GEOG/GEOL 4093/5093 162. EMR INTERACTION WITH MATTER2.1.1. Radiative properties of natural surface• Radiation incident upon a surface must either be transmitted(τ) through it, reflected (α) from the surface, or be absorbed(ξ).• For solar radiation α is referred to as the surface albedo• If we consider only part of the EM spectra α is referred to asspectral reflectance2.1.2. Albedo and EmissionTransmissivity + Reflectivity + Absorptivity = 1GEOG/GEOL 4093/5093 172.1.3. Transmission• Incident radiation passes through matter without attenuation• Change of EMR is given by index of refraction21sinsinθθ=n2.1.4. Reflection (specular reflection)• Surface is smooth relative to wavelengths• Mirror like surface are called specular reflectors2.1.5. Scattering (diffuse reflection)• Surface rough relative to wavelengths• EMR velocity and wavelength are not affected2.1.6. Absorption• Substance is opaque to the incident radiation• Portion of EMR is converted to heat energy (re-radiated)GEOG/GEOL 4093/5093 18EM energy is attenuated by its passage through the atmosphere⇒ scattering (radiation is redirected)• Wavelength dependent• Decreases with increase in radiation wavelength• Rayleigh, Mie, Non selective scattering2.2.1. Rayleigh scattering (Molecular scattering)• Molecules and particles << EM wavelength• Primarily by oxygen and nitrogen molecules•Intensity is proportional to λ-4Example:UV radiation (0.3 µm) is scattered 16 times as readily as redwavelength at 0.6 µm⇒ (0.6/0.3)4 = 16⇒ explains why clear sky is blue2.2. ATMOSPHERIC EFFECTS - SCATTERINGGEOG/GEOL 4093/5093 19ATMOSPHERIC EFFECTS - SCATTERING2.2.2. Mie scattering• Particles that have a mean diameter 0.1 to 10 times EMwavelengths• Water vapor, smoke particles, salt crystals• Intensity is proportional between λ-4 and λ0Clear atmosphere is a medium for both Rayleigh and Miescattering - their combined influence is between λ-0.7 and λ-2Sunset shorter wavelengths scattered away (blue and green),leaving only red wavelengths to reach our eyes.2.2.3. Non selective scattering• Aerosol particles much larger than EM wavelengths(> 10 times)• Water droplets, ice crystals• Independent of wavelengths ⇒ λ0Clouds, smog and fog will cause the color of the sky to go fromblue to grayish white.GEOG/GEOL 4093/5093 20ATMOSPHERIC EFFECTS - SCATTERINGVarious magnitudes of atmospheric scattering are shown as afunction of wavelengths. Shaded portion: scattering typical foratmospheric conditions in mid latitudesGEOG/GEOL 4093/5093 21In the atmosphere EM radiation is absorbed by• H2O Water vapor• CO2 Carbon dioxide• O2 Oxygen (not effective absorber)• O3 Ozone2.3.1. Vibrational process• Small displacements of the atoms from their equilibriumposition• N atoms ⇒ 3N possible vibrational modes2.3. ATMOSPHERIC EFFECTS - ABSORTIONGEOG/GEOL 4093/5093 22ATMOSPHERIC ABSORPTIONThe water molecule has the three classical frequencies ν1, ν2, ν3which correspond to the three wavelengths:• λ1 3.106 µm (symmetric OH stretch)• λ2 6.08 µm (HOH bend)• λ3 2.903 µm (asymmetric OH stretch)An example of combination:ν = ν3+ ν2 1/λ = 1/λ3 + 1/λ2⇒ λ = 1.965 µmMajor spectral regions pertinent to remote sensingGEOG/GEOL 4093/5093 23ATMOSPHERIC ABSORPTIONThe nature and amount of absorption depends on the• Absorption spectra of the atmospheric gases• Clouds• Other aerosolsGEOG/GEOL 4093/5093 24ATMOSPHERIC ABSORPTIONSpectral distribution of solar radiation at the top of theatmosphere and at sea level for average atmospheric conditionsand for the sun at the zenithShaded areas: Absorption by various atm. GasesUnshaded areas between the two curves: proportion of solarenergy backscattered by the air, water vapor, dust, aerosols, andreflected by clouds.Integration over the top curve is the solar constant 1364 W m-2GEOG/GEOL 4093/5093 25The combined effects of scattering and absorption on EMradiation• Attenuation coefficient = extinction coefficient• Factor by which radiation through a medium is reducedin traveling a distance of one wavelengths in themedium2.4.1. Spectral Windows• EM energy of certain wavelengths can be fully transmitted0.3 - 0.7 µm visible light3 - 5 µm emitted thermal energy from the earth8 - 11 µm emitted thermal energy from the earth1 mm - 1 m radar and microwave energy2.4. ATTENUATION
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