the incoming solar energy. review of last lecture –the standard units of measurements (si)...
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The incoming solar energy
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Review of last lecture– The standard units of measurements (SI)– Earth’s three atmospheres: 1st: 4.6 billion years ago, H, He, magnetic field,
volcano 2nd: 4 billion years ago, CO2, H2O, N2, life, ocean
3rd: 400 million years ago, O2, land
– What is residence time? What is the difference between the permanent and variable gases? Name 3 of each.
– Given that variable gases are so rare, why are they considered at all? How are CO2 and O3 changing?
– Earth’s climate history: ice ages (at least 5 have occurred so far. We’re in an ice age!), 100,000-year cycle, little ice age (1350AD-1850AD)
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• Energy: the ability to do work
• Many forms: electrical, mechanical, thermal, chemical, nuclear, …
• Joule (J): standard unit of energy (1 J= 0.239 calories)
• Watt (W): rate of energy flow (W = 1 J/s)
Energy basics
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Methods of Energy Transfer
• Conduction– Molecule to molecule transfer– Heat flow: warm to cold – e.g. leather seats in a car
• Convection– transferred by vertical movement– physical mixing – e.g. boiling water
• Radiation– propagated without medium (i.e. vacuum)– solar radiation provides nearly all energy – The rest of this chapter deals with radiation
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Radiation• Everything continually
emits radiation
• Transfers energy in waves
• Waves are both electrical and magnetic, hence electromagnetic radiation
• Travels at the speed of light (300,000 km/s). It takes 8 minutes for light from the Sun to reach Earth, and 4.3 years for light from the next nearest star, Proxima Centauri to reach us.
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Radiation Quantity and Quality
• Quantity: how much? Wave height (Intensity).
• Quality: what kind? Wavelength: distance between crest and crest (or trough and trough). generally reported in μm (microns)- one millionth of a meter.
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The Electromagnetic Spectrum
The limitations of the human eye!
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Sun
Wavelength of Sun and Earth Radiation
Sun = “shortwave” (0.4-0.7 μm) Peak 0.5 μm (green)
Earth = “longwave” (4-100 μm) Peak 10 μm (infrared)
blue red
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Composition of sunlight
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Video: The solar cycle
• http://www.youtube.com/watch?v=sASbVkK-p0w&playnext=1&list=PL8EDAE089117E3D77&feature=results_main
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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. Moreover, the Earth is farther away from the Sun in the northern summer!
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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?!?!
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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)
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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 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
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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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Summary
– What is energy? 3 methods of energy transfer
– The names of the 6 wavelength categories in the electromagnetic radiation spectrum
– The wavelength range of Sun (shortwave) and Earth (longwave) radition
– The two basic motions of the Earth– What causes the four seasons: the 23.5
degree tilt of the Earth’s axis, and the 3 ways it affects the solar insolation (change of length of the day, beam spreading, beam depletion)