Cirrus clouds are high-altitude clouds that form at elevations of 20,000 feet (6,000 meters) or more. They are thin and wispy in appearance, often resembling delicate strands of hair. These clouds are composed mainly of ice crystals due to the cold temperatures at such heights. Typically white and often seen in fair weather, cirrus clouds can indicate a change in the weather. When they appear in large numbers, they may signal an approaching storm, as they can precede warm fronts and indicate moisture in the upper atmosphere.