Lacy lines and long streaks decorated a bank of marine stratocumulus clouds off the west coast of the United States in early November, 2013.

These bright white, highly reflective streaks are dense clouds created by particles in ship exhaust. As the ships sail the seas, they emit gasses and tiny airborne particles (aerosols). As the aerosols rise, they act as seeds (nuclei) for cloud formation. Water vapor condenses on them, and clouds are formed.

Exhaust plumes of ships contain large numbers of cloud condensation nuclei, many more than is naturally present in the atmosphere. The high number of nuclei and the type of nuclei increase both the number of droplets formed, as also reduce the droplet size. The result is a more dense and highly reflective cloud than that formed from either dust or sea salt – two common types of condensation nuclei. Because of the higher reflectivity, these clouds appear brighter white than the background clouds. In this way, the tracks of ships can be written on the clouds.