Which cloud form is characterized by forming above 20,000 feet and usually composed of ice crystals?

The cloud form characterized by forming above 20,000 feet and usually composed of ice crystals is indeed cirriform clouds. These clouds are typically thin and wispy, and they consist of ice crystals due to the extremely low temperatures at high altitudes. Cirriform clouds are often indicators of fair weather but can also signify that a change in the weather is approaching. Their formation at such high altitudes means they play a role in high-altitude weather dynamics and can influence satellite communications and aviation operations.

On the other hand, the other cloud types mentioned do not match this high-altitude and ice-crystal composition profile. Cumuliform clouds generally form at lower altitudes and are associated with vertical development, while stratiform clouds are layer-like and typically form at lower levels. Nimbostratus clouds are thick, gray clouds that bring continuous rain or snow and usually occur at lower altitudes as well.