A Comma is a very prominent cloud feature developing and existing in cold air. There are several similar phenomena within cold air: One of them is Enhanced Cumulus cloudiness (see Enhanced Cumulus ). Also a ring of vortices, crescent, oval, solid cloudiness, multiple deep or shallow bands, single deep or shallow bands or swirls in cumulus streets can all be found in cold air.

In order to distinguish the Comma from other features in cold air, the Comma cloudiness is defined as a small to meso-scale cloud spiral consisting of white (i.e. cold) cloud cells partly overlaid by cirrus shields. In most cases the strongest convection can be found in the Comma tail, but sometimes enhanced convection can also be found in the Comma head. In a few cases the Comma head and tail are separated by a narrow cloud-free area or only connected by low clouds (although this may not be apparent during the whole life time of the system). The scale of a Comma lies between 200 and 1000 km, i.e. much smaller than a fully developed depression or a cyclone.

Larger Commas can be a sign of a development process called Cold Air Development (see Cold Air Development ), where a Comma increases in size and finally gains some frontal characteristics.

Appearance in AVHRR imagery

• Images from all channels (below) show more details of the cloud structure when compared to Meteosat 8 images.
• RGB-combinations of channels (below left, second row) provide a quick overview of thin and thick high level cloudiness.
• Channel manipulation (eg. subtraction with thresholding, third row, right) highlights the cloud pattern and physical features.

Concerning the Comma cloud pattern, high clouds are the only features to look at in AVHRR satellite images. The characteristics of such cloud are shown in the table below.

In the images above, there is a northwesterly flow over the North Sea. A Cold Front from Iceland to Scandinavia is slowly moving to the east. During passage of the Comma cloud heavy snow showers are reported in The Netherlands and in Germany. The Comma feature was already seen over the British Isles (following images), but the Comma shape is still recognizable and looks like a Cold Air Development (CAD). The fuzzy outlines (turquoise in image above left) indicate thin cold cloudiness and are the result of strong outflow at higher levels. In the RGB-images above, the most important cloudiness of the Commas appear to be white: cold, thick, high level clouds. The borders are rather fuzzy. The Commas are located at the left exit of several jetstreaks and some jet fibres are seen over Ireland (in these images, the jet core is too far to the west to be seen properly). In the image above left, the fuzzy edges of the Comma cloudiness are quite remarkable. In above right, the outlines of the Commas are classical examples. Radar images and surface reports from E. England and Scotland indicate heavy rain, hail and snowshowers and are in agreement with the features shown in the image above right.