METR 104: Our Dynamic Weather (Lecture w/Lab) Reading Assignment #2 (for classes starting Wednesday, Sept. 26) Dr. Dave Dempsey Dept. of Geosciences SFSU, Fall 2012

## From Introduction to the Atmosphere and the Science of Meteorology, Lutgens and Tarbuck, Chapter 1, "Introduction to the Atmosphere", pp. 28-30.

(See "Reading Assignments" for a description of the purpose and value of these Reading Questions.)

### Extent of the Atmosphere (pp. 28-29)

1. Why can't we identify the "top" of the atmosphere?

2. How does the text say that we can interpret the meaning of atmospheric pressure?
[Note that the text is quite misleading on this point! Pressure arises in any material (solid, liquid, or gas) because the molecules of all material are in constant, random motion, vibrating, rotating, and (in gases) zipping around from place to place. As a result of these motions, molecules inevitably collide with surrounding molecules, and by colliding with them, exert a force on them, pushing against them. The total force exerted by molecules of a material colliding against each unit of surface area of the surrounding material, is what we call pressure. In particular, the total force exerted by air against each unit of surface area of surrounding air or the earth's surface is called atmospheric or barometric pressure.

In contrast, the force exerted on any object by the earth's gravity is called the object's weight. This is completely different from pressure! However, it is true that the pressure exerted by air at any particular altitude is approximately equal to the total weight of air above a unit of surface area above that altitude. That's actually what the text means when it says, incorrectly, that "atmospheric pressure is simply the weight of the air above".

As a consequence of this relationship between air pressure and the weight of air above a unit of area, air pressure necessarily decreases with increasing altitude. As you go higher, there is simply less and less air still above you, so the total weight of air still above you is less and less.

3. What is a typical sea level pressure (expressed in millibars)? What is this pressure equivalent to, in terms of weight of air above a surface area of one square centimeter (about the area of one of your fingernails) and above 1 square inch (which is 6.25 times larger than 1 square centimeter)?
4. To what altitude would you have to go so that 50% of all air molecules (and hence 50% of the weight of the atmosphere) is below you? How high would you have to go so that 90% of air is below you?
5. Where in the atmosphere does air pressure decrease the most rapidly as you go up? [See Figure 27.]
6. What does the text mean when it says that air is highly compressible?

### Thermal Structure of the Atmosphere (pp. 29-30)

Troposphere

1. What behavior of air distinguishes the troposphere (lowest atmospheric layer) from higher layers?
2. How rapidly does temperature decrease with increasing altitude in the troposphere, on the average (that is, what is the average environmental lapse rate)?
3. What is a temperature inversion?
4. How deep is the troposphere, on the average? Is it the same depth everywhere?
5. In what layer do all important weather phenomena occur?

Stratosphere

6. What is the tropopause?
7. How does temperature vary with increasing altitude in the stratosphere?
8. How high does the stratosphere extend? What is the stratopause?
9. Why do temperatures in the stratosphere stop decreasing with altitude and start increasing?

Mesosphere (optional)
Thermosphere (optional)

### Vertical Variations in Composition (pp. 30-31)

(optional)

Chapter Summary (pp. 32-33): 12th and 13th bulleted items
Vocabulary Review (p. 34): A few of these (especially if they appear in italics in the questions above).
Review Questions (p. 34): #'s 5-6 and 16.

## From Introduction to the Atmosphere and the Science of Meteorology, Lutgens and Tarbuck, Chapter 2, "Heating Earth's Surface and Atmosphere", pp. 39-47.

Earth-Sun Relationships (pp. 40-47)

1. In what respects does solar energy received by the earth vary with spatial position and with time?
2. What are consequences of unequal heating of the earth by the sun?
3. How do ocean currents and winds affect the distribution of heat created by unequal heating by the sun?
4. How does the text interpret "weather" in the context of the previous three questions? What would happen to "weather" the sun were to go dark?

Earth's Motions

5. What are rotation and revolution, the two principle motions that the earth undergoes?
6. What is the average distance between the earth and the sun?
7. What is the range of distances between the earth and the sun over the course of a year? When during the year is the earth closest to the sun? When is it farthest?
8. How large a role do variations in distance between the earth and the sun play in producing seasonal temperature variations? What is one piece of evidence for this conclusion?

