meteorological optical range [v]

length of the path in the atmosphere that is required to attenuate by 95 % the luminous flux in a collimated beam from a light source at a colour temperature of 2 700 K

NOTE 1 The value of the attenuation has been chosen so that the term provides an approximate measure of the commonly used concept of (meteorological) visibility, which is the greatest distance at which a black object of suitable dimensions can be recognized by day against the horizon sky.

NOTE 2 The meteorological optical range, v, is related to the atmospheric transmissivity, T, assumed to be uniform, by the formula


where d0 is the length specified in the definition of T.

These formulae are sometimes written


with d0 set to unity.

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