Water evaporates (as water vapor) from the surface of the earth(sea, rivers, transpiration from plants etc.) and rises into theatmosphere (as clouds). When it condenses from
…its gaseous state into a liquid state andovercomes the effect of gravity and wind it falls to the earth inthe form of rain. The transition from a gas to a liquid state (condensation) candepend on a number of things, but is normally temperature relatedin the case of rain. When water vapor cools the the moleculescontract and reach what is known as saturation point, the vaporchanges from gas to liquid and the small droplets coalesce to forma rain drop. Our world is warm enough that water evaporates, and most of theworld is covered in water, so a lot of water is evaporating, andwhen there is enough water vapor in the air, it can easily condenseinto rain. Moisture in the atmosphere always exists, but when it coolssufficiently it can condense around small particles and pour down. The water on earth will flow up into the sky by the sun, makingtiny droplets, which also creates clouds. The droplets are stillsmall and stay in the sky, floating. But soon tiny droplets willform together and make big ones, which will drop back to earth. If the droplets dropped when they passed mountains (high ones),they'll form into crystal ice, but melt when meeting the warm air.If the temperature was at freezing point, the droplets will turn tohailstones and fell down. Heat from the Sun causes water to evaporate (turn into a gas). Thewater vapor mixes (sort of dissolves) in the air. When the aircools the amount of water vapor that the air can hold reduces andthe excess vapor condenses as water droplets and falls to the earthas rain. (MORE) 
