What Cause the Four Seasons?
Review of last lecture
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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):
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Wavelength of radiation (Wein’s law):
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The wavelength range of Sun (shortwave)
and Earth (longwave) radition
The 11-year solar cycle
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I=T4
max = b/T
Earth’s orbit around the Sun
Distance from the sun accounts for only 7% change in radiation
receipt, which is not the cause of seasons.
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?!?!
The 4-seasons is caused by the Earth’s
23.5o tilt from the line perpendicular to its
orbit plane (toward the sun during summer)
Video: Mechanism of the seasons
• http://www.youtube.com/watch?v=
WLRA87TKXLM
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)
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
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)
Beam Depletion
The 4-seasons is caused by the Earth’s
23.5o tilt from the line perpendicular to its
orbit plane (toward the sun during summer)
What caused the ice ages and
extinction of mammoth?
Change of Earh’s 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 Earth’s
orbit affect Earth’s climate
Milutin Milankovic (1879-1958)
• 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
Summary
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The two basic motions of the Earth
What causes the four seasons: the
Earth’s tilt and the 3 ways it affects the
solar insolation (change of length of the
day, beam spreading, beam depletion)
Change of the Earth’s orbit at longer time
scales (Milankovitch cycles): eccentricity,
axial tilt, and precession