As ships crisscross the world’s oceans they can leave behind a trail of clouds.
As ships move across the water, pollution particles are released into the air. These particles, such as sulfates, serve as seeds around which water condenses. These seeds are called cloud condensation nuclei.
The end result are long, narrow clouds across the waters that follow the path that the ship took known as ship tracks.
The clouds can be seen crisscrossing oceans from satellite imagery, particularly in busy shipping corridors found in the Atlantic Ocean and the Pacific Ocean. Ship tracks are most common in places with low-lying stratus and cumulus clouds.
This Aqua satellite image shows bright lines of ship track clouds over the Pacific Ocean off the coast of California.
Clouds that form around ship exhaust tend to have more and smaller water droplets. This type of cloud formation scatters sunlight and makes the clouds look brighter than other marine cloud formations.
NASA researchers used remote sensing to measure the size of droplets in cloud ship tracks compared to natural clouds using satellite imagery from the Terra satellite.
The ship tracks are brighter than regular clouds because the cloud particles in them are smaller (yellow and peach) yet more frequent than those in natural clouds (lavender to dark purple).
Faster ships will leave ship tracks that will be narrower, longer, and less diffuse. Ships that travel at a slower rate will leave shorter, broader, and more dispersed ship tracks. Ship tracks frequently reflect the wind’s direction and speed as well as the ship’s direction and speed.
Ship Track Clouds Over Time
When ship tracks first form, they tend to be narrow and straight. Over time, the ship track clouds become broader and wavier.
This satellite image of the Atlantic Ocean off the coast of Portugal shows ship tracks. Some of the criss-crossing clouds stretch for hundreds of kilometers.
What is the Cooling Effect of Ship Track Clouds?
Researchers have looked at the cooling effect of these narrow clouds. One research team led by University of Washington looked at ship tracks from ten-years worth of satellite imagery for the southeast Atlantic shipping corridor that connects Europe to southern Africa and Asia.
The researchers found that about 2 Watts of solar energy were blocked from reaching each square meter of ocean surface in the shipping lane due to the clouds. Globally, the research team extrapolated that short-term cooling effects from ships track clouds blocks about 1 Watt of energy per square meter.
According to Michael Diamond, an atmospheric scientist, “Cloud changes caused by industrial pollution have produced a global cooling effect that is about one-third as strong as the warming from increased greenhouse gases.”
References
Diamond, M. S., Director, H. M., Eastman, R., Possner, A., & Wood, R. (2020). Substantial cloud brightening from shipping in subtropical low clouds. AGU Advances, 1(1), e2019AV000111. https://doi.org/10.1029/2019AV000111
Lindsey, R. (2009, March 12). Ship tracks south of Alaska. NASA Earth Observatory. https://earthobservatory.nasa.gov/images/37455/ship-tracks-south-of-alaska
Voiland, A. (2012, March 8). Ship tracks off the California coast. NASA Earth Observatory. https://earthobservatory.nasa.gov/images/77345/ship-tracks-off-the-california-coast
Voiland, A. (2018, January 21). Signs of ships in the clouds. NASA Earth Observatory. https://earthobservatory.nasa.gov/images/91608/signs-of-ships-in-the-clouds
