Water vapour in the atmosphere comes through evaporation from the oceans, lakes, rivers, ice-fields and glaciers all comprising about 75% of earth, surface. Besides these, evaporation from the moist ground, transpiration from plants, and animal respiration also contribute moisture to the atmosphere.
Water vapour evaporated from these sources is carried for long distances on land from the oceans by wind and convective movements. Under favourable conditions it condenses and precipitates over the earth’s surface as rain, snow and hail. Precipitation over oceans completes the cycle rapidly to follow another one.