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Lecture 5 -- Blackbody Radiation/ Planetary Energy Balance Abiol 574 Electromagnetic Spectrum visible light 1000 100 10 1 0.1 0.01 0.7 to 0.4 m (m) Electromagnetic Spectrum visible light ultraviolet 1000 100 10 1 0.1 0.01 (m) Electromagnetic Spectrum visible infrared light ultraviolet 1000 100 10 1 0.1 0.01 (m) Electromagnetic Spectrum visible microwaves infrared light ultraviolet x-rays 1000 100 10 1 0.1 0.01 (m) Electromagnetic Spectrum visible microwaves infrared light ultraviolet x-rays 1000 100 10 1 0.1 0.01 Low High Energy Energy (m) Blackbody Radiation Blackbody radiation—radiation emitted by a body that emits (or absorbs) equally well at all wavelengths The Planck Function • Blackbody radiation follows the Planck function Basic Laws of Radiation 1) All objects emit radiant energy. Basic Laws of Radiation 1) All objects emit radiant energy. 2) Hotter objects emit more energy than colder objects. Basic Laws of Radiation 1) All objects emit radiant energy. 2) Hotter objects emit more energy than colder objects. The amount of energy radiated is proportional to the temperature of the object. Basic Laws of Radiation 1) All objects emit radiant energy. 2) Hotter objects emit more energy than colder objects. The amount of energy radiated is proportional to the temperature of the object raised to the fourth power. This is the Stefan Boltzmann Law F = T4 F = flux of energy (W/m2) T = temperature (K) = 5.67 x 10-8 W/m2K4 (a constant) Basic Laws of Radiation 1) All objects emit radiant energy. 2) Hotter objects emit more energy than colder objects (per unit area). The amount of energy radiated is proportional to the temperature of the object. 3) The hotter the object, the shorter the wavelength () of emitted energy. Basic Laws of Radiation 1) All objects emit radiant energy. 2) Hotter objects emit more energy than colder objects (per unit area). The amount of energy radiated is proportional to the temperature of the object. 3) The hotter the object, the shorter the wavelength () of emitted energy. This is Wien’s Law max 3000 m T(K) Stefan Boltzmann Law. F = T4 F = flux of energy (W/m2) T = temperature (K) = 5.67 x 10-8 W/m2K4 (a constant) Wien’s Law max 3000 m T(K) We can use these equations to calculate properties of energy radiating from the Sun and the Earth. 6,000 K 300 K T max region in F spectrum (K) (m) (W/m2) Sun 6000 Earth 300 T max region in F spectrum (K) (m) (W/m2) Sun 6000 0.5 Earth 300 10 Electromagnetic Spectrum visible microwaves infrared light ultraviolet x-rays 1000 100 10 1 0.1 0.01 Low High Energy Energy (m) T max region in F spectrum (K) (m) (W/m2) Sun 6000 0.5 Visible (yellow?) Earth 300 10 infrared • Blue light from the Sun is removed from the beam by Rayleigh scattering, so the Sun appears yellow when viewed from Earth’s surface even though its radiation peaks in the green T max region in F spectrum (K) (m) (W/m2) Sun 6000 0.5 Visible (green) Earth 300 10 infrared Stefan Boltzman Law. F = T4 F = flux of energy (W/m2) T = temperature (K) = 5.67 x 10-8 W/m2K4 (a constant) T max region in F spectrum (K) (m) (W/m2) Sun 6000 0.5 Visible 7 x 107 (green) Earth 300 10 infrared 460 Solar Radiation and Earth’s Energy Balance Planetary Energy Balance • We can use the concepts learned so far to calculate the radiation balance of the Earth Some Basic Information: Area of a circle = r2 Area of a sphere = 4 r2 Energy Balance: The amount of energy delivered to the Earth is equal to the energy lost from the Earth. Otherwise, the Earth’s temperature would continually rise (or fall). Energy Balance: Incoming energy = outgoing energy Ein = Eout Eout Ein (The rest of this derivation will be done on the board. However, I will leave these slides in here in case anyone wants to look at them.) How much solar energy reaches the Earth? How much solar energy reaches the Earth? As energy moves away from the sun, it is spread over a greater and greater area. How much solar energy reaches the Earth? As energy moves away from the sun, it is spread over a greater and greater area. This is the Inverse Square Law So = L / area of sphere So = L / (4 rs-e2) = 3.9 x 1026 W = 1370 W/m2 4 x x (1.5 x 1011m)2 So is the solar constant for Earth So = L / (4 rs-e2) = 3.9 x 1026 W = 1370 W/m2 4 x x (1.5 x 1011m)2 So is the solar constant for Earth It is determined by the distance between Earth (rs-e) and the Sun and the Sun’ luminosity. Each planet has its own solar constant… How much solar energy reaches the Earth? Assuming solar radiation covers the area of a circle defined by the radius of the Earth (re) Ein re How much solar energy reaches the Earth? Assuming