Cirrus clouds and contrails in satellite imagery

Similarly to the ground view, even when looking at the satellite imagery it is rather difficult to decide about the origin of the large cirrus field above Bohemia and Austria. It is probably a mixture of both - natural cirrus, and remains of older dissipating condensation trails (contrails). Some of the contrails (more recent ones) are sharply defined, while some other (older ones) are already dissipating, merging with the surrounding cirrus. 

Meteosat-8 (MSG-1) rapid scan loops, RGB bands HRV and IR10.8, 05:00 - 07:00 UTC in  MPG | MOV  formats basically show the movement of the clouds only. The bluish color represents high cold clouds - "natural" cirrus and contrails, and midlevel altocumulus clouds. Yellowish tint indicates the lowest, relatively warm stratus clouds.

NOAA-16 satellite imagery (AVHRR instrument), 06:59 UTC (about one hour later, after the end of the timelapse movie):

• RGB composite image of bands 1, 2 and 4 (pseudo-real colors) - the same color assignment as for the MSG loop above.

• Band 3 image (3.7 µm) composed of both, emitted and reflective radiance. Highly reflective stratus is shown in black (higher total outgoing radiance), while cold, low-reflective cirrus clouds are shown in white (low total radiance). Since contrails are composed of very small ice particles, their 3.7 µm reflectivity is quite high, thus also the total radiance is high. For this reason the contrails appear as dark here. Please note that 3.7/3.9 µm images can be plotted both ways - as radiance image (this case) with high radiance represented in black, or as reflectivity image with high radiance represented in white. Thus the actual appearance of contrails in this band depends on what representation is used for this band - in "reflectivity" image they will be shown in white.

• Band 4 image (10.7 µm), standard IR window image, with low temperatures depicted in white.

• Brightness Temperature Difference, BTD image of bands 3 and 4. Subtracting the brightness temperature (BT) of band 4 from BT of band 3 generates an image, corresponding (more or less) to the reflectivity in band 3 (3.7 µm).

• BTD of bands 4 and 5. This product gives highest BTD values either for very thin cirrus clouds, of for clouds composed of very small particles. In this case it very nicely reveals the contrails, and also altocumulus clouds in the western part of the image.

• RGB composite image of band 4 (red), BTD b3-b4 (green) and BTD b4-b5 (blue). This RGB product brings together the properties of the individual images above, creating a false-color image, nicely discriminating the contrails (bluish) from the cirrus clouds around (reddish). However, given the fiber-like structures of some of the (reddish) cirrus clouds, it appears very likely that these are actually remains of old contrails. As the contrails age, they grow - both in horizontal size (result of ambient turbulence), as well as in their individual particle size (if the layer in which these form is sufficiently moist, the initially small contrail particles can keep growing, reaching the size of "natural" cirrus particles around).

The "aging" of contrails might be revealed in a corresponding RGB loop from the MSG satellite; however its somewhat lower resolution is insufficient for this. Hopefully, it should be possible with the Meteosat Third Generation, which will have resolution comparable to that of the present AVHRR instrument flown on the NOAA/POES and MetOp satellites.

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Satellite data sources: CHMI and EUMETSAT. Further information on the MSG RGB composite products is available here (brief) or here (detailed), and references to scientific publications on contrails are here.

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