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TURN YOUR CLICKER ON

TURN YOUR CLICKER ON. 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.

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TURN YOUR CLICKER ON

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  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 email 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

  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. 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. 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. 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. Winter solsticeNorthern HemisphereSummer solsticeSouthern Hemisphere Around December 21 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. Summer Solstice Northern HemisphereWinter SolsticeSouthern 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 MIDNIGHT SUN LOWEST IN SKY DUE NORTH

  24. 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 23.5°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 http://www.southernskyphoto.com/planet_earth 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. 63-82 (8th ed.) pp. 65-84 (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

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