Cirrocumulus (Cc) is a high-altitude cloud consisting of small, regularly arranged white patches or ripples, composed entirely of ice crystals. Found at 6–12 km, the individual cloudlets appear very small (less than 1° of arc, about the width of a little finger at arm's length). The pattern resembles fish scales, giving rise to the term "mackerel sky" — from the traditional sailor's saying "Mackerel sky, mackerel sky, not long wet, not long dry".
Cirrocumulus forms when shallow instability or wave motions affect a thin layer of air at high altitude, creating the rippled pattern. It is one of the rarest cloud genera — far less common than cirrus or altocumulus — and often appears briefly before transforming into one of these. The key distinction from altocumulus is altitude and size: cirrocumulus elements are much smaller and appear at higher levels.
A true mackerel sky of cirrocumulus is a beautiful but fleeting phenomenon. It can indicate the arrival of moisture at upper levels, sometimes heralding the approach of a warm front within 12–24 hours. However, cirrocumulus can also appear in fair weather with no significant change following. It does not produce precipitation and never creates halos (unlike cirrus), because its ice crystals are too small and randomly oriented for optical effects.