An Imperial Cumulus is an ancient fossilized cloud which remains in the sky. It serves as the base for the White and White-White Seas, the oceans where Skypiea is located 10,000 meters above (blue) sea level.
According to Montblanc Cricket's theory, the Imperial Cumulus is a mass of clouds so dense and ancient (possibly thousands of years old) that it could be described as "petrified". Because of this density, the Imperial Cumulus does not have an atmospheric current and can never condense into rain, but it still manages to float on the sky somehow. It appears to move around the region of Jaya in a predictable pattern (as Cricket was able to deduce its location a day in advance), and, with luck, timing and good planning, it is possible to travel into the Imperial Cumulus by riding the Knock Up Stream once it is aligned with the cloud mass.[1]
Also, due to it being so dense, sunlight cannot penetrate it, meaning that any region in the Blue Sea caught below the Imperial Cumulus's range will darken nearly instantly, almost as if nighttime had suddenly come. Sometimes, a mysterious phenomenon will happen where gigantic silhouettes of winged warriors appear among the clouds; however, Cricket has explained this as merely the result of the shadow of someone up in the Imperial Cumulus being projected down to the Blue Sea due to strong sunlight.[2]
Trivia[]
The phenomena explained by Cricket where people located high up in the sky have their shadows projected down on ground level due to being directly struck by sunlight from above is based on the real life effect known as the Brocken spectre.