Image: Summer ship tracks in the Pacific
In August 2018, long, narrow clouds stood out against the backdrop of marine clouds blanketing much of the North Pacific Ocean. Known as聽ship tracks, the distinctive clouds form when water vapor condenses around the聽tiny particles聽emitted by ships in their exhaust. Ship tracks typically form in areas where thin, low-lying聽stratus聽and聽cumulus clouds are present.
Some particles generated by ships (especially sulfates) are soluble in water and serve as the seeds around which聽cloud droplets聽form. Clouds infused with ship exhaust have more and smaller droplets than unpolluted clouds. As a result, the light hitting the polluted clouds scatters in many directions, making them appear especially bright and thick.
The聽Moderate Resolution Imaging Spectroradiometer聽(MODIS) on Aqua captured this natural-color image of several ship tracks extending northward on August 26, 2018. The clouds were located about 1,000 kilometers (600 miles) west of the California-Oregon border. Similar environmental conditions also triggered the formation of ship tracks in this part of the Pacific on聽August 27聽and聽28.
An analysis of聽one year of satellite observations聽from the聽Advanced Along Track Scanning Radiometer聽(AATSR) on the European Space Agency's聽Enivisat聽indicates that very low clouds are most often present off the west coasts of North and South America.
The large number of ships聽traversing the North Pacific, combined with all of the low clouds, make ship tracks more common here than anywhere else in the world. Roughly two-thirds of the world's ship tracks are found in the Pacific, according to聽the study. Other ship track hotspots were in the North Atlantic, off the west coast of southern Africa, and off the west coast of South America.
The research team also detected a clear seasonality in their occurrence: they are most often observed in May, June, and July, and only occasionally present in December, January, and February. Ship traffic is roughly constant throughout the year, so the cycle is mostly due to seasonal changes in the abundance of very low clouds.
Provided by NASA