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 rain, and the sunlight cannot 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 Cumuloregalis.

It was first mentioned in Chapter 229 and Episode 149.

Millenium_Cumulonimbus