The Sun’s heat causes water in the oceans, rivers and lakes to evaporate and form water vapour. The warm water vapour rises upwards, and as it rises, it cools down. As a result, the water vapour in the air condenses to form clouds containing tiny droplets of water. These droplets grow larger in size and finally fall down as rain. Sometimes, the temperature is so low that these droplets freeze into ice crystals and fall down as snow.