High level clouds mainly composed of ice crystals?

High-level clouds, primarily composed of ice crystals, form at altitudes above 20,000 feet (6,000 meters). These clouds include cirrus, cirrostratus, and cirrocumulus, and are typically thin and wispy in appearance. They often indicate fair weather but can also signal an approaching storm. The presence of ice crystals allows them to produce halos and other optical phenomena when illuminated by sunlight.