• The satellite image shows in the initial stage a Cold Front band (see Cold Front ) and a Comma-like cloud system (see Comma ) in the cold air mass behind the Cold Front. During the different stages of development the Comma approaches the cloud band of the Cold Front without any appreciable increase. During this approach a Wave - like feature develops at the front. In the end stage Comma and frontal cloudiness merge in the area of the wave bulge leading to the typical appearance of an Occlusion cloud spiral.
• In contrast to the cloud band of the Cold Front, the Comma is characterized by a cellular appearance, indicating embedded CB cloudiness. A predestined area for convective developments is the area between the Cold Front and the approaching Comma, where the troposphere is characterized by a potentially unstable stratification as well as WA and PVA (see Meteorological physical background , Key parameters and Weather events ).
• Often also the surrounding of the Comma feature is characterized by the development of convective cloudiness, which is called open cellular convection in literature.
• In the IR and VIS image the comma is white and structured indicating thick multi-level cloudiness.
• In the WV image the area of the Comma often is characterized by dark grey to grey pixels during the initial stages. Within the different stages of development the Comma becomes brighter and brighter.

The satellite image at 09.00 UTC (left image top) shows the cloud band of a Cold Front extending from the Atlantic north-west of the Iberian Peninsula across England to south Scandinavia. Within the Cold Front above the Atlantic (approximately 52N/22W) a wave development takes place. Behind the Cold Front an approaching Comma - like cloudiness can be observed above the Atlantic (approximately 52N/33W).
Three hours later (right image top) the Wave has developed more and the Comma - like cloudiness has approached closer to the rear cloud edge of the front. The Comma - like cloudiness is characterized by a cellular appearence indicating embedded Cb cloudiness.
At 15.00 UTC (left image bottom) the process of the Instant Occlusion has reached the merging stage of development (see Meteorological physical background).
The image at 18.00 UTC (right image bottom) shows still the merging stage of the Instant Occlusion. Between the Comma - like cloudiness and the frontal cloud band some mid-level cloudiness with warmer cloud tops can be observed. This is the area of the so-called shallow moist zone. The Comma - like cloudiness is again characterized by embedded Cb cloudiness (due to a potential unstable stratification of the troposphere) especially at the cloud edge which is oriented to the frontal cloud band (approximately 57N/23W). The image at 21.00 UTC (left image top) shows that increased cloud development in the area between the Comma - like cloudiness and the frontal cloud band (shallow moist zone) has taken place during the last three hours.
Three hours later (right image top) the Comma - like cloudiness has further developed.
At 03.00 UTC (left image bottom) the process of the Instant Occlusion has reached its mature stage of development. The former Comma - like cloudiness has the appearance of an Occlusion. The IR image (left image top) shows the cloud band of a Cold Front extending from the Atlantic to the west coast of the Iberian Peninsula. Behind the Cold Front above the Atlantic a Comma - like cloudiness can be observed at approximately 50N/30W.
The WV image (right image top) is characterized by a black stripe at the rear edge of the Cold Front situated at the cyclonic side of the jet. The Comma - like cloudiness appears black in the WV image within the area of influence of the dry stratospheric air which is situated at the cyclonic side of the jet. Outside or at the edge of that area the Comma - like cloudiness is characterized by dark grey shades.
In the VIS image (left image bottom) the Comma - like cloudiness is characterized by high pixel values indicating high albedo. In contrast to the cloud band of the Cold Front which has a more smoothed appearance, the Comma - like cloudiness has a structured appearance typical for embedded convective cells. Six hours later the process of the Instant Occlusion has reached the merging stage of development.
The IR image (left image top) shows that the Comma - like cloudiness has approached towards the rear cloud edge of the Cold Front. The cloud tops of the shallow moist zone and the Comma - like cloudiness are darker (warmer) than the cloud tops of the Cold Front. This gradient is caused by the dry air situated at the cyclonic side of the jet. The lowest cloud tops can be observed in the area close to the Cold Front (on the cyclonic side of the jet). The cloud tops are continously increasing towards the Comma head. In contrast to the cloud band of the Cold Front, which has a more smoothed appearance, the Comma - like cloudiness is characterized by a structured appearance indicating embedded convective cells.
The WV image (right image top) shows a black stripe at the rear of the Cold Front which is caused by the dry stratospheric air at the cyclonic jet side. The Comma tail and the shallow moist zonewith the area of the cyclonic jet side is black. The area of the Comma head, outside the area of influence of the dry air at the cyclonic jet side, appears dark grey.
At 00.00 UTC the Comma - like cloudiness has merged with the frontal cloud band.
In the IR image (left image bottom) the Comma - like cloudiness is characterized by darker grey shades within the area of the Comma tail (dry air at the cyclonic side of the jet) and brighter grey shades within the area of the Comma head. As mentioned before, the frontal cloud band is mainly characterized by a smoothed appearance, the Comma - like cloudiness by a structured appearance.
As already described for the IR image, in the WV image (right image bottom) the Comma - like cloudiness is also characterized by dark grey shades within the area of the Comma tail (dry air at the cyclonic side of the jet) and brighter grey shades within the area of the Comma head.