Aircraft contrails are a familiar feature of modern life, but ships too can leave traces of their passage in the sky. As this satellite image shows (R. J. Ferek et al., in J. Geophys. Res. 103, 23199-23206; 1998), diesel-powered vessels leave behind ‘ship tracks’ — narrow lines of perturbed regions in the marine atmosphere resembling bright bands of cloud.

Ship tracks were first reported more than 30 years ago. They are caused by the interaction of particles emitted from ships with ambient air, which results in a higher cloud droplet concentration and lower average droplet size. The consequence is increased cloud radiance at 3.7 μm,as detected by the advanced very-high-resolution radiometers (AVHRR) on satellites of the US National Oceanic and Atmospheric Administration.

On 26 A ugust 1992, the region off the coast of Washington state was covered by a uniform layer of low-lying stratus clouds — the perfect weather to look for ship tracks on satellite images, and direct an aircraft to collect microphysical and chemical data in and around them. The two tracks spotted by Ferek et al., seen in this picture as a narrow, inverted V, are caused by plumes from the Forest Wave and Al Alamira on their way northwest across the North Pacific Ocean.

The data collected during repeated aircraft crossings of the tracks, at about 210 m above the sea surface, show the expected strong perturbation of microphysical cloud properties: the initially very high total concentration of particles in the tracks decreases over time, while the cloud droplets (which form inhe presence of activated particles known as cloud-condensation nuclei) are about six times more abundant and half as large as those in the surrounding unperturbed clouds. Surprisingly, the higher droplet concentrations persist for a long time.

The radiative properties of clouds influence the global radiation budget, and depend sensitively on droplet size and concentration. Anthropogenic pollutants such as sulphur dioxide, volatile organic compounds and particulates can initiate the formation of cloud-condensation nuclei, and thereby perturb these radiative properties. Ship tracks seem an ideal setting for tackling the challenge of quantifying the correlation between emissions and cloud perturbations, and so may help to improve understanding of Earth's radiation budget and the factors affecting it.