Why do clouds form?
The short answer
Clouds form when water vapor in rising air cools and changes into tiny droplets or ice crystals.
How it works
Warm air rises and expands where the air pressure is lower. As it expands, it cools. When it becomes cool enough, water vapor condenses around tiny particles into droplets.
A familiar example
Water droplets on the outside of a cold drink are a small example of water vapor cooling and condensing.