Diagnosis of the Causes of Systematic Errors in Atmospheric Transparency Measurements and Some Experimental Verifications in Po Valley Fogs

Abstract

Abstract An extended view of the systematic errors in calibrating transmissometers together with their causes is given. It is shown that systematic errors can be quantified by comparing transparency and luminance contrast data. The proper measurement conditions for such a comparison are described. Instrumental characteristics as well as a method of comparing experimental data are also given. A large series of measurements of both luminance contrast and atmospheric transparency performed in Po Valley fogs by using a standard transmissometer and two instrumental prototypes, namely a transmissometer not requiring calibration in “clean air” and an automatic luminance contrast meter, is presented. A detailed analysis of the results so obtained shows that atmospheric transparency, as given by both transmissometers, may be overestimated by up to 20–30%. Unless the systematic errors in measuring atmospheric transparency are held to much lower values, the calculated extinction coefficients are of little value.