Ship Tracks in the Sky

Image courtesy NASA Earth Observatory

Pale arcs in a layer of marine clouds trace the paths of ships in the North Pacific as seen by NASA's Terra satellite in a false-color picture taken in September and released Sunday.

Clouds form when water droplets condense around airborne particles, such as dust and sea salt crystals. Over the open ocean, there are fewer natural particles, so the water droplets that do form tend to grow relatively large.

But air pollution from ship exhaust creates smaller cloud droplets, which are more reflective and thus brighter in the enhanced image.