As ships crisscross the world’s oceans, they can leave behind a distinct and visible trail of clouds, known as ship tracks. These clouds are not merely a byproduct of natural atmospheric processes but are directly influenced by human activity—specifically, the emissions produced by the ships themselves.
How ships create cloud condensation nuclei
When ships move across the water, they release pollution particles into the air, including sulfates and other aerosols. These particles act as seeds, or cloud condensation nuclei (CCN), around which water vapor condenses. This process is similar to how natural clouds form, but the presence of additional CCN from ship emissions increases the number of droplets, leading to the formation of long, narrow clouds that follow the ship’s path.
The formation of ship tracks
The clouds formed by this process, known as ship tracks, are distinctive in their appearance. They are typically long and narrow, stretching across the water in the same direction as the ship’s movement. These clouds are particularly visible in satellite imagery, where they appear as bright lines against the darker background of the ocean. Ship tracks are most commonly observed in areas with low-lying stratus and cumulus clouds, which are prevalent in many of the world’s busiest shipping corridors.
Where ship tracks are most visible
Ship tracks are often seen in the Atlantic and Pacific Oceans, particularly along major shipping routes. These areas are characterized by frequent low-lying cloud cover, which provides the ideal conditions for ship tracks to form and persist. Satellite images of these regions, such as those captured by NASA’s Aqua satellite, frequently show bright lines of ship track clouds, especially off the coast of California where shipping activity is high.
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.
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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.
How ship track clouds change 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
