Clouds are mostly made of water-vapor, but a dash of something extra is needed to complete their formation.

As air rises through the atmosphere, it cools.  When it reaches its dew point, it is ready to form a cloud if it can find a condensation nuclei.  These are free floating microscopic particles in the air, like dust, salt, or smoke . A cloud droplet is born when a water-vapor molecule lands on one of these tiny non-gaseous surfaces and condenses.   As cloud droplets gather together in massive numbers, they become visible.

There are a variety of potential condensation nuclei in the air. Their size, composition, and capacity to attract and hold water molecules impact their ability to form clouds. Salt particles, for example, are very good at absorbing water while carbon based particles are not. Without the presence of condensation nuclei in the air, cloud development would be limited to areas of significantly colder air temperatures.