In August of this year, long, narrow white stripes stood out against the backdrop of clouds covering most of the northern Pacific Ocean. These are ship tracks - a peculiar form of clouds that form when water vapor condenses around micro-particles of exhaust gases from ships. They usually form in areas where low stratus and cumulus clouds are present. The ejected microparticles are composed of water-soluble elements, so they can combine with steam to form clouds that can be seen from space.
The clouds formed as a result of the exhaust gases from ships contain more droplets, and the droplets themselves are smaller than in unpolluted clouds. As a result, light entering them is scattered in many directions, making them especially bright and dense.
The MODIS instrument on the Aqua satellite captured this image in natural colors on August 26, with ship tracks extending northward. The clouds were about 1000 kilometers west of the California-Oregon border. Similar environmental conditions also triggered the formation of ship tracks in this part of the Pacific on August 27 and 28. Analysis of the results of annual observations using the AATSR instrument on the ESA satellite Enivisat shows that very low clouds are most often present off the western coast of the Americas.
The large number of ships crossing the North Pacific, combined with low clouds, makes ship tracks more common in this part of the globe than anywhere else. According to the study, approximately two-thirds of ship tracks are formed in the Pacific Ocean. They are also seen in the North Atlantic, off the west coast of South Africa and off the west coast of South America. The research team also found clear signs of seasonality in the formation of these clouds: they are most often observed in May, June and July, and only rarely present in December, January and February. The movement of ships is more or less constant throughout the year, so the cycle is mainly driven by seasonal changes that affect the formation of low clouds.