1. TURN YOUR CLICKER ON

2. Announcements
TAs will go over lab experiments today. Kits distributed Friday.
I will not be taking individual student questions after class at the front of the room on Mondays and Wednesdays. Please see me during office hours.
If you did not get assigned to an assignment group, you may go in whichever group you like.
Note taker is still needed for two students. If you would be interested, please see or me. You will receive recognition for your efforts from the DRC.

3. Review Question: A greenhouse gas is called that because____________________
It mainly reflects and scatters shortwave radiation
It mainly absorbs and emits shortwave radiation
C) It mainly reflects and scatters longwave radiation
D) It mainly absorbs and emits longwave radiation
E) All of the above

4. Survey Question: In June, in which direction will the sun set in Tucson, AZ?
Southwest
West
C) Northwest

5. Summary of Lecture 6
Radiation in the atmosphere has four possible fates: transmitted, reflected, scattered, or absorbed. A perfect absorber and emitter of radiation is called a blackbody.
The atmosphere selectively reflects, scatters and absorbs radiation at certain wavelengths, which depend on the specific gas constituents.
Absorbed radiation increases the internal energy by changes on the molecular and atomic level. Terrestrial radiation is associated with translational, rotational, and vibrational energy transitions on the molecular level. Solar radiation is associated with electronic energy transitions on the atomic level.
Greenhouse gases are those which absorb and emit very effectively in the infrared, like water and CO2. Because of them the atmosphere is very opaque to terrestrial radiation and the Earth’s surface temperature is maintained. Reviewed the atmospheric energy budget to prove the point.
Though the atmosphere is fairly transparent to solar radiation, scattering and reflection of solar radiation is important. Scattering of visible light is why the sky is blue!

6. NATS 101 Section 4: Lecture 7
The Seasons

7. The Importance of Seasons
The seasons govern both natural and human patterns of behavior. Some big and small examples:
Planting and harvesting of crops
Migratory patterns of animals
Deciduous trees
Types of sports
What kinds of clothes you wear
Where you go on vacation
The Importance of Seasons

8. Human beings throughout the world figured out a long time ago that seasons were related to astronomical changes they consistently observed—and this became a central theme in many cultures

9. Stonehenge, on the Salisbury Plain in southern England, is essentially an astronomical observatory, build thousands of years ago to detect the first day of summer. We’ll see how it works a bit later…

10. So what causes the seasons?
To answer that, first we have to understand some astronomical characteristics of the Earth’s orbit around the Sun.

11. Three orbital parameters of the Earth
Eccentricity
Precession
Obliquity
Which one is responsible for the occurrence of the seasons?

12. Eccentricity: Elliptical character of orbit
APHELION
PERIHELION
The Earth has an elliptical orbit around the Sun (not circular)
Earth actually gets 7% MORE solar radiation in January than July! So this cannot possibly explain why July could be warmer (at least in our part of the world)….

13. Is this really a popular belief out there?
So the reason it gets warmer in summer is NOT because the Earth is closer to the sun!
Is this really
a popular belief out there?

14. Precession: Change in Time of Perihelion and Aphelion
PRESENT
DAY
PERIHELION
APHELION
ABOUT 11,000 YEARS IN THE FUTURE
PERIHELION
APHELION
Position of perihelion and aphelion reverse

15. Obliquity: Tilt of the Earth with respect to its orbital plane
NORTH POLE
23.5° Angle of tilt
EQUATOR
SOLAR RADIATION
Orbital Plane
SOUTH POLE
As the Earth rotates around the sun, it’s axis of rotation is tilted at an angle of 23.5°. This is the factor that is responsible for the seasons.

16. The Moon is the obliquity “stabilizer”
Interesting aside:
The Moon is the obliquity “stabilizer”
The gravitational presence of the moon helps maintain the Earth’s obliquity at a fairly constant angle. With out it, the obliquity would vary wildly, wreaking havoc on Earth’s climate!
Subject of a documentary program entitled “What if the Earth Had no Moon?”, narrated by Patrick Stewart (a.k.a. Captain Jean-Luc Picard)

17. The Zenith Angle
Zero zenith angle
Zero zenith angle
Large zenith angle
Large zenith angle
Intensity of solar energy depends the angle it strikes the earth. This is called the zenith angle.
Solar beam perpendicular = Zenith angle is zero
Solar energy most intense
Solar beam tilted = Large zenith angle
Solar energy weaker

