Satellite cloud photography occasionally reveals anomalous cloud lines in the form of plumes often 500 km long and up to 25 km wide. These lines, found over the ocean, are composed of liquid particles. Hollow polygonal, convective or thin stratiform, cloud patterns in varying amounts are associated with the lines.Numerous lines within a 10-deg square have been observed which cover up to 4 per cent of the total area.The most likely cause of the cloud lines stems from the exhaust of ocean going vessels. Large numbers of Aitken nuclei form in this exhaust. These are carried upward by the buoyancy of the hot gases and `ships air wake' to form droplets at slight supersaturation. The phenomenon does not appear related to special characteristics of the vessel's power plant but to a critical condition of the atmosphere. As far as is known, this condition may be described as having 1) a convectively unstable layer from the surface to a low-level stable layer, 2) saturation or slight supersaturation near the top of the convective layer, and 3) a convective layer, presumably deficient in cloud forming nuclei.Nuclei are produced in sufficient quantity by an average size ship, moving at 10 m sec1 relative to the wind, to generate 200 droplets cm3 throughout a depth of 200 m and width of 1.9 km. The theoretical increase in albedo of the new cloud over that of the old maritime cloud is about 25 per cent, an amount comparable to that indicated in the photographs.