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A bank of clouds off North America’s west coast featured a series of white trails in early October 2009. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured this true-color image on October 5, 2009. Although the white trails look vaguely like contrails left behind by airplanes, they actually result from ship exhaust.

The exhaust trails’ appearance—whiter than those of the surrounding clouds—results from them having smaller, but more numerous cloud droplets. Compared to the surrounding air, ship exhaust contains more particles, and each particle can act as a nucleus around which water vapor condenses. Because the available water is divided up among a greater number of particles, the resulting ship tracks consist of cloud particles that are smaller and more abundant than those of the surrounding clouds.

The cloud bank shows a fairly sharp boundary along its eastern edge, and the ship tracks end with the other clouds. The cloud-free area to the east apparently holds drier air, which lacks sufficient moisture to lead to cloud formation.

NASA image by Jeff Schmaltz, MODIS Rapid Response Team, Goddard Space Flight Center. Caption by Michon Scott.