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The oceans are the chief source of rain, but lakes and rivers also contribute to it. The Sun's heat evaporates the water. It remains in the atmosphere as an invisible vapor until it condenses, first into clouds and then into raindrops. Condensation happens when the air is cooled.

Air cools either through expansion or by coming into contact with a cool object such as a cold landmass or an ice-covered expanse. When air passes over a cold object, it loses heat and its moisture condenses as fog, dew, or frost. Air also cools as it rises and expands. The water vapor in the cooling air condenses to form clouds and, sometimes, rain.