What type of cloud is typically thin, composed of ice crystals, and forms above 20,000 feet?

The type of cloud that is typically thin, composed of ice crystals, and forms above 20,000 feet is cirriform. Cirrus clouds are high-altitude clouds that generally appear wispy and thin, often indicating fair weather but can also suggest that a change in weather may occur. Their formation occurs at high altitudes, usually above 20,000 feet, where temperatures are low enough for water to exist in a crystalline form rather than as liquid.

Cumuliform clouds are generally puffy and indicate upward air movement but are not characterized by the thin, wispy structure associated with cirriform clouds. Stratiform clouds are more extensive and layered, found at lower altitudes, and do not have the characteristics of thinness or high altitude like cirrus clouds. Nimbostratus clouds are thick, dark, and associated with continuous precipitation, also occurring at lower altitudes, not above 20,000 feet.