The clouds in Patterns are blocky non-physical objects that drift across the sky. They are sometimes low enough to pass through some of the structures.
There is also a layer of fog drifting above the void. There are small hills and indentations in the fog that move along with it. When behind the fog, any structures or objects, such as the player, will lose their texture and colour and only show outlines and some shading.
It is possible to go below the fog, especially with a hill passing by, but if you go too low you will be teleported back to the spawn point, even if walking down a slope.
You can see some distant light-emitting objects below the fog in the void.