An exceptional case of ship trails was observed over the Bay of Biscay during a high-pressure situation on 26-27 January.
21 October 2020
26 January 2003
Cloud-forming aerosols mostly originate from natural sources, like sea salt, volcanic ash, desert dust and biomass burning. However, an increasing quantity of aerosols is being produced by humans through the burning of fossil fuels. By doing so, we are making the clouds above us brighter (reflecting more sunlight), altering their lifetime and possibly creating huge changes in our environment. It is known that polluted clouds live longer and have a higher albedo than unpolluted clouds due to the smaller and more numerous droplets.
In NOAA AVHRR RGB images (red=visible channel, green=near-infrared channel, blue=infrared channel) the polluted clouds appear with a more yellowish colour, as compared to unpolluted clouds that have a reddish colour (see NOAA AVHRR example below).
Testing theories of man-made cloud formation is not an easy task. In most urban areas, it is difficult to discern exactly how pollutants contribute to forming clouds because the atmosphere over the land is normally too well mixed.
As an alternative, researchers have turned to studying ship trails — clouds caused by enhanced cloud droplet nucleation on particles associated with smoke coming from large ships (burnt diesel oil). While, individually, not significant sources of pollution, ships release their exhaust fumes into the relatively clean and still marine air, where it is easy to measure the effects of fossil fuel emissions on cloud formation.
The exhausts from ships introduce aerosol and cloud nucleation particles that normally don't exist in marine air. The numerous nuclei result in small droplets that do not form drizzle (low-level clouds with large droplets tend to drizzle out and the cloud dissipates). When there is no drizzle, the trails can spread and persist for days. They twist and turn with the local winds resulting in the wild patterns. The longer they persist, the more spread out they become.
The visible images below show a large number of persistent ship trails indicating the regular shipping lanes in the Eastern Atlantic. Most trails can be observed close to Brittany where ships enter or exit the English Channel.
Looking carefully at the animated images, one can actually see the movement of some ships at the most narrow, pointed ends of the trails. One can also see how the trails are affected by the wind.
Persistent ship trails usually form under a low-level inversion in air with high humidity or even low-level clouds (stratus or stratocumulus). The soundings from Brest and Bordeaux (see below) indeed indicate indeed a strong inversion at low levels (200 m at Brest, 500 m at Bordeaux) with wet air at the lowest levels (spread close to 0 °C) and extremely dry air at higher levels.
The infrared imagery helps to determine whether the observed trails are ships trails or condensation trails (contrails) caused by aircraft. Ship trails (low clouds) are darker than contrails (high clouds) in IR imagery. In this case, the trails are not visible in the infrared imagery because their temperatures are close to the temperature of the sea surface.
Meteosat-7 (visible channel), 27 January 2003 (12:00 UTC)
Surface Chart, 27 January 2003 (00:00 UTC)
(Source: Deutscher Wetterdienst)
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