NATS 101 Intro to Weather and Climate Lecture 7 Seasonality

1. NATS 101Intro to Weather and ClimateLecture 7Seasonality

2. 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)

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

4. 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?

5. 147 million km 152 million km Ahrens, Fig. 2.17

6. 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

7. 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

8. Schematic Ignores Earth’s Curvature Beam Spreading

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

10. Length of Day Lutgens & Tarbuck, p33

11. 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

12. 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

13. Noon Zenith 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

14. Incoming Solar Radiation (Insolation) at the Top of the Atmosphere W C C W Wallace and Hobbs, p346

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

16. 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

17. Summary • Tilt (23.5o) is primary reason for seasons Tilt changes two important factorsAngle at which solar rays strike the earth Number of 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

18. Assignment for Lecture • Ahrens Pages 55-64 Problems 3.1, 3.2, 3.5, 3.6, 3.14