Ship Trails Over the Eastern Pacific
December 27, 2007
This visible satellite image, acquired at 2230Z (aka 22:30 UTC) on December 5, 2007, shows ship trails off the coast of California. Ship trails are low-level clouds that typically form within a layer of ocean stratus (high relative humidity) as exhaust from ships leave trails of cloud condensation nuclei -- the particles on which water vapor condenses.
Over the eastern Pacific Ocean, condensation nuclei are limited since the marine air is relatively unpolluted. It follows that generic ocean stratus typically consist of relatively large water drops. That's because there's not much competition for available water vapor, so the diameters of cloud drops tend to be larger. Within the exhaust plume of ships, however, there are many more condensation nuclei that compete for available water vapor. Thus, ship trails contain abundant cloud drops that have smaller diameters compared to the surrounding ocean stratus. Clouds containing these small water drops backscatter (reflect) more sunlight than clouds comprised of larger drops. Thus, on visible satellite imagery, ship trails tend to be brighter than the surrounding low clouds. In other words, ship trails stand out from the surrounding ocean stratus.