Cirrus clouds are high-altitude clouds that form at altitudes of 20,000 feet (6,000 meters) or more. They are thin and wispy in appearance, often resembling delicate strands of hair. Composed mainly of ice crystals, cirrus clouds typically indicate fair weather, but they can also signal that a change in the weather may be on the way. These clouds are part of the cloud classification system and are categorized as high-level clouds. Cirrus clouds can sometimes appear before a warm front or a storm, as they may indicate moisture in the upper atmosphere. Their presence can also create beautiful sunsets, as they reflect sunlight in vibrant colors.