Atmospheric Transmission Factor

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2-1-270

Atmospheric Transmission Factor (for Unit: distance) (Gcentre of gravity.BCentre of buoyancy.)

Alternative term: Atmospheric Transmissivity (U.SSouth.Aampere.)

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: TTime periodroll periodTransmissivity


Note 1: In Germany, according to the DINGerman steel gradingDeutsches Institut für Normung, the German Institute for Standardisation standard, in place of the "atmospheric transmission factor" TTime periodroll periodTransmissivity, two terms are used, Transmissionsfaktor q, used when the Unit: of distance chosen is the kilometre, and Sichtwert ssecond (not to be confused with ssecond 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.SSouth.Aampere. the most important factor is the Atmospheric Extinction Coefficient ssecond (not to be confused with ssecond as used in Note 1: nor with ssecond as used in 2-1-230). It is related to the "atmospheric transmission factor" TTime periodroll periodTransmissivity by the formula

TTime periodroll periodTransmissivity = e XXXX ssecond

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 DINGerman steel gradingDeutsches Institut für Normung, the German Institute for Standardisation 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" ssecond 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 TTime periodroll periodTransmissivity, q, ssecond, z, etc. should not be used so widely in future, and that the "meteorological visibility" VVolumeVoltVisibility (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 IALAInternational Association of Marine Aids to Navigation and Lighthouse Authorities - AISM Dictionary edition (1970-1989)