As NASA said, ship tracks typically are formed in areas where low-lying stratus and cumulus clouds are present.
Some of the pollution particles generated by ships are soluble in water and serve as the seeds around which cloud droplets form. Clouds filled with ship exhaust have more and smaller droplets. This caused the light hitting the clouds scatter, making them appear brighter.
"Visualizations of ship traffic show numerous ships sailing in the Mediterranean, with many of them sailing in the Iberian Peninsula. The large volume of ships along the coast appear to have brightened the clouds so much that it is difficult to distinguish individual ship tracks. The more visible tracks are several hundred kilometers offshore, and many of these appear to be created by ships heading out of the Mediterranean Sea toward North America. Others are probably the result of ships from South America and Africa charting courses toward northern Europe," NASA said.
The Moderate Resolution Imaging Spectroradiometer (MODIS) the image depicted above, on January 16, 2018.
"Some of the criss-crossing clouds stretch hundreds of kilometers from end to end. The narrow ends of the clouds are youngest, while the broader, wavier ends are older," NASA concluded.