* Water droplets form when water vapor in the air cools and condenses around tiny particles in the atmosphere, called condensation nuclei. These nuclei can be things like dust, salt, or smoke.

* Ice crystals form when the temperature in the cloud drops below freezing.

The size and shape of the water droplets or ice crystals determine the type of cloud. For example, cumulus clouds are puffy and white because they are made of large water droplets, while cirrus clouds are wispy and feathery because they are made of ice crystals.