Transmittance, Atmospheric

Transmittance, Atmospheric 

(also, atmospheric transparency), the capacity of the atmosphere to transmit electromagnetic energy. Atmospheric transmittance is dependent on the air mass penetrated by rays, as well as on the amount of water vapor and dust in the air. It varies for radiations of different wavelengths; the smaller the atmospheric absorption and scattering of light, the greater the atmospheric transmittance.

Atmospheric transmittance may be characterized either by the transmission coefficient, which is equal to the fraction of radiation that passes through the atmosphere when rays fall vertically, or by the turbidity factor, which indicates the extent to which the transmittance of an actual atmosphere under given conditions differs from the transmittance of an ideal (ideally clean and dry) atmosphere.

The atmospheric transmittance of the rays of a celestial body decreases as the body approaches the horizon because the length of the path of the rays in the air increases, which is expressed by the optical air mass.

Atmospheric transmittance varies at different points on the earth’s surface. The most transmissive air is in polar countries because it contains the least amount of dust and moisture. Changing humidity and the dust content in the air at a given point throughout the year determine the annual atmospheric transmittance at that point. Thus, for example, in Pavlovsk, near Leningrad, the minimum atmospheric-transmission coefficient, which is equal to 0.759, is observed in July, and the maximum transmission coefficient, which is equal to 0.806, is observed in December (for the same optical air mass, equal to 2). Thus, the atmosphere is most transmissive in winter and least transmissive in summer.

A significant decrease in atmospheric transmittance is observed as a result of increasing air pollution, especially when the dust content increases.

REFERENCES

Kondrat’ev, K. Ia. Aktinometriia. Leningrad, 1965.
Berliand, M. E., and K. Ia. Kondrat’ev. Goroda i klimat planety. Leningrad, 1972.
Zuev, V. E. Rasprostranenie vidimykh i infrakrasnykh voln ν atmosfere. Moscow, 1970.