Ferek, R.J., Hegg, D.A., Hobbs, P.V., Durkee, P. and Nielsen, K. 1998. Measurements of ship-induced tracks in clouds off the Washington coast. Journal of Geophysical Research 103: 23,199-23,206.
What was done
Two ships that left the mouth of the Columbia River in the early morning of 26 August 1992 (one heading to Pakistan with grain and the other carrying wood chips to Japan) created distinct ship tracks in a uniform layer of marine stratus clouds. These cloud features were observed in three successive images obtained from Advanced Very High Resolution Radiomerers carried on U.S. Government satellites, as well as by in situ measurements obtained during repeated aircraft flights through the tracks. These efforts resulted in "the most complete data set described to date of ship-track formation produced by specific ships."
What was learned
Chemical analyses of cloud water in and out of the ship tracks showed that cloud condensation nuclei in the effluents from the ships were responsible for the ship tracks. Furtheremore, cloud droplet spectra measured in the ship tracks showed higher number concentrations and lower cloud droplet effective radii than spectra measured in ambient clouds.
What it means
These findings demonstrate that natural clouds can be modified by effluents from anthropogenic sources, which can make them brighter and more persistent, both of which consequences tend to cool the planet. They also suggest that if we now do this inadvertantly, we could probably develop workable and economically viable techniques to do so purposefully, if we ever needed to counteract any future global warming that might go beyond what may yet be beneficial. As an aid in this regard, we note that anything that promotes more robust plant growth, such as the ongoing rise in the air's CO2 content, will lead to the production of more biogenic cloud condensation nuclei that will function in much the same fashion.