# Atmospheric Transmission Factor

**2-1-270**

Atmospheric Transmission Factor (for Unit: distance) (G.B.)

Alternative term: Atmospheric Transmissivity (U.S.A.)

A quantity characterising the transparency of the atmosphere. It is the ratio of the luminous flux transmitted without change of direction by the atmosphere over Unit: distance to the luminous flux which would be transmitted along the same path in a vacuum. The Unit: of distance chosen is usually the kilometre or the nautical mile (sea-mile).

Symbol: T

Note 1: In Germany, according to the DIN standard, in place of the "atmospheric transmission factor" T, two terms are used, Transmissionsfaktor q, used when the Unit: of distance chosen is the kilometre, and Sichtwert s (not to be confused with s used as Symbol: for "diffusion factor" 2-1-230), used when the Unit: of distance chosen is the sea-mile.

Note 2: It is also possible to use other related factors and practice in this respect has differed in various countries.

In Britain and the U.S.A. the most important factor is the Atmospheric Extinction Coefficient s (not to be confused with s as used in Note 1: nor with s as used in 2-1-230). It is related to the "atmospheric transmission factor" T by the formula

T = e XXXX s

where e is the base of natural logarithms.

In France, geophysicists use the Optical Density (for a thickness of 1 kilometre).

In Germany, according to the DIN standard, the factor Schwachungskoeffizient z is used. This is similar to the atmospheric extinction coefficient but is identical with it only if the Unit: of distance chosen is the kilometre. It is related to the 'Transmissionsfaktor" q and to the "Sichtwert" s by the formulae

q = e-z

= e-1.852s

where e is the base of natural logarithms.

Note 3: It is suggested that the various quantities described above T, q, s, z, etc. should not be used so widely in future and that the "meteorological visibility" V (2-1-280 and 2-1-290, Note:) should be used instead.

*Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)*