Altocumulus (Ac) is a mid-level cloud (base at 2–6 km) consisting of white or grey patches, layers, or rolls made of water droplets (sometimes mixed with ice crystals at higher altitudes). The individual cloudlets typically subtend an angle of 1–5° when observed at 30° elevation — larger than cirrocumulus elements but smaller than stratocumulus.
Several notable species exist. Altocumulus castellanus — turreted towers rising from a common base — is one of the best visual indicators that afternoon thunderstorms are likely. When seen in the morning, it indicates mid-level instability and moisture that may fuel convection later. Altocumulus lenticularis (the lenticular cloud) forms in standing waves downwind of mountains and is prized by photographers for its smooth, UFO-like appearance.
Altocumulus is extremely common and appears in a wide variety of situations: ahead of cold fronts, in the warm sector of depressions, on summer mornings before convective development, and in post-frontal airmasses. A sky covered with rippling altocumulus (the "mackerel sky" at mid-level) is aesthetically striking, with light playing through the gaps to create crepuscular rays. The cloud rarely produces significant precipitation — at most virga (rain streaks that evaporate above the ground).