Altocumulus (Alto, "high", cumulus, "heaped") is a cloud belonging to a class characterized by globular masses or rolls in layers or patches, the individual elements being larger and darker than those of cirrocumulus and smaller than those of stratocumulus. Like other cumulus clouds, altocumulus signifies convection. It is usually white or grey, and often occurs in sheets or patches with wavy, rounded masses or rolls. A sheet of partially conjoined altocumulus perlucidus is sometimes found preceding a weakening warm front, where the altostratus is starting to fragment, resulting in patches of altocumulus perlucidus between the areas of altostratus, or more rarely, an entire sky of the perlucidus variety. Altocumulus is also often seen in a fragmented form behind a frontal system alongside stratus fractus or stratocumulus. Altocumulus is also commonly found between the warm and cold fronts in a depression, although this is often hidden by lower clouds. Towering altocumulus, known as altocumulus castellanus, frequently signals the development of thunderstorms later in the day, as it shows instability and convection in the middle levels of the troposphere, the area where towering cumulus clouds can turn into cumulonimbus. It is therefore one of three warning clouds often recorded by the aviation industry, the other two being towering cumulus and cumulonimbus. Altocumulus generally forms about 6,500 feet to 20,000 feet (2,000 to 6,100 meters) above ground level, a similar level to altostratus formations and satellite photography has revealed that the two types of cloud can create formations that can stretch for thousands of square miles. Extensive altocumulus formations, particularly if they take the form of undulatus are often referred to as altocumulus mackerel sky.