The temperature of a thin turbid layer above the ground is expressed as a function of time by considering the processes of nocturnal radiation and eddy conduction. It is found that under certain conditions the layer will cool until the lapse rate below the turbid layer becomes unstable. It is suggested that vertical mixing then may transform the thin layer into a thick layer extending to the ground. At the same time increased stability above the turbid layer may result in a sharply defined upper boundary. The temperature distribution in a thick turbid layer is found to depend on the free-air and ground temperatures (treated as constants), the infra-red absorptivity of the layer, and the eddy conductivities within and above the layer. If certain critical conditions are satisfied, the lapse rate within the turbid layer will become unstable and the lapse rate within the non-turbid air will become stable. Observations of the vertical temperature distribution before and after fog formation support this conclusion. Comparison of the theoretical and observed temperature distributions suggests that the coefficient of eddy conductivity within the turbid layer increases at about the time the layer achieves neutral stability. Application to the problem of air pollution suggests that the occurrence of fog is of great importance in the development of high concentrations of stack effluents.