What Cause the Four Seasons?

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 (Wein’s law): max = b/T

– The wavelength range of Sun (shortwave) and Earth (longwave) radition

– The 11-year solar cycle

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 PeriodEquinoxes 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

– 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