The Seasons

9. What are two aspects of solar radiation that vary seasonally? How does each vary over the course of the year?
10. How does the concentration of solar radiation on a horizontal surface vary with the angle between the sun and the horizontal surface? [This is the "spreading out" effect due to variations in sun angle.]
11. How does the distance that solar radiation travels through the atmosphere vary as the sun angle varies (see Figure 4)?
12. What can happen to solar radiation as it travels through the atmosphere?
13. What consequence does your answers to the previous two questions have for how the intensity of solar radiation on a horizontal surface at the earth's surface varies as sun angle varies? Does this consequences reinforce the "spreading out" effect, or does it compensate for it?
14. On any particular day, how much of the earth experiences the sun directly overhead during the day (at "solar noon")? Why?
15. How does the sun angle at noon vary with latitude the farther you are from the latitude where the sun angle at solar noon is 90°?
16. What are the two most important reasons why the amount of solar radiation received during a day at any particular location varies over the course of a year?

Earth's Orientation

17. Why do the sun angle at solar noon and the length of daylight vary over the course of the year?
18. What is the earth's axis of rotation? What is the angle between the axis of rotation and the plane of the earth's orbit around the sun? If this angle were 0°, what would the seasons be like?
19. How does the orientation of the axis of rotation relative to distant stars vary as the earth revolves around the sun?
20. How does the orientation of the axis of rotation relative to the sun vary as the earth revolves around the sun? What is the range of angles toward and away from the sun that the axis of rotation tilts (or "leans")?
21. What is the range of latitudes where the sun can appear directly overheat at solar noon?
22. How much can the sun angle at solar noon vary over the course of the year at the latitude of San Francisco (and New York and all other midlatitudes and some low and high latitudes, too)? [Note: the sun angle at solar noon in San Francisco reaches its maximum of 76° above the horizon on June 21 or 22 each year, and it reaches its minimum on December 21 or 22. How low is the lowest noon-time sun angle here?]

Solstices and Equinoxes

23. What is special about June 21 or 22 each year (the date of one of the two solstices)? What is special about the Tropic of Cancer (23.5°N latitude)? When is the first day of summer in the Northern Hemisphere? (What day is this in the Southern Hemisphere?)
24. What is special about December 21 or 22 each year (the date of the other solstice)? What is special about the Tropic of Capricorn (23.5°S latitude)? When is the first day of winter in the Northern Hemisphere? (What day is this in the Southern Hemisphere?)
25. What is special about March 21 or 22 and September 21 or 22 (the dates of the two equinoxes)? When is the first day of spring in the Northern Hemisphere? (What day is this in the Southern Hemisphere?)
26. What is the circle of illumination? How can we use it to determine whether the length of daylight at any particular latitude is more than, less than, or equal to 12 hours?
27. What can we say about the length of daylight everywhere in the Northern Hemisphere at the June solstice? What about in the Southern Hemisphere at that time? [Note: Your answers would be the same for the six months from after the March equinox until the September equinox.]
28. What can we say about the length of daylight everywhere in the Northern Hemisphere at the December solstice? What about in the Southern Hemisphere at that time? [Note: Your answers would be the same for the six months from after the September equinox until the March equinox.]
29. How does the length of daylight vary with increasing latitude in the Northern Hemisphere at the June solstice?
30. The text lists four facts about the June solstice as they apply in the Northern Hemisphere (where it is the first day of summer). What would these facts be on the same date for the Southern Hemisphere (where it is the first day of winter)?

[Note: The text says that "It should now be apparent why a midlatitude location is warmest in the summer. It is then that the days are longest and the angle of the sun is highest." However, the length of daylight is greatest and the sun is highest at the summer solstice, which is the day summer begins (and spring ends). After the summer solstice, both length of daylight and sun angle decrease over the course of the summer, which ends at the fall (autumn) equinox, the first day of fall, when the length of daylight is 12 hours everywhere. Similarly, at the spring equinox, the length of daylight is 12 hours and the noon sun angle is the same as it is at the fall equinox, and both increase until they reach their maximum at the summer solstice. Hence, length of daylight and sun angle at solar noon, averaged over the whole season, are the same during the spring and during the summer! Conclusion: it isn't quite so apparent after all why a midlatitude location is warmest in the summer. Why not late spring and early summer, when the length of daylight and sun angles are equally great?]

31. What does "equinox" mean? (What is the length of daylight everywhere on earth at the equinoxes?)
32. According to Figure 9, during what month(s) are temperatures at several locations in the Northern Hemisphere reach their maximum? (In each case, does this month coincide with the time when length of daylight and sun angle are greatest?)
33. According to Figure 9, during what month are temperatures at Capetown, South Africa (in the Southern Hemisphere) the lowest? Does this month coincide with the time when length of daylight and sun angle are the least there (at the June solstice)?

Chapter Summary (pp. 64-65): First three bulleted items
Vocabulary Review (p. 65): Several of these (especially if they appear in italics in the questions above).
Review Questions (p. 66): #'s 1-3.

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