Although the impact on climate remains controversial, the recent rediscovery of such phenomena as ship tracks has stimulated research into the effects of cloud-aerosol interaction on climate. These two satellite images show a stratocumulus cloud system off the West Coast of the United States. The large image was constructed from NOAA-9 AVHRR data on June 27, 1987, at a shortwave infrared wavelength. The smaller inset (top left) was constructed from reflected solar radiation measurements at a visible wavelength. The MODIS instrument on the EOS AM and PM satellite platforms will provide a substantially improved set of measurements for identifying and studying cloud-climate processes, including the ship track phenomena.
The streaks revealed in the images are due to a reduced droplet size in clouds contaminated by the exhaust from ships. Particles emitted by ships increase concentration of cloud condensation nuclei (CCN) in the air. Increased CCN, in turn, increase the concentration of cloud droplets and reduce the average size of the droplets.
Increased concentration and smaller particles reduces production of drizzle (100 ┬╡m radius) droplets in clouds. Liquid water content increases because loss of drizzle particles is suppressed. Therefore, along ship tracks, clouds are OPTICALLY both thicker and brighter, and cloud single scattering albedo increases.
In the words of John Aitken (1880), ΓÇ£...when water vapor condenses in the atmosphere, it always does so on some solid nucleus. The dust particles in the air form the nuclei on which it condenses. If there were no dust in the air, there would be no fog, no clouds, no mist, and probably no rain.ΓÇ¥
