Chapter 4: Radiation

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Chapter 4 Radiation

• Basic Relationships (p. 160, Eq 4.1)
• Energy ??T4 (Steffan-Boltzmann Law)
• ? - emmissivity ? - Steff.-Botlz. constant

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Solar Electromagnetic Spectrum
Temperature of the sun 6000 K (10,500 F)
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• Basic Relationships (p. 160, Eq 4.2)
• ?max 2.88 x 10-3 / T (Plancks Law)
• ?max Peak Wavelength of EM Radiation

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The Radiation Budget
Outs
Ins
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What determines the amount of solar energy
received at the surface of the Earth?

• Angle of the sun
• Zenith angle
• Length of day
• Scattering / reflection / absoprtion in
  atmosphere
• What causes these factors to change over the
  course of a year?

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The Only Reason We Have Seasons is Because the
Earth is Tilted!

• Average Orbital Characteristics
• Elliptical Orbit 365.25 day period
• Average distance (radius) 150 x106 km
• Perihelion - Earth is closest to sun (January -
  147x106 km)
• Aphelion - Earth is furthest from sun (July -
  152x106 km)
• Earth rotates on its axis 1 spin per 24 hrs
• Average Sunlight Day length 12 hrs

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Equinox Point(s) in the Earths orbit where the
sun is directly over the
equator (12hrs light 12 hrs of darkness
everywhere) Solstice Point in orbit where the
sun is displaced farthest (N or S)
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Important Latitude Bands

• Equator central latitude
• Tropics (23.5 NS) Latitudes of maximum
  displacement of the sun
• Tropic of Cancer 23.5 N (Summer N. Hem.)
• Tropic of Capricorn 23.5 S (Summ. S. Hem.)
• Arctic/Antarctic Circles 66.5 NS Latitudes
  where there is 24 hrs of daylight(nighttime) at
  summer(winter) solstices

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The Directness of Insolation
Main Point Higher the sun, more direct the
insolation
Insolation In-coming Sola-r Radia-tion
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Arctic circle 66.5 N
Tropic of Cancer 23.5 N
Equator 0
Tropic of Capricorn 23.5 S
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Noon zenith angle Sun latitude - local
latitude
Noontime Sun
What does azenith angle gt 90 deg mean?
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Fig. 4.8 Daily-total radiant energy received on a
horizontal surface
Noontime Sun
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The Diurnal Cycle of Temperature and Radiation
Diurnal Cycle daily cycle
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Daytime Heating (from insolation)
profile
Note Temperatures almost always decrease with
height
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Nighttime Cooling (from longwave emission or
radiational cooling)
Radiation Inversion Temperatures increase with
height
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Absorption of solar and Earth radiation in the
atmosphere Selective Absorbers
Wavelength (?m)
Total Absorption Spectrum All gases
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The Radiation Budget
Solar Constant Top of atmo. incoming radiation
1367 W/m2
Ins
Albedo Percent of incoming radiation that is
reflected
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Effects of Radiation
Driver of Weather
Climate
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The Radiation Budget
(255 K)
With No H2O or CO2 Its COLD!
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The Radiation Budget
H2O CO2
(288 K)
Warm with Interest !!!
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The Global Radiation Tax Return