Ships at sea, like high-flying jets, can leave contrails -- cloudlike swaths of condensed water vapor -- hundreds of miles long in the atmosphere above them.

The phenomenon, sometimes photographed by satellites, works a bit differently for ships than planes, however.

Jet contrails (a contraction of "condensation trails") are largely the result of water vapor in the plane's exhaust, which condenses and freezes into clouds of ice particles after hitting cold air.

Ship contrails occur when the vessels move under a fairly calm atmosphere and microscopic particulates from the ship's exhaust stacks rise into the middle atmosphere. The particles become nuclei around which water vapor condenses. Although the condensation presumably occurs regardless of winds, calm air preserves the trail long enough to yield the impression of an ocean aswarm with ships.

James A. Coakley Jr. and two colleagues wrote in last week's Science that space-based study of ship contrails could lead to a better understanding of the possible climatic effects of pollutant particles released into the atmosphere.

Coakley, a scientist at the National Center for Atmospheric Research in Boulder, Colo., said his findings show that although the particulates absorb sunlight and add to atmospheric heating, the effect is offset by the sunlight-reflecting properties of the droplets that form around the particles.

An increase in particulates spewed into the air, Coakley said, could be expected to increase the atmosphere's ability to reflect sunlight back into space. He suggested that the solar heat repelled by this phenomenon might offset the increased retention of the heat that does get through by "greenhouse gases," such as carbon.