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Understanding Solar and Terrestrial Radiation: Energy Flow and Laws

Discover how energy flows into and out of the Earth-Atmosphere system through solar and terrestrial radiation, including key laws and concepts such as electromagnetic spectrum, radiation laws, Earth's orbit, solar intensity, and greenhouse effect.

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Understanding Solar and Terrestrial Radiation: Energy Flow and Laws

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  1. Chapter 3 Solar and Terrestrial Radiation

  2. Driving Question • How does energy flow into and out of the Earth-Atmosphere system? • Law of Energy Conservation – Energy cannot be created nor destroyed (First Law of Thermodynamics)

  3. Electromagnetic Spectrum • Earth is continuously bombarded by electromagnetic radiation from the sun • All objects emit electromagnetic radiation (except at absolute zero) • Types: • Radio waves • IR • Visible • UV • X and Gamma Rays • Together these form the electromagnetic spectrum

  4. Waves • Wavelength – the distance between successive waves crests or troughs • Frequency – the number of crests or troughs that pass a given point in a given amount of time (1 second) • 1 cycle/1 second = 1 Hertz

  5. Radiation Laws • Blackbody – an object at a constant temperature that absorbs all radiation incident on it and emits all radiation at every wavelength • Perfect absorber and perfect emitter • Earth and Sun are NOT blackbodies, but they are close enough that blackbody laws can be applied

  6. Radiation Laws • Wien’s Displacement Law • The wavelength of most intense radiation is inverse to the temperature of the object • λmax = C/T • λ: wavelength (μm) • C: constant = 2897μm K • T: Temperature (K) • The sun emits short wave radiation • The earth emits long wave radiation

  7. Radiation Laws • Stefan-Boltzman Law - Law relating the temperature of a blackbody to the amount of energy emitted • E = σT4 • E: Energy Emission (W/m2) • σ: Stefan Boltzman Constant = 5.67e-8 W/m2K4 • T: Temperature (K) • Average T earth = 288K • Average T sun = 6000K

  8. Earth’s Orbit • Earth’s orbit is slightly elliptical • Closest to the sun in early January (91 million miles (perihelion) • Farthest from the sun in early July (94 million miles (aphelion) • Earth’s axis is tilted 23 degrees 27 minutes

  9. The angle of the sun above the horizon Greatest = 90 Lowest = 0 Solar intensity is greatest when the solar altitude is at 90 degrees Solar Altitude

  10. Important Latitude Lines • Equator • Tropic of Cancer: line where solar altitude is 90 degrees at summer solstice (June 21) • Tropic of Capricorn: line where solar altitude is 90 degrees at winter solstice (December 21) • Arctic Circle: At winter solstice, 24 hours of darkness northward • Antarctic Circle: At summer solstice, 24 hours of darkness southward

  11. First day of spring/fall Solar altitude is 90 degrees at equator Night and day are generally equal at 12 hours Equinox – “Equal Night”

  12. First day of summer in NH Solar altitude is 90 degrees at the Tropic of Cancer 24 hours of darkness south of Antarctic Circle Summer Solstice

  13. First day of winter in NH Solar altitude is 90 degrees at Tropic of Capricorn 24 hours of darkness north of Arctic Circle Winter Solstice

  14. Why is it colder in the winter if the earth is closer to the sun? Tilt and Solar Altitude • Less Daylight – decrease in amount of solar energy

  15. Why is it colder in the winter if the earth is closer to the sun? • Decreased Solar Intensity in Atmosphere

  16. Why is it colder in the winter if the earth is closer to the sun? • Decreased Solar Intensity at Surface

  17. Solar Radiation • Reflection • Occurs when radiation hitting a surface is reflected • Law of reflection: angle of incidence equals the angle of reflection

  18. Solar Radiation • Scattering • A particle (gas molecule, aerosol) disperses solar radiation in all directions • Scattering is wavelength dependent • Oxygen and Nitrogen tend to scatter blue/violet light – reason why the sky is blue • Water and ice crystals scatter light equally at all wavelengths – reason why clouds are white

  19. Solar Radiation • Absorption • Process where some of the radiation on an object is converted to heat • Different from reflection and scattering: energy conversion and not energy redirection • Absorption by atmospheric gases varies greatly by wavelength • O3 < 0.3μm H2O > 0.8μm

  20. Solar Radiation

  21. Albedo • The fraction of incident radiation that is reflected by a surface • Albedo = (reflected radiation/incident radiation) • Recall that 30% of solar radiation was “lost to space” • Earth’s Albedo is 30% or 0.30 • Dark objects have low albedos and bright objects have high albedos • Moon’s albedo is about 7% - no atmosphere to reflect the radiation

  22. Greenhouse Effect • Global radiative equilibrium keeps the planet’s temperature in check – emission of heat to space in the form of infrared radiation balances the solar radiation’s heating. • Solar radiation and terrestrial radiation emit at different wavelengths – allows “trapping” of radiation • Recall that different gases absorb radiation at different wavelengths

  23. Greenhouse Effect • Without the greenhouse effect the earth’s surface temperature would be about 0oF – too cold • Average temperature of earth’s surface is about 59oF • Most IR radiation escapes through atmospheric windows • Gases that prevent IR radiation from entering space are greenhouse gases • Water vapor, carbon dioxide, ozone, nitrous oxide, methane

  24. Greenhouse Warming Examples • The Gulf Coast and desert Southwest • Similar solar radiation and daytime highs, but different morning lows – why? • Reason: amount of water vapor • More water vapor exists near gulf coast and it traps IR radiation leaving the surface. Drier air in the southwest does not trap radiation allowing temperatures to drop • Clouds – generally composed of water droplets • Cloudy nights are warmer – trap IR radiation • Cloudy days are cooler – block solar radiation

  25. Ozone • Unstable molecule of 3 oxygen atoms that has positive and negative effects • Positive: blocks harmful UV rays in the stratosphere from reaching the surface • Negative: smog at the surface • Chemical reactions (UV) in the stratosphere account for the destruction and creation of ozone • Destruction by CFC’s (banned in US in 1979)

  26. Global Warming • Increasing CO2 concentrations have been observed • Enhances the natural greenhouse effect • Methane and Nitrous Oxide concentrations also increasing

  27. Global Warming • Possible Effects • Shifting climate zones • Melting of ice sheets and glaciers leading to an increase in sea level • Positives • Longer growing season • Less energy used (warmer in winter months) • http://www.intellicast.com/DrDewpoint/Library/1213/ProGlobal/ • http://www.intellicast.com/DrDewpoint/Library/1213/ChallengingGlobal/

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