Thursday, January 17, 2008

EATS 1011 Lecture 5


 

  • Co2 has less of an impact on global warming than waper vapour, but CO2 can be controlled, water vapour isn't something humans can control
  • The surface temperature would be exactly the same as the effective temperature if we had no atmosphere but 30% albedo
  • Teffective =
  • Greenhouse effect
    • Dilute solar radiation heats (mostly) surface
    • Surface heats and emits mid-IR radiation which is absorbed(mostly) by atmospheric gases and clouds
      • The gases such as water vapour(clouds), carbon dioxide are what cause the greenhouse effect because they absorb radiation from the sun
    • Atmosphere re-radiates
      • Up to space
      • Back down to earth
    • Surface is heated "twice" (solar + mid-IR)
      • Figure 212 on slide
    • Red skys are caused by Rayleigh scattering

      •  
        • Where lambda is the wavelength
    • Albedo
      • Refers to visible part of the light spectrum
      • Albedo depends on the surface the light is directed to
      • Fresh snow is most reflective, 75-95% light reflected
      • Earth is about 30% 'albedo'
        • Not sure how to use the word 'albedo'
      • 6% reflected from atmosphere
      • 20% reflected by clouds
      • 4% is reflected naturally by the earth's surface
      • 51% of solar energy heats the Earth's surface
      • Total energy budget

      • Most infrared is absorbed by the earth's atmosphere instead of being released into space
      • When the sun goes down at night we don't freeze because infra red energy from the atmosphere continues heating the earth while the sun is down
      • 70 units of solar coming in , 50 heating the surface, after all the processing we have 70 units of energy leaving in the mid infrared

Earth's Energy Budget

  • Top of atmosphere
    • 70 units of solar in balanced by 70 units of Mid-IR emitted
  • Surface
    • Heated twice
    • Solar 51 units and 96 mid-IR units
    • We are heated by mid-IR at night
  • Read slides for more details on the web

Seasons

  • What are the drivers of the seasons
    • In the tropics there are no seasons
    • Seasons are at mid latitudes
  • orbital parameters
    • ellipticity of orbits
      • Perihelion
        • 147 x 106 km, 4th january
      • Ephelion
        • 153 x 106 km, 4th July
      • 7% change in insolation
    • Insolation:
      • The suns energy averaged over 24 hours
    • Obliquity: tilt
      • Rotation axis – pole star
      • 23.5 degrees
      • Solstice
        • N pole sun 21st June, sun overhead at noon at tropic of Cancer(23.5N)
        • S pole sun 21st December, sun overhead at noon at the tropic of Capricorn (23.5S)
        • Regions of perpetual day and night
      • Equinox
        • Axis 90 to sun – earth line
        • Vernal 21st march
        • Autumnal 21st September
        • Effects the length of the day
  • So what causes the seasons
    • Ellipticity
      • 7% so little impact
        • Except over eons
    • Obliquity
      • Large effect at mid- and polar latitudes
    • Latitude
      • In the tropics changes driven by overhead sun
      • Wet and dry seasons
      • Twice a year
  • Length of day light for various latitudes


 

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