Ship trails & industrial plumes
Atlantic Ocean, Pacific Ocean, Belarus, Spain, Portugal, USA
Meteosat-6 to 11, NOAA-20, Suomi NPP, GOES
SEVIRI, VIIRS, MVIRI, MODIS
Night Microphysics RGB, Cloud Phase RGB, Day-Night Band, Cloud Type RGB, Day Microphysics RGB, OCA Cloud Effective radius product, Visible, Infrared, Water Vapour, AVHRR, HRV, Natural Colour RGB
Particles that are emitted from ships and factories are often visible, with a similar appearance, in satellite images.
15 December 2022
25 March 2021
By Maria Putsay and Dóra Cséke (Hungarian Meteorological Service), Natasa Strelec Mahovic (EUMETSAT), Ivan Smiljanic (CGI), HansPeter Roesli (Switzerland), Nuno Moreira (Instituto de Meteorologia, Portugal) and Jochen Kerkmann (EUMETSAT), Edwin Thema, Dan Lindsey
The emitted particles act as condensation nuclei, forming small-droplet clouds which appear as linear structures within low-level clouds that are made of larger droplets.
Somewhat similar in shape to contrails, when seen in the satellite imagery (as long, thin cloud structures), ship trails form when ship exhaust interacts with clouds in the lower troposphere. Ship trails or tracks are particularly visible in the environment of low water clouds (stratocumulus). This is because of the difference between the particle (droplet) size of the clouds and the size of droplets (re-)formed under the influence of the ship exhaust. Low water clouds over the ocean usually consist of large droplets due to the limited number of condensation nuclei (i.e. less air pollution). As the ships emit large amounts of aerosols that act as condensation nuclei, the droplets (re-)forming on these nuclei are smaller than the surrounding cloud droplets, making them more reflective in near-infrared (NIR) satellite channels. Therefore, ship trails can be best seen in the RGBs composed with channels sensitive to particle sizes, such as the Day Microphysics RGB (see example in Figure 1), the Night Microphysics RGB, and the Cloud Phase RGBs (see the RGB Quick Guides for more information).
By comparing with higher resolution VIIRS imagery (pixel size up to 750x750m) we know that for SEVIRI (pixel size nominally 3x3km) one pixel may have mixed contributions — from clouds and from underlying water body/sea. So, in such a scenario for SEVIRI data, the colour contrast between a ship track and its surroundings is affected by both the droplet size difference and the cloud/water ratio. Adding to that, due to reduced spatial resolution, in the SEVIRI image the cellular structure of ship track cloud lines is not visible.
The example in Figure 2 shows ship trails as seen in different VIIRS and SEVIRI RGB combinations. The different structure of the ship trails compared to the neighbouring clouds is clearly seen as relatively broad continuous lines in the cellular structured background. The line-shaped ship trail clouds consist of smaller particles than the surrounding cellular structured water clouds, as can be seen from the colour shades in the images.
Water clouds with small and large droplets appear in different colours, for instance light yellow against pink in the Cloud Phase RGB. The highest colour contrast can be found in the Cloud Phase RGB image, which is more sensitive to the cloud top particle sizes. Note that the VIIRS Day Microphysics RGB, shown in Figure 2, was created with the NIR1.6 channel instead of the IR3.7 channel. The VIIRS Day Microphysics and Cloud Type RGBs use one microphysical channel, the NIR1.6. Cloud Phase RGB uses two microphysical channels, one of which is NIR2.25 which is more sensitive to the particle size than NIR1.6, and will be a new channel with FCI on board MTG-I satellites.
NWC SAF Cloud Microphysics products (CMIC), in particular Cloud effective radius (CMIC 2) and Cloud Optical thickness (CMIC 3), also reveal the microphysical characteristics of the ship trail clouds. As shown in the animation in Figure 3, ship trails have smaller cloud drop effective radius, i.e. they are made of smaller droplets, than the neighbouring water clouds (Figure 3, left). The cloud optical thickness product (Figure 3, right), on the other hand, shows that the clouds created through the interaction of the ship’s exhaust and the existing low-level water cloud have larger optical thickness, meaning they are more dense then the neighbouring clouds.
Ship trails south of Alaska - 2019
Ship trails can be tracked at night using the Night Microphysics RGB or the 24-hour Microphysics RGB.
Small droplets emit less radiation (lower emissivity) in the IR3.9 and IR8.4 bands (compared to large droplets), so that at night the brightness temperature difference (BTD) IR10.3–IR3.9 (or IR10.3–IR8.4) of ship trails is more positive than the brightness temperature difference of stratocumulus clouds with large droplets.
