- An area of divergence aloft and mid tropospheric vertical motion is located over the maritime polar air mass in the cold air northwest of cold front or quasistationary front. Although the area is characterized by weak to moderate cold advection, the synoptic forcing is usually dominated by differential positive vorticity advection. In addition, the air masses in this region tend to be conditionally unstable. Thus, the area of divergence is associated with the development of a coherent band of cumulonimbus. Since the area is also characterized by strong cyclonic shear and, often, cyclonic curvature vorticity, the cloud band achieves a comma-shape (used to be known as a COMMA CLOUD to old-time forecasters).
- If the upper flow is directed in such a manner so that the area of divergence associated with the comma cloud approaches the main frontal band, it will appear as if the main frontal band widens and bows back towards the comma, as the divergence stimulates a new wave on the front.
- If the upper flow is such that the area of divergence aloft associated with the comma directly overspreads the frontal band, the developing cyclone will appear to "join" the comma cloud, producing a major weather system that has the appearance and thermal structure of a classic occlusion. The weather system thus produced is termed an "Instant Occlusion".
- The satellite image shows in the initial stage a cold front band and a comma-like cloud system 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 precipitation fields often show dramatic evidence of the process, as is indicated by the animated gif below.