Sea Clouds, also called Ocean Clouds, are large water-like clouds. They form seas in the sky. One can almost sail on it as if it were water except that Sea Clouds are slightly less dense than water, and as such, things that are drop into it will sink faster than in normal water. Another difference of seas made of Sea Clouds from seas made of water is that there is no sea floor. However, they still affect Devil Fruit users like a regular body of water would. These clouds form two known seas: the White Sea and the White White Sea.
Sea Clouds, as well as Island Clouds, are created when an element contained in the Seastone called 'pyrobloin', ejected into the air by volcanic eruptions, comes in contact with water vapor.[1]The density of the water vapor determines whether a Sea Cloud or an Island Cloud will be formed. Both these types of clouds are not normally able to survive on the Blue Sea.[2] However, after the timeskip, Nami is shown to be able to create a Sea Cloud that is capable of doing just that.
Fanciful creatures inhabit these clouds. In order to adapt themselves to a buoyant force that is less than normal water, they became less dense: some became flat and some became like balloons.
The White Sea (白海,Haku-kai?) is the lower part of sky ocean, located 7000 meters above the Blue Sea, comprised of Sea Cloud. It also had strange, rubbery clouds that bounce objects off of them and can be used as trampolines for those who are creative. There are different ways to get to the White Sea from the Blue Sea. The only way that has been shown so far is the unorthodox and dangerous way in which the sailors have to take a Knock Up Stream. Another way, mentioned by Gan Fall is the High West, as well as a couple of unnamed islands. However, these routes are apparently very treacherous, as Gan Fall states that, "If you had brought 100 people, it would be a gamble as to how many would survive".[3] The section first appeared in Chapter 237 and Episode 153.
The White-White Sea (白々海,Hakuhaku-kai?) is the upper part of sky ocean. It is located 10,000 meters above the Blue Sea. Skypiea (among other sky islands) is located here. This section first appeared in Chapter 239 and Episode 154.
Trivia
Sometimes the shadows of people from the White-White Sea are visible projected on the sky from the Blue Sea and are often feared by the sailors as they seem humongous giant creatures. This effect also happen in real life and is called Brocken spectre.
