Clouds are large, cotton and glue structures built to hide UFOs and other flying alien structures. in order to avoid suspicion, most clouds are empty and formed mostly out of ghosts (also known as fog). There are several types of cloud, the appearance being mostly decided by the preference of the driver. It is possible to walk on them if you practice, but the government is spreading rumors saying that you can't so that we won't try to escape and live on the moon (it would be quite easy, mostly because there is, in fact, air in space. There is also gravity, but you can just hitch a ride on a meteor).