Published on: 1.10.2019
Information produced by Finnish Environment Institute
The water cycle
It is raining, and the whole world is wet for a short while. Soon the rain eases off and the puddles disappear. Where did the rain come from, and what happened to the rainwater?
Water is in constant motion, moving from place to place and changing its form on the way. The rain that falls onto the ground starts off as water vapour in the atmosphere. In suitable conditions it condenses into droplets that fall down as rain, or snow when the temperature is low enough.
On the ground surface, part of the rainwater is infiltrated into the soil and penetrates ever lower, producing groundwater. As it is absorbed through the soil layers, the water is purified, which is why groundwater is very clean. Groundwater flows deep underground and bubbles up in springs.
If the rain is heavy or the ground is already wet, some of the rainwater stays on its surface. It flows downhill, accumulates into brooks and finally ends up in rivers, lakes and the sea. On the way it may pick up all types of dirt.
The water cycle does not end even in the ocean. Water constantly evaporates from the sea and from other water and ground surfaces into the atmosphere. As it evaporates, water turns into vapour in the same way as it does in a kettle. Evaporation is maintained by the warmth of the sun. Even in the cool Finnish climate, approx. 30 centimetres of water evaporate from the surface of lakes a year. Currents carry the water vapour in the air, and it builds up into clouds. Soon it will be raining again somewhere.