Imperial Cumulus

A Sekiteiun (積帝雲) is an ancient fossilized cloud which remains in the sky.

This cloud piles up very high, it does not have atmospheric current, it can never let loose any rain, and the sunlight can't penetrate it. Skypiea is located 10,000 meters above sea level at the top of this type of cloud. In the Funimation dub, it is called Cumuloregalus.

It was first mentioned in Chapter 229 and Episode 149.