18. atmospheric attenuation of solar energy
Zenith angle and
atmospheric attenuation of solar energy
The presence of the Earth’s atmosphere also weakens the amount of incoming solar radiation.
If the zenith angle is large, the solar beam has to pass through more atmosphere to reach the surface
So more absorption and scattering of solar radiation.
Large zenith angle
Sun low in the sky
Longer beam path
Zero zenith angle
Sun directly overhead
Shortest beam path

19. Obliquity and Seasonal Cycle
The variation in the amount of solar radiation through the year due to the obliquity of the Earth is what causes the seasons. Two ways this occurs:
Change in the zenith angle
Change in the length of day

20. Seasonal Change in Day Length
Lutgens & Tarbuck, p33

21. Around
December 21
Winter solstice Northern Hemisphere Summer solstice Southern Hemisphere
ARCTIC CIRCLE: 66.5°N
Limit of permanent darkness
TROPIC OF CAPRICORN: 23.5°S
Sun directly over head
ANTARCTIC CIRCLE: 66.5°S
Limit of permanent sunlight

22. Northern Hemisphere Winter Solstice Southern Hemisphere
Summer Solstice
Northern Hemisphere Winter Solstice Southern Hemisphere
Around
June 21
ARCTIC CIRCLE: 66.5°N
Limit of permanent sunlight
TROPIC OF CANCER: 23.5°N
Sun directly over head
ANTARCTIC CIRCLE: 66.5°S
Limit of permanent darkness

23. Alaska: Land of the Midnight Sun
SUN LOWEST IN SKY
DUE NORTH

24. Autumn: Summer to Winter Spring: Winter to Summer
Autumn equinox:
September 22
Spring equinox:
March 20
Equinox
Autumn: Summer to Winter
Spring: Winter to Summer
EQUATOR: 0°
Sun directly over head
Day and night are each equal to 12 hours at every point on Earth.

25. The farther you are away from the tropics (23. 5°S to 23
The farther you are away from the tropics (23.5°S to 23.5°N), the lower in the sky the Sun will be.
The figure here is for the Northern hemisphere. Flip the image and you’ll get what happens in the Southern Hemisphere.
For Tucson (~32° N):
Summer solstice:
Day length: 14 hours
Zenith angle: 8°
Winter solstice:
Day length: 10 hours
Zenith angle: 55°
Danielson et al., p75

26. Sun rises due East
SUMMER SOLSTICE PATH
Sun rises
due Southeast
Sun rises due
Northeast
EQUINOX PATH
WINTER SOLSTICE PATH
Sun sets
due West
Sun sets
due Southwest
Sun sets due
Southwest
SUN ALWAYS TO THE SOUTH AT SOLAR NOON ALL YEAR

27. Now let’s see how Stonehenge is actually an ancient astronomical observatory…

28. Stonehenge and the summer solstice
BBC image
Stonehenge Aoteoroa in New Zealand on Dec. 21
Photo by C.J. Picking

29. Considering all the concepts we’ve discussed today, let’s get a brief preview of how this understanding helps us to understand weather and climate.

30. Because of the variation in zenith angle through the year and with latitude, amount of solar energy absorbed at the top of the atmosphere varies….

31. This means there is an imbalance of incoming vs. outgoing radiation.
Summer hemisphere has a net surplus of radiation
Winter hemisphere has a net deficit of radiation.
Lutgens & Tarbuck, p51

32. Earth’s Net Radiation Balance
The equator doesn’t keep getting warmer and warmer.
The high latitudes don’t keeping getting colder and colder.
Therefore there must be ways that heat is transferred from equator to pole.

33. Summary of Lecture 7
The three orbital parameters of the Earth are the eccentricity, precession, and obliquity. Most relevant to the discussion of the seasons is the obliquity, or tilt of the Earth with respect to its orbital plane (at 23.5°).
The intensity of solar energy depends on the zenith angle. If the sun is directly overhead the zenith angle is equal to zero and the solar energy is most intense.
Solar energy is further attenuated at high zenith angles due to the fact that the solar bean has more atmosphere to pass through.
Earth’s obliquity causes variation in solar radiation by changes in the zenith angle and length of day through the year—and thus is the cause of the seasons.
Special latitudes are associated with the solstices and equinoxes. Know what these special latitudes are and what they physically mean. Know dates when the solstices and equinoxes occur.

34. Reading Assignment and Review Questions
Ahrens, Chapter 3, pp (8th ed.)
pp (9th ed.)
Chapter 3 Questions
Questions for Review: 1,2,3,4,5
Questions for Thought: 1,2,3,4,5
Problems and Exercises: 3