Clouds form when humid air rises and cools, creating a pocket of air that is supersaturated with water vapor (1). As simple as it may seem, cloud formation requires a precise set of conditions to occur, and it begins with the sun’s energy.
Cloud formation begins when heat from the sun causes liquid water to evaporate into gaseous water. The atoms in liquid water are held close together by polar bonds, but heat from the sun helps bump the water molecules into the atmosphere (2). Evaporation will continue until the atmosphere becomes fully saturated: at this point the atmosphere cannot hold any additional water vapor. The term relative humidity if often used in weather forecasts and it indicates the degree of atmospheric saturation. Evaporation stops when relative humidity reaches 100% (1).
Evaporation is the first step in cloud formation, but it does not guarantee that a cloud will form. At 100% saturation, water molecules are evaporating and condensing at equal rates. However, saturated air that is cooled quickly, as happens when warm air rises high into the atmosphere, will become supersaturated. This means the pocket of air contains more water vapor than it can hold. At this point the water vapor needs a tiny nudge to condense. This nudge is provided by airborne dust particles called condensation nuclei. Much like a cold beverage glass on a hot day, the dust particles give the water vapor a surface to condense upon. This tiny amount of condensation sets off a chain-reaction, and all the water vapor condenses at once (3). A cloud is born. This is also why contrail clouds form in the wake of jet exhaust: dust particles in the exhaust give water vapor a place to condense.
(1) University Corporation for Atmospheric Research
“Clouds and Cloud Formation”
(2) USGS Water Science School
The Water Cycle: Evaporation
(3) NASA Earth Observatory
Aerosols and Clouds