Some clouds on fancy graphics
Clouds are a aspect of a map that appear close to the top of the map, they have no practical service and are merely aesthetically pleasing. They do however, float towards the north.
Appearance
Clouds will appear differently on different graphics settings, and have a set pattern.
Fancy Graphics
On fancy graphics, clouds appear as about four block tall transparent prisms. In the nighttime they turn a shade of blue and become more transparent. These clouds can be very frustrating for tall towers, as they appear at layers 108-112, and are capable of passing through blocks. The smallest horizontal dimensions are 12 x 12 blocks wide.
The pattern clouds have
Fast Graphics
On fast graphics, clouds appear as opaque planes. They have no daytime/nighttime rotation. They appear at layer 121, making them less of a nuisance while building. The smallest horizontal dimensions are 8 x 8 blocks wide.
Pattern
Unlike many things in minecraft, clouds are not procedurally generated. With a texture pack, clouds' color can be changed to anything, meaning you could have pink clouds if you so desired. The file can be resized to any size, meaning it would be possible to have a different cloud pattern each day.