 Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere . When atmospheric conditions permit an uplift of warm , humid air , this water condenses and falls to the surface as precipitation . Most of the water is then transported to lower elevations by river systems and usually returned to the oceans or deposited into lakes . This water cycle is a vital mechanism for supporting life on land , and is a primary factor in the erosion of surface features over geological periods . Precipitation patterns vary widely , ranging from several meters of water per year to less than a millimeter . Atmospheric circulation , topographic features and temperature differences determine the average precipitation that falls in each region . 
