A maze of long white clouds is interwoven into the uneven field of white that covers the Atlantic Ocean off the east coast of the United States. Though these clouds may resemble airplane contrails, the streaky clouds of condensation that follow in the wake of jet airplanes, they are actually ship tracks, clouds that form around the exhaust released by ships into the still ocean air. Water molecules collect around the tiny particles (aerosols) from exhaust to form a cloud seed. More and more water accumulates on the seed until a visible cloud is formed. In the case of ship tracks, the cloud seeds are stretched over a long narrow path where the wind has blown the ship's exhaust, so the resulting clouds resemble long strings over the ocean. Ship tracks provide important clues about how human-produced aerosols affect cloud formation. Though the exhaust released by ships is not a significant source of pollution, it does modify clouds, and that could have an impact on climate. When a large number of aerosol particles are in the atmosphere, water condenses onto a large number of ''seeds'' instead of gathering around fewer, larger seeds. As a result, ship tracks are brighter or more reflective, carry more water, and may inhibit rainfall. For more information about ship tracks, read http://earthobservatory.nasa.gov/Study/ShipTracks/ ''Every Cloud has a Filthy Lining' on the Earth Observatory. This image of ship tracks was taken by the Moderate Resolution Imaging Spectroradiometer ( modis.gsfc.nasa.gov MODIS ) on NASA's terra.nasa.gov/ Terra satellite on May 11, 2005.
