## Scattering

Atmospheric scattering is accomplished through absorption and immediate re-emission of radiation by atoms or molecules. Rayleigh scattering occurs when radiation interacts with particles or molecules much smaller in diameter than the wavelength of the radiation. This type of scattering takes place in the upper 4 to 5 km of the atmosphere. The degree of Rayleigh scattering is inversely proportional to the fourth power of the wavelength. As a result, short wavelength radiation tends to be scattered much more than long-wave radiation. The wavelength dependence and lack of directional dependence of Rayleigh scattering results in the appearance of "blue" sky.

If the atmospheric particulates are of roughly the same size as the wavelength of incident radiation, Mie scattering becomes important. This type of scattering takes place in the lower atmosphere and can affect longer wavelength radiation. Smoke and dust particles are the dominant sources of Mie scattering. The orange and red light visible in the sky during sunrise and sunset are the result of this type of scattering. The greater the amount of smoke and dust particles in the atmosphere, the more that violet and blue light will be scattered away and only the longer orange and red wavelength light will be visible. Unlike Rayleigh scattering, most of the incident radiation is scattered in a forward direction as a result of Mie scattering.

Visible satellite imagery is suseptible to Mie scattering, especially looking with some component towards the sun. The effect is a hazy image looking towards the sun as can be seen in the center of this image.

A less hazy image can be seen looking away from the sun in the center of the second image. These two images were taken over Argentina and Chile from the GOES-7 and METEOSAT-5 satellites, respectively. To reduce the haze effect, a longer wavelength such as the near infrared channel aboard the AVHRR instrument on the NOAA polar orbiting satellites should be selected. Note that even though the lower image is brightened, the cloud free areas are still darker than the top image.

Non-selective scattering takes place in the lowest portions of the atmosphere where there are particles greater than 10 times the wavelength of the incident electromagnetic radiation. All wavelengths of light are scattered. Thus, the water droplets and ice crystals that comprise clouds scatter all visible light wavelengths equally well. As a result, clouds appear white because of this type of scattering as can be seen with the above two images.

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