the right figure the concentration of greenhouse gases has increased (due to human activities). The ground has warmed even more (global warming) and is now emitting 4 units of energy and still only getting 2 from the sun. But 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 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

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.

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 you moved to the other side of the scene and looked back toward the sun. 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.

We barely had enough time to cover the top of p. 76

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.

We viewed a couple of short video experiments showing the solar irradiance experiment (Experiment #3). You'll find some information about that experiment here.