The altitude range for these clouds is 16,500-45,000 ft (5,032-13,725 m) but they usually form between 20,000-25,000 ft (6,000-7,500 m). There are three genera of high level clouds and they are all labeled with the term cirrus. Cirrus clouds are the highest clouds, forming around 30,000 ft (9,150 m). They are totally made of ice crystals (or needles of ice) because they form where freezing temperatures prevail. Pure cirrus clouds look wispy, with a slight curl, and very white. Because of their appearance, they are often called mares' tails. Cirrocumulus clouds, the least common cloud, are small, white or pale gray, with a rippled appearance. Sometimes they appear like a sky full of fish scales; this effect is called a mackerel sky. These clouds usually cover a large area. They form around 20,000-25,000 ft (6,000-7,500 m) and are made of either supercooled water droplets or ice crystals. Cirrostratus also form at 20-25,000 ft, but are made completely of ice crystals. They usually cover the sky as a thin veil or sheet of white. These clouds are responsible for the halos that occur around the sun or moon. The term "on cloud nine" (feeling of euphoria) is derived from the fact that the highest clouds are labeled category nine.