The propagation of radiation through a cloud depends on the concentration and size distribution of drops. A series of aerosol measurements in various types of fogs and hazes shows a direct relationship between a subset of the dry aerosol and fog drops. These measurements allow identification of the specific aerosol component (fog condensation nuclei, FCN) upon which fog droplets condense. The extensive variability of these particle concentrations can be seen in the preliminary FCN climatology which is presented. It is shown that the variability in fog drop concentration is largely due to differences in FCN concentrations which are characteristic of various air masses.