A hydrosphere is the total amount of water on a planet. The hydrosphere includes water that is on the surface of the Earth, underground and in the air.
Water in the air is mainly in the form of clouds. Clouds are basically made up of water vapor and/or ice. Water vapor is essentially tiny droplets of water suspended in the air.
The suspended water droplets congregate together when they do, they form a cloud. Therefore clouds are water and are thus part of the hydrosphere.
