There are only five cloud berries available in Ko. If fed to a Pokemon, the cloud berry will max out their speed stat. While held, the cloud berry will bring the Pokemon's weight down to zero.
A cloud seed will appear after a Pokemon eats a cloud berry. Once five seeds are planted at a certain point in Ko (only reachable in the Sunrise Version), a cloud tree will grow.
Cloud trees are thousands of feet in height. (It is said that the Banyan tree in sun city is a Cloud Berry tree - although it doesn't bear fruit.) Climbing the cloud tree might lead you to a secret area!