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A practical method of determining the visibility number v at night
Author(s) -
Gold Ernest
Publication year - 1939
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49706528003
Subject(s) - visibility , metre , scale (ratio) , filter (signal processing) , meteorology , mathematics , computer science , environmental science , optics , geography , physics , computer vision , cartography , astronomy
The paper contains an account of a method of ascertaining the visibility number V of the International Scale of Visibility from observations of lights at fixed distances smaller than the actual distance of visibility. The method depends on the use of a meter with a filter in which the obscuring power varies continuously from zero to a value large enough to obscure the selected lights when the atmosphere is clear. The construction of the meter and its method of use are described. A unit of obscuring power is defined such that 100 units reduce the intensity of light to 1/1000th part of its incident value. This unit is of a convenient practical size for use in the graduation of the meter. In view of its utility the name “nebule” is proposed for this unit. The paper also contains an account of a method of estimating the distance of “sure” visibility of lights of different candle power at different distances in conditions of atmospheric turbidity corresponding with the different numbers of the standard scale of visibility, and suggestions for the construction of a more logical scale of visibility than the existing International Scale.