With the availability of new satellite instruments like the Advanced Himawari Imager (AHI) on Himawari-8/9 and the Advanced Baseline Imager (ABI) on GOES-16/17, the ability to detect ship trails has significiantly improved. The higher spatial resolution (2km), and the higher radiometric resolution and lower noise, lead to a larger colour contrast between ship trails (green) and non-polluted stratocumulus clouds (light magenta).
This can be seen in the animation of the GOES-17 Night Microphysics RGB from 02:00 to 09:00 UTC (night). The ship trails appear as a bright green colour. It is possible to follow some of the ships by tracking the leading edges of the trails, resulting in speed estimates of around 18–20kts.
Ship trails over the southern Atlantic - 2015
The Atlantic high pressure system was situated over the southern Atlantic, west of Southern Africa. There was also a cut-off low system developing over the north-western parts of South Africa.
Ship trails, which are formed by aerosols emitted from the ship funnels, are trapped in small water drop usually in a stratocumulus clouds in a stable atmosphere.
During the day, small water droplets have a higher reflectivity in the NIR1.6 and the IR3.9 channels, so bright trails are visible on the Meteosat-10 Day Microphysics RGB image (Figure 5).
The Meteosat-10 Day Microphysics RGB animation, 26 June 08:00–13:00 UTC shows the ships trails rotating anti-clockwise around the Atlantic high pressure system.
The ship trails can also be tracked at night using the Night Microphysics RGB or the 24-hour Microphysics RGB.
Small droplets emit less radiation (lower emissivity) in the IR3.9 and IR8.7 bands (compared to large droplets), so the brightness temperature difference (BTD) IR10.8-IR3.9 (or IR10.8-IR8.7) of ship trails is more positive than the brightness temperature difference of stratocumulus clouds with large droplets.
This can be seen in the Night Microphysics RGB image (Figure 6, left) and the IR10.8-IR3.9 BTD (green component of the Night Microphysics RGB) (Figure 6, right) where the ship trails appear brighter compared to the 'normal' stratocumulus around.
Ship trails over the eastern Atlantic - 2013
Typical cloud patterns over the eastern Atlantic in a high pressure situation in July 2013.
Stratus/fog can be seen on the Portuguese coast, and stratocumulus fields and ship trails over the Atlantic. The image is a composite (blending) of the Natural Colour RGB and the HRV image (range: 2 to 20% reflectivity).
Ship trails over the Gulf of Biscay - 2012
Ship trails over the Atlantic - 2011
Numerous ship trails over the North Atlantic, off the coast of Spain and Portugal. The ship trails are visible as thread-like stripes embedded in magenta-coloured stratocumulus cloud fields.
Atlantic ship trails - 2010
The ship trails are visible as pale blue stripes embedded in magenta-coloured stratocumulus cloud fields.
Ship trails over the northern Atlantic - 2007
Ship trails can be easily detected in MSG images during day-time using RGB combinations that make use of the IR3.9 channel. It is also well known that ship trails may be tracked during night-time using the fog RGB composite (IR12.0–IR10.8, IR10.8–IR3.9, IR10.8).
However, it is less well known that ship trails also show up in the 24-hour microphysics RGB composite (IR12.0-IR10.8, IR10.8-IR8.7, IR10.8), as shown below. The advantage is, as the name implies, that this RGB composite can be used during day- and night-time for monitoring clouds, and in particular ship trails.
As with the IR 3.9 channel, the 24-hour microphysics RGB takes advantage of the fact that the brightness temperature of water droplets in ship trails is lower in IR8.7 than in IR10.8 (as water droplets emissivity is lower in IR8.7), therefore yielding positive values of the difference IR10.8-IR8.7.
Furthermore, this difference depends on the droplet size and is generally larger for small water droplets. Thus, as this difference is allocated to the green colour beam, ship trails can be traced as green stripes over the ocean.
Ship trails over the eastern Atlantic - 2006
A large number of ship trails were observed over the north-eastern Atlantic on 17–18 January. They were embedded within a north-eastern flow at low and medium-levels associated with a high pressure system that persisted over the Azores for some days. On the following day (19th), as a low pressure area replaced the high pressure system that weakened and moved northeast, ship trails could no longer be seen on MSG images (see image from 19 January).
As shown below, ship trails can be easily detected in Meteosat-8 images during day-time using the high-resolution visible channel (lowest image) or by using RGB combinations that make use of the IR3.9 channel (Figure 12). As regards the visible channels, the detectability is due to the higher reflectivity of water clouds formed of small particles (ship trails) as compared to maritime stratus and stratocumulus clouds that are usually formed of larger water droplets.
To put this into numbers, assuming for example a water cloud with a water content of 1g/m3 and a geometrical cloud depth of 100m, the reflectivity in the VIS0.6 channel varies from about 60% (small droplets of about 12 microns) to 30% (large droplets of about 30 microns).
