Clouds of the Day - Altocumulus stratiformis- Thursday, September 14, 2023

Photo copyright by Craig Johnson, Weather Briefing LC, September 2023

There are many forms of Altocumulus. This version forms a layer of individual cloud elements that are cellular in shape. They are also flat. The cellular structure sets them apart as Altocumulus even though they have the flat characteristic of stratus.

This is an example of why some clouds are may not be easily identified. While we assign names to clouds based on their height. shape, and texture sometimes clouds have a mixture of these characteristics. The altocumulus in the photo above is one example.

These clouds have the characteristics of cumulus and stratus. They are mostly Altocumulus so that is their primary name. However, in cloud naming there is room to add other descriptive names. To better describe them these clouds are Altocumulus stratiformis.

The primary cloud type is Altocumulus and the secondary type is stratus. They are Altocumulus because they are in the middle level of the atmosphere between 6,500 ft (2000 m) and 20,000 feet (5000 m) above sea level. They are also cellular which is a feature of cumulus. The cloud forms by columns of rising air up through the cloud with sinking air surrounding it. The clouds look flat because the air is weakly rising and a temperature inversion (air is warmer above the cloud) which limits the upward motion. There is likely sinking motion pressing downward to the cloud level. The cloud forms where the rising air cools to its condensation temperature. The top of the cloud is limited by the stable layer of air above which is warmer and drier than the air below. All middle level clouds are given the prefix Alto which means “high.” Altocumulus are higher in the atmosphere than Cumulus which are found in the low levels below 6500 ft (2000 m).