1. Electromagnetic Radiation: Interactions in the Atmosphere

2. Sun’s Radiant Energy Distribution Spectral RegionWavelength Range [  m] Percent of Total Energy Gamma and X-rays< 0.01Negligible Far Ultraviolet0.01 - 0.20.02 Middle Ultraviolet0.2 - 0.31.95 Near Ultraviolet0.3 - 0.45.32 Visible0.4 - 0.743.5 Near Infrared0.7 - 1.536.8 Middle Infrared1.5 - 5.612.0 Thermal Infrared5.6 - 10000.41 Microwave> 1000Negligible Radio Waves> 1000Negligible

3. Solar Emittance Curve Radiation leaving the surface of the sun: 6.4 ´ 107 Wm -2 Solar radiation at sea level: 1370 Wm -2 Blackbody radiation curve By comparison, the Earth at 288K emits only 390 Wm -2

4. Terminology (yuck!) Radiant flux , [W, Js -1 ] Irradiance (flux density), E [Wm -2 ] (its is called Exitance, M when it is away from the surface) Radiance, L, [Wm -2 sr -1 ] Note, all can be functions of wavelength with additional units [  m -1 ]

5. Electromagnetic Radiation Interactions with Matter Conservation of energy: radiation at a given wavelength is either –reflected (  ) -- property of surface or medium is called the reflectance or albedo (0-1) –absorbed (  ) -- property is absorptance or emittance (0-1) –transmitted (  ) -- property is transmittance (0-1) Radiation Budget Equation:     

6. Transmission,  Absorption,  (and Emission) Reflection, 

7. Reflection + Absorption + Transmission = 1 For the Earth’s surface, we ignore Transmission Reflection + Absorption = 1

8. Reflection –diffuse reflection (rough surface) –specular reflection (smooth surface) Diffuse Specular

9. Atmospheric Effects EMR is attenuated by its passage through the atmosphere Attenuation = scattering + absorption –Scattering is the redirection of radiation by reflection and refraction –Attenuation is wavelength dependent

12. –scattering by molecules and particles whose diameters are << –primarily due to oxygen and nitrogen molecules –scattering intensity is proportional to -4 –responsible for blue sky Rayleigh Scattering

13. A Clear Blue Sky Blue radiation ( = 0.46) Red radiation ( = 0.66) (0.66/0.46) 4 = 4.24 Blue is scattered 4  more than red radiation

15. Mie Scattering –Spherical particles that have a mean diameter 0.1 to 10 times the incident wavelength –Examples for visible light: water vapor, smoke particles, fine dust –Scattering intensity is proportional to -4 to 0 (depending on particle diameter) Clear atmosphere has both Rayleigh and Mie scattering. Their combined influence is between -0.7 to 

16. Red Sky at Night At sunset, solar radiation must traverse a longer path through the atmosphere. Viewing a setting sun, the energy reaching the observer is largely depleted of blue radiation, leaving mostly red wavelengths (Rayleigh). Dust/smoke adds additional scattering with a wavelength dependence that increases the red sky effect (Mie)

17. Non-selective Scattering –aerosol particles much larger than the wavelength (> 10x) –examples: water droplets, ice crystals, volcanic ash, smog –independent of wavelength: 0

19. Atmospheric Absorption Absorption is the process whereby radiant energy is absorbed by atmospheric constituent and converted to thermal energy Atmospheric absorbers are primarily: H 2 Owater vapor, water droplets CO 2 carbon dioxide O 2 oxygen O 3 ozone Dust and soot

20. Absorption Energy incident on a molecule causes –small displacements of the atoms from their equilibrium position (vibrations and rotations) –N atoms 3N possible vibrational modes O HH O HH O HH 2 = 6.08  m 1 = 3.106  m 3 = 2.903  m

21. Water Vapor Absorption The water molecule has three classical frequencies 1, 2, 3 that correspond to the three wavelengths: 1 3.106 mm (symmetric OH stretch) 2 6.08 (HOH bend) 3 2.903 mm (asymmetric OH stretch) An example of combination: = 2 + 3 1/  1/   1/    m

22. Absorption Bands EM spectrum within which radiant energy is absorbed by substances such as water (H 2 O), carbon dioxide (CO 2 ), oxygen (O 2 ), ozone (O 3 ), and nitrous oxide (N 2 O), dust, soot, etc.An absorption band is a portion of the EM spectrum within which radiant energy is absorbed by substances such as water (H 2 O), carbon dioxide (CO 2 ), oxygen (O 2 ), ozone (O 3 ), and nitrous oxide (N 2 O), dust, soot, etc.

23. Atmospheric “Windows” Regions in the EM spectrum where energy can be fully transmitted 0.3-0.7  m UV and visible light 3-5  m emitted thermal energy from Earth 8-11  m emitted thermal energy from Earth 1 mm-1 m radar and microwave energy