Turbidity networks to obtain daily values of haze attenuation from measurements of solar radiation, mostly by means of sun photometers, were established in 1961 in the USA by the National Center for Air Pollution Control, Cincinnati, Ohio, and in Western Europe from 1963 to 1967 by the author. The course of turbidity in the two networks during interesting periods is presented. Discussion of synoptic variations of turbidity is rather difficult, when referring to periods of rapid change of air masses, but consistency over large parts of the network is often observed during periods of quiet, sunny weather. Other data show that industrial activity over Central Europe is resulting in a daily increase of vertical optical thickness τD ≅ 0.02, while τD = 0.23 in the average. Thus, the often observed high values of turbidity (τD ≅ 1.1) must normally have other causes and may, in the climate of North America and Europe, be related mostly to cloud physics.