The water cycle

The Sun is the main source of heat for the Earth. Wet clothes dry faster on a sunny day because the Sun's heat makes water evaporate faster. The same heat makes water evaporate from oceans and lakes.

Water exists in three states in nature:

• liquid in oceans, rivers and lakes;
• solid as snow, ice sheets and glaciers in the mountains and polar regions;
• gas as water vapour in the air.

In summer, some snow and ice melts in the Sun's radiation and flows down as rivers, finally into the oceans. Fresh snow in winter replaces the melted ice.

Water in oceans, rivers and lakes is heated by the Sun and evaporates as water vapour. Plants give off water vapour too, through their leaves; this is called transpiration.

As water vapour rises, it cools down and condenses into tiny drops that form clouds. Clouds bring rain, snow and hail back down; this is called precipitation.

Water keeps moving like this: up as vapour, down as precipitation, through soil, rocks and plants, and back into water bodies. This endless loop is the water cycle.

The water cycle refills rivers, lakes and oceans, and it keeps the total amount of water on the Earth the same. Rainwater either flows into ponds, lakes, rivers and oceans, or seeps into the ground.