Electromagnetic Radiation: Interactions in the Atmosphere

Sun’s Radiant Energy DistributionSpectral Region Wavelength Range

[m]Percent of Total

Energy

Gamma and X-rays < 0.01 Negligible

Far Ultraviolet 0.01 - 0.2 0.02

Middle Ultraviolet 0.2 - 0.3 1.95

Near Ultraviolet 0.3 - 0.4 5.32

Visible 0.4 - 0.7 43.5

Near Infrared 0.7 - 1.5 36.8

Middle Infrared 1.5 - 5.6 12.0

Thermal Infrared 5.6 - 1000 0.41

Microwave > 1000 Negligible

Radio Waves > 1000 Negligible

Solar Emittance CurveRadiation 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

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]

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:

Transmission,

Absorption, (and Emission)

Reflection,

Reflection + Absorption + Transmission = 1

For the Earth’s surface, we ignore TransmissionReflection + Absorption = 1

Reflection– diffuse reflection (rough surface)– specular reflection (smooth surface)

Diffuse Specular

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

– 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

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

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

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)

• Non-selective Scattering– aerosol particles much larger than the wavelength

(> 10x)– examples: water droplets, ice crystals, volcanic

ash, smog

– independent of wavelength: 0

Atmospheric Absorption

Absorption is the process whereby radiant energy is absorbed by atmospheric constituent and converted to thermal energy

Atmospheric absorbers are primarily:• H2O water vapor, water droplets• CO2 carbon dioxide• O2 oxygen• O3 ozone• Dust and soot

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

H H

O

H H

O

H H

2 = 6.08 m1 = 3.106 m

3 = 2.903 m

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

Absorption Bands

• An absorption band is a portion of the EM EM spectrum within which radiant energy is spectrum within which radiant energy is absorbed by substances such as water (Habsorbed by substances such as water (H22O), O),

carbon dioxide (COcarbon dioxide (CO22), oxygen (O), oxygen (O22), ozone ), ozone

(O(O33), and nitrous oxide (N), and nitrous oxide (N22O), dust, soot, etc.O), dust, soot, etc.

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