Condensation, gas changing to liquid, is the foundation of cloud formation.
When warm, generally moist air rises, it cools. When there are particles in the air for the water vapor to adhere to, the vapor converts to water droplets, between 0.02 and 0.6 millimeters (0.000788 and 0.02364 inches) wide.
The particles, which can be dust, pollutants, ice or salt crystals, or chemical compounds, are called condensation nuclei. Without them, the water vapor either remains gaseous or converts directly to ice crystals through sublimation.
Sublimation is the process of a gas turning into a solid without going through the liquid stage. Given cold enough temperatures, water vapor will instantly crystallize.
The resultant ice crystals provide condensation nuclei for the vapor to condense on.