What Cause the Four Seasons?

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What Cause the Four Seasons?
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Review of last lecture

• What is energy? 3 methods of energy transfer
• The names of the 6 wavelength categories in the
  electromagnetic radiation spectrum
• Intensity of radiation (Stefan-Boltzman law)
  I?T4
• Wavelength of radiation (Weins law)
  ?max b/T
• The wavelength range of Sun (shortwave) and Earth
  (longwave) radition
• The 11-year solar cycle

3
Earths orbit around the Sun
Distance from the sun accounts for only 7 change
in radiation receipt, which is not the cause of
seasons.
4
Revolution vs. Rotation

• Revolution about a body (the earth revolves
  around the sun i.e., a year)
• Rotation about an axis (The earth rotates about
  its axis i.e., a day)
• So what causes the seasons?!?!

5
The 4-seasons is caused by the Earths 23.5o tilt
from the line perpendicular to its orbit plane
(toward the sun during summer)
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Video Mechanism of the seasons

• http//www.youtube.com/watch?vWLRA87TKXLM

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How the changing orientation of the Earth
directly affects the receipt of insolation

1. Length of Daylight period
2. Angle at which sunlight hits the surface (Beam
   Spreading)
3. Thickness of atmosphere through which sunlight
   must travel (Beam Depletion)

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Length of Daylight Period
Equinoxes
June Solstice

• Day length increases from equator to pole in
  summer hemisphere
• opposite in winter
• At least one day of 24-hour sunlight/darkness
  above Arctic/Antarctic circles, depending on
  season

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Beam Spreading

• Beam spreading the increase in surface area over
  which radiation is distributed in response to a
  decrease of solar angle
• The greater the spreading, the less intense the
  radiation (and vice versa)

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Beam Depletion
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The 4-seasons is caused by the Earths 23.5o tilt
from the line perpendicular to its orbit plane
(toward the sun during summer)
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What caused the ice ages and extinction of
mammoth?
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Change of Earhs orbit at longer time scales -
The Milankovitch cycles

• Named after Serbian geophysicist and astronomer
  Milutin Milankovic, who mathematically theorized
  that variations in eccentricity, axial tilt and
  precession of the Earths orbit affect Earths
  climate

Milutin Milankovic (1879-1958)
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• Eccentricity shape of orbit varies between a
  circle and a thin oval on a 100,000 year cycle
• Axial tilt with respect to orbit plane varies
  between 21.5o and 24.5o on a 41,000 year cycle
• Precession direction of axis varies on a 20,000
  year cycle

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Summary

• The two basic motions of the Earth
• What causes the four seasons the Earths tilt
  and the 3 ways it affects the solar insolation
  (change of length of the day, beam spreading,
  beam depletion)
• Change of the Earths orbit at longer time scales
  (Milankovitch cycles) eccentricity, axial tilt,
  and precession