Appearance in Satellite Data

The cloud feature called Instant Occlusion forms when a Comma cloud merges with a frontal cloud band. Comma is situated within an cold air mass behind a Cold Front, so it is in unstable environment, which leads to the formation of convective cloudiness. The process of forming an Instant Occlussion is one where stratified frontal cloudiness and convective cloudiness merge.

The appearance of Instant Occlusion in different channels:

  • VIS and IR: In the initial, separate Comma stage, the frontal cloud band is white and the Comma cloud and the surrounding convective cloudiness are white to light grey. In the final, merged stage, the whole cloud system is white.
  • WV: In the initial stage the Comma cloud is initially light grey to grey, and the frontal cloudiness is white with some light grey structure. In the final stage the whole cloud system is white with some light grey structure.
  • Air mass combination WV6.2-WV7.3,IR9.7- IR10.8,WV6.2: In the initial stage the frontal cloudiness is white, while the Comma cloud appears light orange/yellow, being in the reddish very cold and dry air mass descending from the stratosphere behind the Cold Front. In the final stage the whole cloud system is white.

In the following pictures the initial stage is on the left, the final stage on the right side.

On the 1st of December a Comma cloud was located over Ireland, and a Cold Front extends from Norway to England and further southwest. The Instant Occlusion process took place during the next 12 hours.
In the following images the initial and final stages are shown in different channels.

01 December 2006/12.00 UTC - Meteosat 8 IR10.8 image
02 December 2006/12.00 UTC - Meteosat 8 IR10.8 image
01 December 2006/12.00 UTC - Meteosat 8 WV6.2 image
02 December 2006/12.00 UTC - Meteosat 8 WV6.2 image
01 December 2006/12.00 UTC - Meteosat 8 WV6.2-WV7.3,IR9.7- IR10.8,WV6.2
02 December 2006/12.00 UTC - Meteosat 8 WV6.2-WV7.3,IR9.7- IR10.8,WV6.2