Electromagnetic Spectrum and Laws of RadiationSlide 2Frequency and wavelengthElectromagnetic spectrumBlackbody radiationMeasuring energyRadianceElectromagnetic radiationStefan-Boltzmann LawPlanck’s FunctionPlanck’s functionPlanck curveWein’s Displacement LawIntensity and Wavelength of Emitted Radiation : Earth and SunRayleigh-Jeans ApproximationEmissivity and Kirchoff’s LawKirchoff’s LawSolar ConstantSlide 19Slide 20Radiative TransferProcesses:transmissionreflectionReflection from smooth surfaceScatteringMolecular scattering (or other particles)Scattering from irregular surfaceAbsorption (attenuation)Slide 30RefractionRefraction in two different mediaGradually changing mediumDispersionThe “classic” exampleDiffractionSlide 38Atmospheric Constituents:Optical phenomenaAtmospheric StructureSlide 57Atmospheric windowsSlide 59Where are the windows?Slide 61Size parameterSlide 63Mie scatteringRayleigh scatteringSlide 66Beer’s LawBeer’s Law for AirBeer’s Law: A more general formInverse Squared LawElectromagnetic Spectrum and Laws of RadiationSatellite Satellite Meteorology/ClimatologyMeteorology/ClimatologyProfessor Menglin JinProfessor Menglin JinHow much energy is emitted by some How much energy is emitted by some medium? medium? What “kind” of energy (what What “kind” of energy (what frequency/wavelength) is emitted by frequency/wavelength) is emitted by some medium?some medium?What happens to radiation (energy) as it What happens to radiation (energy) as it travels from the “target” (e.g., ground, travels from the “target” (e.g., ground, cloud...) to the satellite’s sensor?cloud...) to the satellite’s sensor?Frequency and wavelengthv =c Frequency (Hz)WavelengthSpeed of light1 hertz (Hz) = one cycle per secondc = 3.0 x 108 ms-1Electromagnetic spectrum0.001m1m1000 m1m1000m1,000,000 m = 1mGammaX raysUltraviolet (UV)Infrared (IR)MicrowaveRadio wavesRed(0.7m)Orange(0.6m)YellowGreen(0.5m)BlueViolet(0.4m)VisibleLonger waves Shorter wavesBlackbody radiationExamine relationships between Examine relationships between temperature, wavelength and temperature, wavelength and energy emittedenergy emittedBlackbody: A “perfect” emitter and Blackbody: A “perfect” emitter and absorber of radiation... does not absorber of radiation... does not existexistMeasuring energyRadiant energy: Total energy emitted in Radiant energy: Total energy emitted in all directions (J)all directions (J)Radiant flux: Total energy radiated in all Radiant flux: Total energy radiated in all directions per unit time (W = J/s)directions per unit time (W = J/s)Irradiance (radiant flux density): Total Irradiance (radiant flux density): Total energy radiated onto (or from) a unit area energy radiated onto (or from) a unit area in a unit time (W min a unit time (W m-2-2))Radiance: Irradiance within a given angle Radiance: Irradiance within a given angle of observation (W mof observation (W m-2-2 sr sr-1-1))Spectral radiance: Radiance for range in Spectral radiance: Radiance for range in RadianceToward satelliteSolid angle, measured in steradians(1 sphere = 4 sr = 12.57 sr)Normalto surfaceElectromagnetic radiationTwo fields:Two fields:•Electrical & Electrical & magneticmagneticTravel Travel perpendicular & perpendicular & speed of lightspeed of lightProperty & Property & behaves in behaves in predictable waypredictable wayFrequency & Frequency & wavelengthwavelengthPhotons/quantaPhotons/quantaC=3*108=v * Stefan-Boltzmann LawM BB = T 4 Total irradianceemitted by a blackbody(sometimes indicated as E*)Stefan-Boltzmann constantThe amount of radiation emitted by a blackbody is proportional to the fourth power of its temperatureSun is 16 times hotter than Earth but gives off 160,000 times as much radiationPlanck’s FunctionBlackbody doesn't emit equal amounts Blackbody doesn't emit equal amounts of radiation at all wavelengthsof radiation at all wavelengthsMost of the energy is radiated within a Most of the energy is radiated within a relatively narrow band of wavelengths. relatively narrow band of wavelengths.  The exact amount of energy emitted at The exact amount of energy emitted at a particular wavelength a particular wavelength lambdalambda is given is given by the Planck function:by the Planck function:Planck’s functionB  (T) = c1-5 exp (c2 / T ) -1 Irridance:Blackbody radiative fluxfor a single wavelength at temperature T (W m-2)Second radiation constantAbsolute temperatureFirst radiation constant Wavelength of radiationTotal amount of radiation emitted by a blackbody is a function of its temperaturec1 = 3.74x10-16 W m-2 c2 = 1.44x10-2 m °KPlanck curveWein’s Displacement LawmT = 2897.9 m KGives the wavelength of the maximum emission of a blackbody, which is inversely proportional to its temperatureEarth @ 300K: ~10 mSun @ 6000K: ~0.5 mIntensity and Wavelength of Emitted Radiation : Earth and SunRayleigh-Jeans ApproximationB (T) = (c1 / c2) -4 T When is this valid: 1. For temperatures encountered on Earth 2. For millimeter and centimeter wavelengthsAt microwave wavelengths, the amount of radiation emitted is directly proportional to T... not T4(c1 / c2) -4 Brightness temperature (TB) is often used for microwave and infrared satellite data, where it is called equivalent blackbody temperature. The brightness temperature is equal to the actual temperature times the emissivity.B (T)TB =Emissivity and Kirchoff’s LawActual irradiance bya non-blackbodyat wavelength Emittance: Often referred to as emissivityEmissivity is a function of the wavelength of radiation and the viewing angle and) is the ratio of energy radiated by the material to energy radiated by a black body at the same temperatureabsorbed/ incidentAbsorptivity (r , reflectivity; t , transmissivity)Kirchoff’s LawMaterials which are strong absorber at a particular wavelength are also strong emitter at that wavelengthSolar ConstantThe intensity of radiation from the The intensity of radiation from the Sun received at the top of the Sun received at the top of the atmosphereatmosphereChanges in solar constant may result Changes in solar constant may result in climatic variationsin climatic