The water cycle — Earth science, 14–17 years
Water is constantly moving between the ground, oceans, living things and the atmosphere. The same water can freeze, flow, evaporate and fall as rain many times.
A journey with no finish
Earth’s water does not get used up after one journey. Sunlight helps liquid water evaporate, cooling turns vapour into droplets, and gravity brings water back as rain or snow. It then gathers in rivers, lakes, oceans or the ground.
Why scientists follow it
People need to know where usable freshwater comes from and why it can disappear from a place. The water cycle connects rainfall, droughts, floods, crops and drinking-water supplies, so it explains changing water availability rather than treating rain as random.
One drop, several stages
Imagine a drop in the sea. Sunlight warms it, so it becomes water vapour and rises; high in the air it cools and joins a cloud. Later it falls as rain, runs into a stream, and may soak into soil before returning to the sea.
The water has not vanished
A common mistake is to say that a puddle has disappeared because the water is gone. It is reasonable: you can no longer see it. Usually, some water evaporated into the air and some may have soaked into the ground, so “out of sight” is not the same as destroyed.
Water planning
Water managers use the cycle to decide when reservoirs may fill and when restrictions may be needed. Farmers, cities and flood planners compare rainfall, evaporation and underground storage. The cycle helps them prepare, but it cannot tell them the exact weather on every day.
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