NATS 101 Lecture 5 Greenhouse Effect and Earth-Atmo Energy Balance and the Seasons

1. NATS 101Lecture 5Greenhouse Effect and Earth-Atmo Energy Balanceandthe Seasons

2. Review Items • Heat Transfer • Latent Heat • Wien’s Displacement Law Ramifications • Stefan-Boltzman Law Ramifications

3. New Business • Selective Absorption and Emission • Earth-Atmo Energy Balance

4. Modes of Heat Transfer Williams, p. 19 Latent Heat Convection Conduction Radiation Remember this thought experiment and the incandescent light bulb thru the prism

5. Latent Heat Take 2 Williams, p 63 Takes energy from environment Emits energy to environment

6. General Laws of Radiation • All objects above 0 K emit radiant energy • Hotter objects radiate more energy per unit area than colder objects, result of Stefan-Boltzman Law • The hotter the radiating body, the shorter the wavelength of maximum radiation, result ofWien’s Displacement Law • Objects that are good absorbers of radiation are also good emitters…today’s lecture!

7. Sun’s Radiation Spectrum Planck’s Law Ahrens, Fig. 2.7 Key concept: Radiation is spread unevenly across all wavelengths

8. Sun - Earth Radiation Spectra Ahrens, Fig. 2.8 Planck’s Law Key concepts: Wien’s Law and Stefan-Boltzman Law

9. What is Radiative Temperature of Sun if Max Emission Occurs at 0.5 m? • Apply Wien’s Displacement Law

10. How Much More Energy is Emitted by the Sun than the Earth? • ApplyStefan-Boltzman Law

11. Radiative Equilibrium • Radiation absorbed by an object increases the energy of the object. • Increased energy causes temperature to increase(warming). • Radiation emitted by an object decreases the energy of the object. • Decreased energy causes temperature to decrease(cooling).

12. Radiative Equilibrium (cont.) • When the energy absorbed equals energy emitted, this is calledRadiative Equilibrium. • The corresponding temperature is theRadiative Equilibrium Temperature.

13. Why Selective, Discrete Absorption/Emission? Life as we perceive it: A continuous world! Atomic perspective:A quantum world! Gedzelman 1980, p 103

14. Energy States for Atoms Gedzelman 1980, p 104 Hydrogen Atom Electrons can orbit inonly permitted states A state corresponds tospecific energy level Onlyquantum jumpsbetween states Intervals correspond tospecific wavelengths

15. Energy States for Molecules Molecules can rotate, vibrate But only atspecific energy levels or frequencies Quantum intervals between modes correspond tospecific wavelengths Gedzelman 1980, p 105 H2O molecule H2O Bands

16. Selective Absorption The Bottom Line Each molecule has aunique distribution of quantum states! Each molecule has a unique spectrum of absorption and emission frequencies of radiation! H2O molecule Williams, p 63

17. Absorption Visible IR Visible (0.4-0.7 m) is absorbed very little O2 an O3 absorb UV (shorter than 0.3 m) Infrared (5-20 m) is selectively absorbed H2O & CO2 are strong absorbers of IR Little absorption of IR around 10 m – atmospheric window Ahrens, Fig. 2.9

18. Visible radiation (0.4-0.7 m) is not absorbed Infrared radiation (5-20 m) is selectively absorbed, but there is an emission window at 10 m Total Atmospheric Absorption Ahrens, Fig. 2.9

19. Global Solar Radiation Balance (Only half of Solar Radiation SR reaches the surface) 30% SR reflects back to space Albedo: percent of total SR reflected ~20% absorbed by atmosphere 70% SR absorbed by earth-atmosphere Ahrens, Fig. 2.13 ~50% SR absorbed by surface

20. Atmosphere Heated from Below Ahrens, Fig. 2.11 old ed. Air above ground heats by convection and absorption of some IR from ground Net Effect: Atmosphere is Heated From Below Air contacting ground heats by conduction Ground heats further through absorption of IR from atmosphere Solar radiation heats the ground

21. Global Atmo Energy Balance Ahrens, Fig. 2.14 Solar Atmosphere Ground

22. Summary • Greenhouse Effect (A Misnomer) Surface Warmer than Rad. Equil. Temp Reason: selective absorption of air H2O and CO2 most absorbent of IR • Energy Balance Complex system has a delicate balance All modes of Heat Transfer are important

