Linked from
The 11 pages that link to Cloud formation, each with the reason it gives.
Orographic liftRelated: Cooling during forced ascent can bring air to saturation and produce clouds.
Water vaporRelated: Clouds appear when vapor condenses or deposits onto particles in the atmosphere.
CondensationRelated: Cloud droplets form when rising, cooling air condenses water vapor.
Atmospheric convectionRelated: Convection lifts moist air, which cools and can form clouds.
HumidityRelated: Saturated humid air can condense on particles and form cloud droplets.
Substellar objectRelated: Condensate clouds alter the colors and spectra of many substellar objects.