Here's why:

* Atmosphere: The atmosphere is the layer of gases surrounding the Earth. Clouds are formed from water vapor condensing within the atmosphere.

* Hydrosphere: While clouds are made of water, they aren't part of the hydrosphere (the water on Earth's surface). They are suspended in the atmosphere.

* Geosphere: The geosphere encompasses the solid Earth, including rocks, soil, and mountains. Clouds are not part of the solid Earth.

* Biosphere: The biosphere refers to all living organisms on Earth. While clouds are essential for life, they aren't living organisms themselves.

Therefore, the primary sphere that clouds belong to is the atmosphere.