Ship trails over the Northern Atlantic detected in MSG images during day-time using RGB combinations.
More information and detailed analysis of the feature can be found in the In Depth section.
by Jochen Kerkmann (EUMETSAT)
As shown in the case from 17 January 2006, 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 greenish stripes over the ocean.
Fog RGB Composite (for comparison) (JPG, 436 KB)
Animation Fog RGB Composite
(24 Jan. 21:30 UTC–25 Jan. 04:30 UTC, AVI, 11 MB)
Ship trails over the Eastern Atlantic (17 January 2006)
Ship trails in the region off the Bay of Biscay (27 January 2003)