Altocumulus at Sunset

Altocumulus are "high cumulus". The name originates from "alto," which means "high" and "cumulus" which means heaped. Altocumulus are found in the middle levels of the atmosphere (between 6,000 to 18,000 feet), higher than cumulus which are found in the low levels (below 6,000 feet). Cumulus in the middle levels are therefore altocumulus (high cumulus). 

Cumulus above 18,000 feet are in the cirrus level, a region where temperatures are always below freezing. The clouds consist mainly of ice crystals although water droplets are sometimes found in cirrocumulus, which is the name given to cumulus above 18,000 feet.

In this photo the tops of the altocumulus were illuminated by the setting Sun which helped reveal the vertical structure of the clouds. The photo was taken on the evening of October 5, 2016 at Cedar Falls, Iowa looking northwest. The clouds occurred as moist air in the mid-levels moved into a region of cooler air aloft. The result was an unstable layer that created rising motion in a cellular pattern. The clouds are forming where rising motion cools the air revealing the cloud structure.