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 ) 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, 8 th Ed. 484 pp. Prentice Hall. (ISBN ) Wallace, J. M. and P. V. Hobbs, 1977: Atmospheric Science, An Introductory Survey. 467 pp. Academic Press. (ISBN )

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

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

147 million km152 million km Ahrens, Fig. 2.17

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 Ahrens, Fig Large Area Small Area Short Path Long Path Equal Energy 23.5 o

Ahrens, Fig Large Zenith Angle Zero Zenith Angle Large Zenith Angle Small Zenith Angle Beam Spreading High Sun – Power Spread over Smaller Area Low Sun – Power Spread over Larger Area

Quantifying Beam Spreading Schematic Ignores Earth’s Curvature

Atmospheric Path Length Schematic Ignores Earth’s Curvature Cloud

Length of Day Lutgens & Tarbuck, p33

Day Hours at Solstices - US Sites Summer-Winter Tucson (32 o 13’ N) 14: :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: :46 Gedzelman, p67 Arctic Circle

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., p75

Solar Declination Aguado & Burt, p46 Solstice Equinox

Noon Zenith at Solstices Summer-Winter 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’ Aguado & Burt, p46

Incoming Solar Radiation (Insolation) at the Top of the Atmosphere C C W W

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

Radiation Budget Summer hemisphere shows a surplus, warms Winter hemisphere shows a deficit, cools Equator/S. Pole always shows a surplus/deficit Why doesn’t the equator warm and S. Pole cool? Lutgens & Tarbuck, p51 NH SH

Annual Energy Balance Heat transfer done by winds and ocean currents NHSH Radiative Warming Radiative Cooling Ahrens, Fig 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 a month later Requirement for Heat Transport Done by Atmosphere-Ocean System

Assignment for Lecture Ahrens Pages Problems 3.1, 3.2, 3.5, 3.6, 3.14