The total amount of water on the earth and in its atmosphere does not change, but we have all seen rain and rivers flow, so there must be a motion that transfers water in a never-ending cycle. This cycle is called the 'water cycle' or the 'hydrological cycle'.
Most of the earth's water is in the sea, about 97.5% of the total. The rest is on land or in the air. To start the cycle, the water must get from the sea to the earth's atmosphere. This happens by 'evaporation', the water becomes 'water vapour' which is a gas. It is evaporation that dries clothes and puddles.
Plants also play an important role in converting water into water vapour. Plants transport water from the soil to their leaves to supply nutrients. Water evaporating into the atmosphere from their leaves is called 'transpiration'.
Once in the air, the water vapour is blown by winds and distributed through the atmosphere. Generally as air rises it cools, as it does so it loses its capacity to hold as much water. The water vapour condenses to form the water droplets of clouds. When there is enough water in the air, it rains, and the water returns to earth. The water does not always come to earth as rain though - sometimes it is snow, hail or sleet.
Back on the earth's surface, the water continues its journey. If the water falls on land, it could make its way back to the sea as 'runoff' flowing over the land. If the water is soaked up by the ground, then it can flow underground back to the sea. The water could also fall onto a glacier as snow or ice. Once there it will slowly melt and eventually return to the sea.
All this does not have to happen quickly of course. Water can stay under the ground for 10,000 years, and in a glacier for 40 years. So the water cycle can be more of a slow amble than a sprint.
The distribution of water throughout the earth is not uniform. Some places have far more rainfall than others. There are many reasons for this, such as where the mountains are and where the prevailing winds blow. This rainfall distribution partly explains the differences in vegetation, why some areas are deserts, and some rainforests…and why the UK is neither.
Other features in the Weather Basics series:
- Clouds
- Day and Night
- Fog and Mist
- Fronts
- Frost
- High Pressure
- Humidity
- Jet Streams
- Lightning
- Low Pressure
- Mirages
- Northern Lights
- Rain or Showers?
- Thunder
- Watercycle
