Seeing the Clouds Close-Up

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While satellite instruments can show scientists that clouds are getting brighter from ships’ smokestacks, the images cannot tell them anything about the details of the processes behind the phenomenon. For that, the scientists have to take measurements inside the ship tracks themselves.


King and Coakley collaborated with researchers from the University of Washington to analyze data on solar radiation and cloud properties inside clouds that were modified by ships. "We flew an airplane through the clouds and monitored the pollution from the ships at the same time the satellite went over them," said King. The research plane was equipped with an array of gadgets that measure the light bouncing around inside the clouds, the water content there, the size of the water droplets, and many other variables. To take these measurements, the aircraft traveled 100 kilometers through the middle of the low-lying clouds (Radke et al. 1989).

As the aircraft sped out to sea, it encountered two 10-kilometer strips within the clouds that had very different properties. Across these strips, the number of water droplets per cubic centimeter more than doubled, the radius of the drops decreased by roughly six percent and the amount of water per cubic meter increased twofold (King et al 1993). "The first two results were what we expected to happen with ship tracks, but the third one was not expected," said King.

No one had predicted that this excessive cloud seeding from the ship would cause the clouds to retain more water. King explained that rain forms when cloud drops coagulate and reach a size where gravity can pull them to the ground. Yet, in the ship tracks the scientists observed, the cloud seeding made the drops so small that they could no longer easily merge together to reach the size needed to escape. Since no drizzle came out of the seeded clouds, the liquid water just kept building in the cloud. "This effect worked in exactly the same direction as the other two effects — all three made the cloud brighter and more reflective to incoming sunlight, especially in the near-infrared part of the spectrum," said King. The satellite data they received showed that the clouds that had been enhanced by the ship tracks were 13 percent brighter (King et al. 1993).

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The University of Washington Convair C-131A measured aerosol, ship track, and cloud properties directly. The measurements confirm that sulfate aerosols form the ship tracks with larger numbers of smaller water droplets than contained in unpolluted clouds. (Photograph courtesy Dr. John S. Foot, U.K. Meteorological Office)