"Climate forcing due to aerosol changes is a wild card," concluded James Hansen, Makiko Sato, Reto Ruedy, Andrew Lacis, and Valdar Oinas in Global warming in the twenty-first century: An alternative scenario. "Current trends, even the sign of the effect, are uncertain. Unless climate forcings by all aerosols are precisely monitored, it will be difficult to define optimum policies."*
Hansen and colleagues expected that aerosols will "have to be monitored globally, thus by satellite, because of their heterogeneity." Although most of the aerosol emissions occur over land, air turbulence makes it harder to observe cloud generation and modification by those emissions. However, a ship's exhaust can be monitored as a line cloud in the ship's wake. It was the weather satellite TIROS-7 in mid 1960s to monitor first ship tracks.
Cloud droplets occur when water vapor condenses (or nucleates) onto a small particle such as dust or a salt crystal from the ocean surface or an aerosol particle produced from fossil fuel burning. Emissions from ships cause formation of distinct clouds that are called ship tracks. There are mainly three conditions for ship tracks: (1) small particles in the ship exhaust to serve as nucleation seeds or cloud condensation nuclei; (2) very humid air in the ship's path; and (3) non-turbulent air surrounding the ship.
A pair of images below shows how ship tracks are different from the marine clouds nearby. The source for both is the observations made on March 4, 2009 by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA Terra satellite.
The top image is a natural-color view of the ship tracks in the skies over the northeast Pacific Ocean near Alaska. The image under it reveals the size of the cloud droplets.
Cloud Droplet Radius
NASA Images - Ship Tracks South of Alaska, March 4, 2009. Posted March 12, 2009.
These images can imply qualitatively the following points on cloud generation and modification.
(1) The ship tracks are noticeably brighter than the surrounding marine clouds. They are brighter because the cloud droplets in them are smaller (yellow and peach), but more numerous, than those in the marine clouds (lavender to dark purple).
(a) The more numerous the nucleation seeds, the smaller is the resulting droplet size because the available water molecules form more droplets. The particles released in ship exhaust are more numerous than the aerosols over the ocean. So the ship tracks have more, smaller droplets than the background marine clouds.
(b) The cloud with smaller droplets looks brighter because more light is reflected back to the satellite.
(2) A cloud’s brightness influences the portion of sunlight to be reflected back to space and to reach the surface of the Earth.
(3) The size of the cloud droplets also determines the amount of rain the clouds produce. The smaller the droplets, the less likely they are to collide and form rain drops.
James Hansen, Makiko Sato, Reto Ruedy, Andrew Lacis, and Valdar Oinas (2000) Global warming in the twenty-first century: An alternative scenario. Proc. Natl. Acad. Sci. USA 97(18): 9875–9880.