USA – June 6th, 2013
The track of large ships is sometimes visualised by a trail of shallow stratus clouds. These clouds, known as ‘ship tracks’, form in the wake of ships and are remarkably long-lived. They typically are between 0.5-5 km wide, i.e. wide enough to be seen in visible satellite imagery. Here, several can be seen southeast of California, USA.
Sometimes a ship track appears as a band of enhanced cloud thickness embedded in stratus. Ship tracks are due to cloud condensation nuclei (CCN) in the ship’s exhaust. They are most likely in a near-saturated environment that is otherwise depleted of CCN. Such environment is very common in the marine boundary layer over the subtropical highs. Over these large, quasi-stationary highs, the boundary-layer air is divergent, making it unlikely to draw in CCN-rich continental air.
The nature and climatic effect of ship tracks has been investigated off the central California coast. Ship tracks increase the albedo, yet have very little effect on the long-wave radiation balance, because they are so shallow. Therefore ship-tracks tend to cool the global climate, although the magnitude of this effect is likely to be small (click here for more information).