The nimbus cloud is a cloud people can ride on to get places very quickly.
There was once lots of nimbus clouds in the dragonball world now only a few even exist.
Most disappeared because of the very very few amount of people who are still
pure of heart.
Goku got his nimbus cloud when he helped Master Roshi's Turtle back to the sea
and Master Roshi gave it to Goku because he helped Turtle with thoughts of nothing
in return. Master Roshi himself couldn't ride it anymore because he
had become perverted through the years, thus making him not pure of heart.
Most people near to goku can ride nimbus, including...
Gohan, Goten, Chi Chi, Launch, Krillen(one time to find Launch with goku),
and Nam.
