On March 4, 2009, the skies over the northeast Pacific Ocean were streaked with clouds that form around the particles in ship exhaust.
This pair of images reveals how these ship tracks are different from the natural marine clouds in the same area. The top image is a natural-color (photo-like) view of the ship tracks. The image below reveals more information than a picture: it shows the size of the cloud droplets. Both images are based on observations made by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite.
In the natural-color view of the clouds, the ship tracks are noticeably brighter. What makes one cloud brighter than another? The bottom image reveals one of the important influences on brightness: the size (radius) of the droplets in the cloud. The ship tracks are brighter than the regular clouds as the cloud particles in them are smaller (yellow and peach), but more numerous, than the particles in the natural clouds (lavender to dark purple). The two types of clouds may actually contain the same volume of water, but when the water gets spread out over many, many small droplets, the total reflective surface area increases. (Said another way, as you make a sphere smaller and smaller, the volume decreases faster than the surface area). More light is reflected back to the satellite, and the cloud looks brighter.
Clouds form when water vapor condenses (or freezes) onto a small particle, like a speck of dust or a salt crystal. The bigger the “seed,” the bigger the cloud droplet. The particles in ship exhaust are even tinier than natural sea salt particles, so the clouds that form in the wakes of ships have smaller droplets. A cloud’s brightness impacts how much sunlight gets bounced back to space and how much reaches the surface of the Earth, which influences global climate. The size of the particles also influences the amount of rain the clouds produce; smaller droplets are less likely to collide and form drops that are big enough to fall as rain.
(Credits: NASA images by Robert Simmon and Jesse Allen, based on an image by Jeff Schmaltz, MODIS Rapid Response Team. Caption by Rebecca Lindsey)
