High-level clouds sit high in the sky and are made mainly of ice crystals. The common high-level types include cirrus (thin, wispy strands), cirrostratus (a thin, sheet-like veil that can cover the sky), and cirrocumulus (small, white patches often in a rippled, fish-scale pattern). Cirrus and cirrostratus are seen more regularly because their formation conditions are more common in upper air. Cirrocumulus, however, requires very specific, very cold conditions high in the atmosphere, so it appears much less often. Cumulonimbus is a tall, vertically developed cloud associated with storms and isn’t classified as a high-level type in this group, since its defining feature is vertical growth rather than just altitude. Therefore, the least common high-level cloud is the one that shows as small, fish-scale patches.