A Sekiteiun (積帝雲 Sekitei-un?, literally meaning "Imperial Cumulus") 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 and in the Viz manga it is called Imperiocumulus.
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