Altocumulus clouds illuminated by Sunset

On July 17, a clear spot along the western horizon allowed the setting sun to illuminate a sky filled with altocumulus clouds. They provide a rich photographic texture when each cloud cell reflects orange and red colors. Image © Ronald Smith

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Altocumulus (Alto, “high”, cumulus, “heaped”) is a middle altitude cloud genus that belongs to the stratocumuliform physical category 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 cumuliform and stratocumuliform 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. Altocumulus is also commonly found between the warm and cold fronts in a depression, although this is often hidden by lower clouds.