23. NATS 101Intro to Weather and ClimateNext subject:TheSeasons

24. Supplemental References for Today’s Lecture Aguado, E. and J. E. Burt, 2001: Understanding Weather & Climate, 2nd Ed.505 pp. Prentice Hall. (ISBN 0-13-027394-5) Danielson, E. W., J. Levin and E. Abrams, 1998: Meteorology. 462 pp. McGraw-Hill. (ISBN 0-697-21711-6) Gedzelman, S. D., 1980: The Science and Wonders of the Atmosphere. 535 pp. John-Wiley & Sons. (ISBN 0-471-02972-6) Lutgens, F. K. and E. J. Tarbuck, 2001: The Atmosphere, An Intro-duction to the Atmosphere, 8th Ed. 484 pp. Prentice Hall. (ISBN 0-13-087957-6) Wallace, J. M. and P. V. Hobbs, 1977: Atmospheric Science, An Introductory Survey. 467 pp. Academic Press. (ISBN 0-12-732950-1)

25. Reasons for Seasons • Tilt of Earth’s Axis - Obliquity Angle between the Equatorial Plane and the Orbital Plane • Eccentricity of Earth’s Orbit Elongation of Orbital Axis

26. Eccentricity of Orbit Perihelion Aphelion Ahrens (2nd Ed.), akin to Fig. 2.15 Earth is 5 million km closer to sun in January than in July. Solar radiation is 7% more intense in January than in July. Why is July warmer than January in Northern Hemisphere?

27. 147 million km 152 million km Ahrens, Fig. 2.17

28. Solar Zenith Angle Depends onlatitude, time of day & season Has two effects on an incoming solar beam Surface area covered or Spreading of beam Path length through atmosphere or Attenuation of beam Long Path Large Area Equal Energy 23.5o Small Area Short Path Ahrens, Fig. 2.19

29. Ahrens, Fig. 2.16 Large Zenith Angle Small Zenith Angle Zero Zenith Angle Large Zenith Angle Beam Spreading Low Zenith- Large Area, Much Spreading High Zenith - Small Area, Little Spreading

30. Schematic Ignores Earth’s Curvature Beam Spreading

31. Schematic Ignores Earth’s Curvature Cloud Atmospheric Path Length

32. Length of Day Lutgens & Tarbuck, p33

33. Day Hours at Solstices - US Sites Summer-Winter Tucson (32o 13’ N) 14:15 - 10:03 Seattle (47o 38’ N) 16:00 - 8:25 Anchorage (61o 13’ N) 19:22 - 5:28 Fairbanks (64o 49’ N) 21:47 - 3:42 Hilo (19o 43’ N) 13:19 - 10:46 Arctic Circle Gedzelman, p67

34. Path of Sun Hours of daylight increase from winter to summer pole Equator always has12 hours of daylight Summer pole has 24 hours of daylight Winter pole has24 hours of darkness Note different Zeniths Danielson et al., p75

35. Noon Zenith Angle at Solstices Summer-Winter Tucson AZ (32o 13’ N) 08o 43’ - 55o 43’ Seattle WA (47o 38’ N) 24o 08’ - 71o 08’ Anchorage AK (61o 13’ N) 37o 43’ - 84o 43’ Fairbanks AK (64o 49’ N) 41o 19’ - 88o 19’ Hilo HI (19o 43’ N) 3o 47’ (north) - 43o 13’ Aguado & Burt, p46

36. Is Longest Day the Hottest Day? Consider Average Daily Temperature for Chicago IL: USA Today WWW Site

37. Annual Energy Balance Radiative Warming Radiative Cooling Radiative Cooling Heat transfer done by winds and ocean currents Differential heating drives winds and currents We will examine later in course NH SH Ahrens, Fig. 2.21

38. Summary • Tilt (23.5o) is primary reason for seasons Tilt changes two important factors • Angle at which solar rays strike the earth • Numberof hours of daylight each day • Warmest and Coldest Days of YearOccur after solstices, typically around a month • Requirement for Heat TransportDone by Atmosphere-Ocean System

39. Assignments for Next Lectures • Ahrens (next lecture) Pages 55-64 Problems: 3.1, 3.2, 3.5, 3.6, 3.14