The concentrations of cloud condensation nuclei (CCN) in the plumes from coal-fired electric power plants are generally about 2 to 5 times greater than in the ambient air unaffected by the plumes. However, if the ambient air is very clean, the concentrations of CCN in a coal power plant plume can be up to ∼80 times greater than in the ambient air. The rates of production of CCN due to gas-to-particle (g-to-p) conversion in the plume from one of the plants studied were measured on different occasions to be ∼2 × 1015 and ∼5 × 1013 CCN h−1 per mole of SO2. The maximum current of CCN to be expected in the plume from a coal power plant is ∼1017 CCN s−1. After a travel time of ∼1 h, most of the CCN in power plant plumes have been produced by g-to-p conversion rather than emitted directly from the stack. The concentrations of ice nuclei in the plumes did not differ significantly from those in the ambient air. The materials in a plume may be transported rapidly in the vertical if the plume is entrained into a convective cloud. The plume may cause a lowering in the altitude of the cloud base, but any effects that the plume may have on the drop size distribution in a convective cloud are often less than the natural variations. By contrast, in stratiform clouds a plume can produce marked increases in the concentration of small drops (∼10–20 μm diameter) and in the total concentrations of drops