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Chapter 2

Chapter 2. The Earth’s Global Energy Balance. Energy Balance. What is energy balance? Energy Balance - equilibrium between flow of energy or radiation reaching Earth and flow of energy or radiation leaving Earth 69% absorbed, 31% reflected

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Chapter 2

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  1. Chapter 2 The Earth’s Global Energy Balance

  2. Energy Balance • What is energy balance? • Energy Balance- equilibrium between flow of energy or radiation reaching Earth and flow of energy or radiation leaving Earth • 69% absorbed, 31% reflected • Importance: controls daily and seasonal variations in temperature, currents, and life

  3. Global Radiation Balance • Absorption of shortwave (solar) radiation must equal emission of longwave (terrestrial) radiation • Globally, some areas have surplus of solar radiation while others have deficit  circulation in atmosphere and oceans • Areas receive different amounts of solar radiation at different times of year  circulation patterns vary through year • Annually, lower latitudes (~35°N to ~35°S) tend to have surplus, while higher latitudes (~35°N to 90°N and ~35°S to 90°S) tend to have deficit

  4. Why does the Earth not receive all of the Sun’s energy? • Absorption-when molecules and particles in the atmosphere intercept and absorb radiation at particular wavelengths (Solar radiation) • Gasses? • Scattering-when incoming solar radiation is turned in a different direction, often deflected back into space • Received vs. Reflected

  5. Electromagnetic Spectrum • Electromagnetic Radiation-Wave-like form of energy radiated by any object either as light (visible) or heat (invisible) • Emitted at different wavelengths • Crest and Troughs • Measured in micrometers (one-millionth of a meter) • Short wavelengths=Higher heat • Longer wavelengths=Lower heat

  6. Wien’s Law • Inverse relationship between object’s temp and maximum wavelength of emitted radiation • Hotter objects  shorter maximum wavelengths • Example: Sun (~5800K) vs. Earth (~288K) Example: The surface temperature of Venus is 773K. At what wavelength does Venus emit radiation? *3.75μm (Infrared)

  7. Stephan-Boltzmann Law • Describes amount of energy emitted by a black body • A black body is an object that absorbs 100% of the radiation striking it and radiates heat at a max. rate (blacktop) • Earth not perfect black body, but can approximate • Amount energy emitted directly related to temp • What does this mean? • Hotter objects emit more energy • T4= E/8.17 x 10-11 • Example: What is the average temperature of a body with an intensity of incoming radiation of 7.29L? • T=547K • CAVEAT-If you are calculating the Temperature of a Planet, you must divide the solar constant by 4 before using it in the Equation

  8. Example • You have crash landed on Mercury and your instruments tell you that the solar constant is 13.64L. What is the average temperature of Mercury? • 1. 13.64/4=3.41 • 2. T4=3.41/8.17 x 10-11 • 3. T=452K

  9. Net Radiation • Net Radiation=Incoming – Outgoing Radiation • Rn=I (1-a) + R↓ - σT4e • Example: What would the net radiation be if insolation were 1200 W/m2; the albedo of the surface was 0.33; emissivity was 0.90, temperature was 325K, and downward longwave radiation was 313 W/m2? • Rn=1200(1-.33)+313-(5.67 x 10-8) x 3254 x .90 • Rn=547.68 W/m2

  10. Global Radiation Balance • Scattering of shortwave vs. longwave radiation • Insolation-interception of solar energy (shortwave radiation) by an exposed surface • W/m²

  11. Insolation • Insolation (incoming solar radiation) is dependent on 4 things: • 1. Angle of Incidence- the angle between the sun’s rays and a line perpendicular to the Earth’s surface (also called the Zenith angle) • 2. Length of daylight • 3. Transparency of atmosphere (dust, smog) • 4. Variation in Solar Irradiance- insolation received at the outer edge of the atmosophere

  12. Insolation • Angle of the Sun (SA)-the angle between the Earth’s surface and the sun’s rays • SA + ZA = 90° • Solar Constant- the maximum potential intensity of insolation available to the Earth at the outer edge of the atmosphere • Solar Constant = 1.94 Langleys • Sine Law • Summarizes the relationship between the SA and radiation intensity • Ib=(Ia) x (Sin SA) • Example: What is the maximum potential intensity of radiation if the SA = 60°? • 1.68L

  13. Net Radiation, Latitude, and Energy Balance • Net radiation: difference between incoming and outgoing radiation • Varies strongly by latitude, but balance on global scale due to heat transfer • Transfer occurs by methods discussed in both atmospheric and oceanic circulation

  14. Terrestrial Radiation • Energy emitted by earth • Earth absorbs certain wavelengths of solar radiation, then is converted to another form and emitted at different wavelength • Wavelength much longer than that of solar radiation (Wien’s Law)

  15. World Latitude Zones • Equatorial (10°N to 10°S): intense annual insolation • Tropical (10°N to 25°N; 10°S to 25°S): some seasonal variation, but still high annual insolation • Subtropical (25°N to 35°N; 25°S to 35°S): more seasonal variation; fairly high annual insolation • Mid-latitude (35°N to 55°N; 35°S to 55°S): strong seasonal contrasts in insolation • Sub-Arctic (55°N to 60°N), Sub-Antarctic (55°S to 60°S): very strong seasonal contrasts in insolation • Arctic (60°N to 75°N), Antarctic (60°S to 75°S): intense seasonal contrasts in insolation • Polar (75°N to 90°N; 75°S to 90°S): greatest seasonal contrasts in insolation... Range is ~500W/m2

  16. Other Types of Heat Transfer • 2 main ways in addition to longwave and shortwave radiation... • 1. Sensible heat • Sensible heat transfer • Conduction OR convection • 2. Latent heat • Latent heat transfer • Occurs during change of state

  17. Composition of the Atmosphere • Important to energy flow and energy balance • Different gases impact heat absorption in air • Nitrogen, oxygen, and argon – three most abundant; low importance to climate • Carbon dioxide, water vapor, ozone, and methane – low content in atmosphere, but very important to climate - “Greenhouse Gasses”

  18. Diffuse Radiation • Diffuse Radiation-when incoming radiation is scattered by dust and other atmospheric particles causing radiation to bounce in various directions • Clouds can reflect a lot of radiation • Direct Absorption • 15% incoming radiation absorbed by CO² and water vapor • 17% absorbed by diffuse radiation • 80% reaches ground

  19. Miscellaneous Topics • Albedo-the proportion of shortwave radiation that is reflected and scattered by an object’s surface • Different surfaces and colors • Counterradiation-LW radiation of the atmosphere directed downward toward the Earth’s surface • CO² and Water vapor

  20. Greenhouse Effect • Greenhouse Effect- the ability of the Earth’s atmosphere to admit most of the insolation and prevent its re-radiation from the Earth • Although the natural “greenhouse effect” enables the Earth to remain livable…increased pollutants have caused an adverse reaction creating an “enhanced effect” • The “enhanced effect” is when human activities increase the ability for the Earth’s atmosphere to absorb radiation causing a net warming of the atmosphere over time • Greenhouse Gasses? • Water Vapor, Ozone, Carbon Dioxide, and Methane

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