# 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 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)*