Altocumulus clouds are midlevel clouds that often presage cold fronts in temperate climates. The bottoms of these clouds can be found around 6,500 to 13,000 feet in the polar regions, and up to 20,000 feet in the tropics.
These clouds are often made of water droplets. They are white or gray and can vary widely in appearance. Sometimes altocumulus clouds are found in rolling lines or waves or round masses that are somewhat shaded around their borders. On the other hand, altocumulus clouds can form great continuous sheets that seem featureless. One famous type of altocumulus produces a mackerel sky, which resembles the markings of the fish.
Altocumulus clouds are created by small, vertical updrafts of air. Because these clouds absorb heat at their base and their top, air rises into them, and water vapor condenses into liquid. However, since the top of the cloud receives direct sunlight, the water evaporates.
The clouds often form at night. More heat is radiated away from the top of the cloud, though heat is still retained at the bottom. Air and water vapor still rise through the cloud, but at night, the water vapor condenses into cold water droplets. These droplets sink back into the cloud. This type of night-forming cloud often goes away by morning when the water droplets evaporate.