1. NATS 101 Intro to Weather and Climate Lecture 7 Seasonality

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

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 on latitude, 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
Short Path
Small Area
Ahrens, Fig. 2.19

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

8. Beam Spreading
Schematic Ignores Earth’s Curvature

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

10. Length of Day
Lutgens & Tarbuck, p33

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

12. Path of Sun Hours of daylight increase from winter to summer pole
Equator always has hours of daylight
Summer pole has hours of daylight
Winter pole has 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) o 47’ (north) - 43o 13’
Aguado & Burt, p46

14. Incoming Solar Radiation (Insolation) at the Top of the AtmosphereW W C
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 Heat transfer done by winds and ocean currents
Radiative Warming
Radiative Cooling
Radiative Cooling NH SH
Ahrens, Fig. 2.21
Heat transfer done by winds and ocean currents
Differential heating drives winds and currents
We will examine later in course

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

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