As regards the RGB composite shown in the top image, the reflectivity of the IR3.9 channel (solar, reflected component) strongly depends on the droplet size, with small droplets reflecting more than large droplets. Thus, the ship trails appear much brighter than the maritime stratus and stratocumulus clouds.
Ship trails may also be tracked during night-time using the Fog RGB composite (IR12.0–IR10.8, IR10.8–IR3.9, IR10.8). This RGB takes advantage of the fact that the brightness temperature of water droplets in ship trails is lower in IR3.9 than in IR10.8 (as water droplets emissivity is lower in IR3.9), therefore, yielding positive values of the difference IR10.8–IR3.9.
Furthermore, this difference depends on the droplet size and is generally larger for small water droplets. As this difference is allocated to the green colour beam, ship trails can be traced during the night as greenish stripes over the ocean (see Figure 13).
Ship trails in the region off the Bay of Biscay - 2003
An exceptional case of ship trails was observed over the Bay of Biscay during a high-pressure situation on 26-27 January.
In NOAA AVHRR RGB images (red=visible channel, green=near-infrared channel, blue=infrared channel) the polluted clouds appear with a more yellow 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 (200m at Brest, 500m 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.
Similar to the trails formed from the exhaust of the ships, clouds with small droplets can also form on the aerosols emitted from industrial hot spots/chimneys. The difference is that the source of the emission is not moving, but the wind causes elongated cloud features, very similar to that of the ship trail, with similar microphysical characteristics. A very good example of that is this case from 22-23 November 2018 over Belarus.
The VIIRS Night Microphysics RGB image taken on 22 November 2018 (Figure 25) shows that the northern and north-eastern region of Belarus was covered by fog or low cloud, consisting of relatively large droplets, appearing in pink in the Night Microphysics RGB. According to the synop observations these clouds were stratus. Within this region several bright yellow-green plumes indicating low clouds with small droplets can be seen spreading southwards. In the southern and south-western region there are water clouds with small droplets, appearing in bright yellow-green. The most probable reason for the largest plume (indicated by the large black arrow) was the factories of Navapolatsk city, the leading producer in the refining and chemical industry business. Besides the largest 'plume', several smaller plumes are seen. The whole region has many such (smaller), probable industry- or city-related hot spots, some of them marked by arrows in Figure 4. The plumes are elongated due to the cloud advection above the sources of nuclei particles.
The main plume is even visible in the (strongly enhanced) Day-Night Band image on 22 November (Figure 26). It is slightly brighter than the surrounding cloud mass due to a smaller droplets.
The Optimal Cloud Analysis – OCA Cloud Effective Radius product reveals a plume of droplets with a smaller effective radius in darker blue shades, (see the white arrow in Figure 27). Note that this product is retrieved from SEVIRI data.
During the day, as the wind changed the direction the main plume of smaller water droplets in Day Microphysics RGB (created with NIR 1.6 instead of IR 3.7 micron channel) spread eastwards, see the bright green plume indicated by the black arrow in Figure 28.
The main plume is still visible the next night on 23 November (Figure 29). During the day the main plume could still be seen (with a hook shape) in the comparison of Day Microphysics and Cloud Phase RGBs (Figure 30).
The cloud top droplet size difference is better seen in the Cloud Phase RGB (light yellow against pink). However, it is still recognisable (albeit dimly) in the Day Microphysics RGB (brighter green colour against a background with slightly pink shades).
In the imagery in this study the industrial plumes were better seen during the night than during the day. The colour contrast between water clouds with larger and smaller droplets were higher during night in the Night Microphysics RGB (see Figures 25, 26, and 29) than during the day in the Cloud Phase RGB (see Figure 30). The reason might be the low Sun angle and high satellite scanning angle (causing generally weaker reflection towards satellite during the day), but also the fact that during night the mixing in the boundary layer is weaker, so the size difference might be larger.
Other sources of ‘microphysical trails’
Trails of altered microphysical signature, most vividly seen in the low laying stratiform clouds, can, likely, have other persistent sources of condensation nuclei (aside from ships and industry hot-spots). One such example is fires.
Fires (if persistent, and with enough intensity and spread) are also a large and a steady source of small particles (smoke, and ash) that can change the microphysical structure of the clouds above. Again, with injection of the bigger concentration of condensation nuclei, elongated linear shapes start to appear in stratiform cloudiness, showing different a spectral signature, i.e. signature of relatively small cloud particles (Figure 31).
Ship Tracks Over the Atlantic (NASA Earth Observatory)
Ship Tracks in the East Pacific (CIMSS Satellite Blog)
GOES-17 Night Microphysics RGB, 5 Nov 2019, 07:20 UTC, ship trails off coast of California