NATS 101 Intro to Weather and Climate Lecture 7 Seasonality

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NATS 101Intro to Weather and ClimateLecture
7Seasonality

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Supplemental References for Todays 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)

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Reasons for Seasons

• Eccentricity of Earths Orbit
• Elongation of Orbital Axis
• Tilt of Earths Axis - Obliquity
• Angle between the Equatorial Plane and
  the Orbital Plane

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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?
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147 million km
152 million km
Ahrens, Fig. 2.17
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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
Small Area
Short Path
Ahrens, Fig. 2.19
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Beam Spreading

• Low Zenith - Large Area, Much Spreading
• High Zenith - Small Area, Little Spreading

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Beam Spreading
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Atmospheric Path Length
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Length of Day
Lutgens Tarbuck, p33
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Day Hours at Solstices - US Sites

• Summer-Winter
• Tucson (32o 13 N) 1415 - 1003
• Seattle (47o 38 N) 1600 - 825
• Anchorage (61o 13 N) 1922 - 528
• Fairbanks (64o 49 N) 2147 - 342
• Hilo (19o 43 N) 1319 - 1046
  

Arctic Circle
Gedzelman, p67
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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
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Solar Declination
Solstice
Equinox
Solstice
Aguado Burt, p46
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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
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Incoming Solar Radiation (Insolation) at the Top
of the Atmosphere
W
C
C
W
Wallace and Hobbs, p346
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Is Longest Day the Hottest Day?
Consider Average Daily Temperature for Chicago IL
USA Today WWW Site
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Radiation Budget

• Summer hemisphere shows a surplus, warms
• Winter hemisphere shows a deficit, cools
• Equator/S. Pole always shows a
  surplus/deficit
• Why doesnt the equator warm and S. Pole cool?

NH
SH
NH
SH
Lutgens Tarbuck, p51
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Annual Energy Balance
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

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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

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Assignment for Lecture

• Ahrens
• Pages 55-64
• Problems 3.1, 3.2, 3.5, 3.6, 3.14