solar radiation covers the area of a circle defined by the radius of the Earth (re) Ein = So (W/m2) x re2 (m2) Ein re How much energy does the Earth emit? 300 K How much energy does the Earth emit? Eout = F x (area of the Earth) How much energy does the Earth emit? Eout = F x (area of the Earth) F = T4 Area = 4 re2 How much energy does the Earth emit? Eout = F x (area of the Earth) F = T4 Area = 4 re2 Eout = ( T4) x (4 re2) Earth Sun 1000 100 10 1 0.1 0.01 Hotter objects emit more energy than (m) colder objects Earth Sun 1000 100 10 1 0.1 0.01 Hotter objects emit more energy than (m) colder objects F = T4 Hotter objects emit at shorter wavelengths. max = 3000/T Earth Sun 1000 100 10 1 0.1 0.01 Hotter objects emit more energy than (m) colder objects F = T4 How much energy does the Earth emit? Eout = F x (area of the Earth) Eout How much energy does the Earth emit? Eout = F x (area of the Earth) F = T4 Area = 4 re2 Eout = ( T4) x (4 re2) Eout How much solar energy reaches the Earth? Ein How much solar energy reaches the Earth? We can assume solar radiation covers the area of a circle defined by the radius of the Earth (re). Ein re How much solar energy reaches the Earth? We can assume solar radiation covers the area of a circle defined by the radius of the Earth (re). Ein = So x (area of circle) Ein re Remember… So = L / (4 rs-e2) = 3.9 x 1026 W = 1370 W/m2 4 x x (1.5 x 1011m)2 So is the solar constant for Earth It is determined by the distance between Earth (rs-e) and the Sun and the Sun’s luminosity. How much solar energy reaches the Earth? We can assume solar radiation covers the area of a circle defined by the radius of the Earth (re). Ein = So x (area of circle) Ein = So (W/m2) x re2 (m2) Ein re How much solar energy reaches the Earth? Ein = So re2 BUT THIS IS NOT QUITE CORRECT! **Some energy is reflected away** Ein re How much solar energy reaches the Earth? Albedo (A) = % energy reflected away Ein = So re2 (1-A) Ein re How much solar energy reaches the Earth? Albedo (A) = % energy reflected away A= 0.3 today Ein = So re2 (1-A) Ein = So re2 (0.7) Ein re Energy Balance: Incoming energy = outgoing energy Ein = Eout Eout Ein Energy Balance: Ein = Eout Ein = So re2 (1-A) Eout Ein Energy Balance: Ein = Eout Ein = So re2 (1-A) Eout = T4(4 re2) Eout Ein Energy Balance: Ein = Eout So re2 (1-A) = T4 (4 re2) Eout Ein Energy Balance: Ein = Eout So re2 (1-A) = T4 (4 re2) Eout Ein Energy Balance: Ein = Eout So (1-A) = T4 (4) Eout Ein Energy Balance: Ein = Eout So (1-A) = T4 (4) T4 = So(1-A) 4 Eout Ein T4 = So(1-A) 4 If we know So and A, we can calculate the temperature of the Earth. We call this the expected temperature (Texp). It is the temperature we would expect if Earth behaves like a blackbody. This calculation can be done for any planet, provided we know its solar constant and albedo. T4 = So(1-A) 4 For Earth: So = 1370 W/m2 A = 0.3 = 5.67 x 10-8 W/m2K4 T4 = So(1-A) 4 For Earth: So = 1370 W/m2 A = 0.3 = 5.67 x 10-8 T4 = (1370 W/m2)(1-0.3) 4 (5.67 x 10-8 W/m2K4) T4 = So(1-A) 4 For Earth: So = 1370 W/m2 A = 0.3 = 5.67 x 10-8 T4 = (1370 W/m2)(1-0.3) 4 (5.67 x 10-8 W/m2K4) T4 = 4.23 x 109 (K4) T = 255 K Expected Temperature: Texp = 255 K (oC) = (K) - 273 Expected Temperature: Texp = 255 K (oC) = (K) - 273 So…. Texp = (255 - 273) = -18 oC (which is about 0 oF) Is the Earth’s surface really -18 oC? Is the Earth’s surface really -18 oC? NO. The actual temperature is warmer! The observed temperature (Tobs) is 15 oC, or about 59 oF. Is the Earth’s surface really -18 oC? NO. The actual temperature is warmer! The observed temperature (Tobs) is 15 oC, or about 59 oF. The difference between observed and expected temperatures (T): T = Tobs - Texp T = 15 - (-18) T = + 33 oC T = + 33 oC In other words, the Earth is 33 oC warmer than expected based on black body calculations and the known input of solar energy. T = + 33 oC In other words, the Earth is 33 oC warmer than expected based on black body calculations and the known input of solar energy. This extra warmth is what we call the GREENHOUSE EFFECT. T = + 33 oC In other words, the Earth is 33 oC warmer than expected based on black body calculations and the known input of solar energy. This extra warmth is what we call the GREENHOUSE EFFECT. It is a result of warming of the Earth’s surface by the absorption of radiation by molecules in the atmosphere. The greenhouse effect: Heat is absorbed or “trapped” by gases in the atmosphere. Earth naturally has a greenhouse effect of +33 oC. The concern is that the amount of greenhouse warming will increase with the rise of CO2 due to human activity.

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