Experimental studies of condensation nuclei and laboratory fog investigations have largely been based on the assumption of adiabatic conditions. This study includes a check on the validity of this assumption under a number of different conditions. The median size and the number of fog droplets were determined during the formation and evaporation of artificial fogs. The effect of nearness to a water surface is also studied. The results for the artificial fogs are compared with measurements of natural fogs. The jet method, used in sampling artificial fogs, is found to yield out‐of‐door measurements that are in line with those obtained by the sedimentation method. The experiments show that, due to the non‐fulfillment of adiabatic conditions in the laboratory, temperature and moisture conditions in an air sample cannot be calculated from measurements of pressure. They show, too, that condensation processes produced in the laboratory are adequate for studying the type and, to some extent, the size distribution of condensation products as they occur in n