Cirrus clouds are the highest in elevation, which form above 20,000 feet. The temperature at that elevation is so cold that cirrus clouds are usually composed of ice crystals. The ice crystals are the result of the freezing of super-cooled water drops. This can only happen when the temperatures reach below –38 degrees Celsius. These high-level clouds are white, thin, feathery, and wispy in appearance. Cirrus clouds often mark the start of a warm front, which is an indication of approaching bad weather. They are the fastest moving in the atmosphere because the wind current is very strong that high up.
Cirrostratus clouds form in 18,000 feet and above. They are composed of ice crystals because of the low temperatures. They are sheet-like and are very transparent. They can be several thousand feet thick, and the sun and moon can still be seen through them. Sometimes the only way to tell that there are cirrostratus clouds in the atmosphere is by seeing a halo around the sun or moon. Refraction of light by the ice crystals causes the halo. Cirrostratus clouds thicken when a warm front approaches, which indicates that more ice crystals are being formed. The halo will not be able to be seen when this happens, and the sun or moon will be less visible because the clouds condense too much for clear visibility.
Cirrocumulus clouds form above 18,000 feet. They are called cirrocumulus clouds because they have features of both cirrus and cumulus clouds. They usually are small white patches that look like small altocumulus clouds. Cloud ripples are associated with cirrocumulus clouds, and they are somewhat transparent. Small spaces can be seen between the clouds, and they have a puffy and patchy appearance. Sometimes they are in the sky as rows. They are composed of ice crystals, and they change slowly over time. These clouds often follow cirrus clouds in a warm front system. Therefore, cirrocumulus clouds indicate good weather.