Altocumulus undulatus cloud

The altocumulus undulatus is a mid-level cloud (about 8000 - 20,000 ft or 2400 - 6100 m), usually white or grey with layers or patches containing undulations that resemble "waves" or "ripples" in water. Elements within the cloud (such as the edges of the undulations) are generally darker than those in cirrocumulus and smaller than those in stratocumulus. These clouds may appear both as patches or as covering the sky. The width of these clouds is generally less than 300 feet (91.44 meters) thick. The presence of altocumulus undulatus may indicate precipitation within the next 20 hours or simply an overcast day.