NATS 101 Intro to Weather and Climate Lecture

NATS 101 Intro to Weather and Climate Lecture 7 Seasonality

Supplemental References for Today’s Lecture Aguado, E. and J. E. Burt, 2001: Understanding Weather & Climate, 2 nd Ed. 505 pp. Prentice Hall. (ISBN 0 -13 -027394 -5) Danielson, E. W. , J. Levin and E. Abrams, 1998: Meteorology. 462 pp. Mc. Graw-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 Introduction to the Atmosphere, 8 th 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)

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

Eccentricity of Orbit Perihelion Aphelion Ahrens (2 nd 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?

147 million km Ahrens, Fig. 2. 17 152 million km

Solar Zenith Angle Equal Energy e rg La ea Ar Long Path o 23. 5 Short Path Small Area Ahrens, Fig. 2. 19 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

Beam Spreading Ahrens, Fig. 2. 16 Large Zenith Angle Zero Zenith Angle Small Zenith Angle Large Zenith Angle Low Zenith - Large Area, Much Spreading High Zenith - Small Area, Little Spreading

Beam Spreading Schematic Ignores Earth’s Curvature

Atmospheric Path Length Schematic Ignores Earth’s Curvature Cloud

Length of Day Lutgens & Tarbuck, p 33

Day Hours at Solstices - US Sites Summer-Winter Arctic Circle Gedzelman, p 67 Tucson (32 o 13’ N) 14: 15 - 10: 03 Seattle (47 o 38’ N) 16: 00 - 8: 25 Anchorage (61 o 13’ N) 19: 22 - 5: 28 Fairbanks (64 o 49’ N) 21: 47 - 3: 42 Hilo (19 o 43’ N) 13: 19 - 10: 46

Path of Sun Hours of daylight increase from winter to summer pole Equator always has 12 hours of daylight Summer pole has 24 hours of daylight Winter pole has 24 hours of darkness Note different Zeniths Danielson et al. , p 75

Noon Zenith at Solstices Summer-Winter Aguado & Burt, p 46 Tucson AZ (32 o 13’ N) 08 o 43’ - 55 o 43’ Seattle WA (47 o 38’ N) 24 o 08’ - 71 o 08’ Anchorage AK (61 o 13’ N) 37 o 43’ - 84 o 43’ Fairbanks AK (64 o 49’ N) 41 o 19’ - 88 o 19’ Hilo HI (19 o 43’ N) 3 o 47’ (north) - 43 o 13’

C W W C Wallace and Hobbs, p 346 Incoming Solar Radiation (Insolation) at the Top of the Atmosphere

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

Annual Energy Balance Radiative Cooling NH Radiative Warming Ahrens, Fig. 2. 21 Radiative Cooling SH Heat transfer done by winds and ocean currents Differential heating drives winds and currents We will examine later in course

Summary • Tilt (23. 5 o) 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

Assignment for Lecture • Ahrens Pages 55 -64 Problems 3. 1, 3. 2, 3. 5, 3. 6, 3. 14