1.11 Transmittance of the Atmosphere

1.11 Transmittance of the Atmosphere

The sunlight's transmission through the atmosphere is affected by absorption and scattering of atmospheric molecules and aerosols. The reduction of sunlight intensity is called extinction. The rate of extinction is expressed as extinction coefficient (see 1.12).

The optical thickness of the atmosphere corresponds to the integrated value of the extinction coefficient at each altitude by the atmospheric thickness. The optical thickness indicates the magnitude of absorption and scattering of the sunlight. The following elements will influence the transmittance of the atmosphere.

a. Atmospheric molecules(smaller size than wavelength):
carbon dioxygen, ozone, nitrogen gas, and other molecules

b. Aerosols (larger size than wavelength):
water drops such as fog and haze, smog, dust and other particles with a bigger size

Scattering by atmospheric molecules with a smaller size than the wavelength of the sunlight is called Rayleigh scattering. Raleigh scattering is inversely proportional to the fourth power of the wavelength.

The contribution of atmospheric molecules to the optical thickness is almost constant spatially and with time, although it varies somewhat depending on the season and the latitude.

Scattering by aerosols with larger size than the wavelength of the sunlight is called Mie scattering. The source of aerosols will be suspended particles such as sea water or dust in the atmosphere blown from the sea or the ground, urban garbage, industrial smoke, volcanic ashes etc., which varies to a great extent depending upon the location and the time. In addition, the optical characteristics and the size distribution also changes with respect to humidity, temperature and other environmental conditions. This makes it difficult to measure the effect of aerosol scattering.

Scattering, absorption and transmittance of the atmosphere are different for different wavelengths. Figure 1.11.1 shows the spectral transmittance of the atmosphere. The low parts of the curve show the effect of absorption by the molecules described in the figure. Figure 1.11.2 shows the spectral transmittance, or conversely absorption, with respect to various atmospheric molecules. The open region with higher transmittance in called "an atmospheric window".

As the transmittance partially includes the effect of scattering, the contribution of scattering is larger in the shorter wavelengths. Figure 1.11.3 shows a result of simulation for resultant transmittance multiplied by absorption and scattering which would be produced for a standard "clean atmospheric model" in the U.S.A. The contribution by scattering is dominant in the region less than 2mm and proportional according to the shorter wavelength. The contribution by absorption is not constant but depends on the specific wavelength.