Following Koschmeider's method of attack an expression for the daytime visibility within a cloud or fog is derived in terms of the size of the suspended drops and the number of drops in a unit volume of air. A comparison of typical data for clouds and fogs shows that clouds contain from 50 to 500 drops per cc. with an average drop diameter of about 20 microns while fogs contain only 1 to 10 drops per cc. with an average drop size of about 50 microns. Simultaneous observations of the visibility and the fog characteristics reveal a good qualitative agreement with the theory. In clouds the visibility tends to be inversely proportional to the liquid water content, indicating a constant average drop diameter. In fogs the visibility is more nearly inversely proportional to the two-thirds power of the liquid water content which suggests that the number of drops in a unit volume tends to remain constant. The value of Fechner's constant, which represents the threshold contrast between the object and the background, was found to be greater in a fog than when measured in the laboratory and still larger in clouds. I t is suggested that this is due to the diffuse boundary between the object and the background in the case of a fog or cloud.