Altocumulus is a middle level cloud (6,000 - 18,000 feet) with cellular vertical development. Stratiformis is a cloud sub-characteristic indicating the cloud also has stratus (flat or layered) structure. This type is typically the thinnest of all altocumulus clouds. Notice the difference in thickness of the cloud layer. Some of the cells, particularly in the lower right quadrant are thicker, showing a more vertical structure. Clouds on the left side of the photo are thinner with a more stratus structure. This cloud type is formed by water vapor that condenses into water droplets.
The Weather Briefing Blog