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Ship Trails of the Pacific: Earth Image of the Week July 16, 2010
Satellite Image
Especially calm air over the eastern Pacific allows ship tracks to hang over the ocean for days.
The white contrails left high in the atmosphere by passing jet aircraft have a maritime counterpart near sea level in at least some regions of the world’s oceans.

The white streaks visible in the NASA image to the right were not caused by aircraft, but were instead left in the wake of ships at sea.

As the MODIS instrument orbiting on the Aqua satellite captured the image at midday on July 3, 2010, the “ship tracks” from several vessels were hovering over the Pacific off the California and Oregon coasts.

This is one of the most heavily traveled areas of the world for maritime shipping, with ships carrying cargo from ports along the West Coast to destinations in Asia and elsewhere around the world.

The ship tracks phenomena form when water molecules gather around the exhaust that ships release into the air. When enough water molecules collect, a visible cloud is created.

Ship track clouds often have a long, rope-like appearance because they become stretched over a narrow path when blown by the wind. Such formations may remain visible for days until they are eventually dispersed by the wind.

The atmosphere has to be just right to allow the tracks to form. They are mainly visible only over parts of the eastern Pacific Ocean. Their appearances in the western Atlantic have been extremely rare, but they have occasionally been seen off the coast of Western Europe.

Ship tracks can only form in a very clean air mass of Arctic origin. A temperature inversion is also needed at about 1,600 to 2,300 feet above the ocean’s surface, allowing the lower atmosphere to remain very stable with very little turbulence or upward motion of the air.

Image: NASA MODIS Rapid Response System