Where does rain come from, and where does the water go?

In numbers

• ≈ 9 daysaverage time water stays in the air
• ≈ 3,200 yearsaverage time water stays in the ocean
• ≈ 2 mma typical raindrop

In five stops

1. 1

   Where were its atoms born?

   The nuclei of a water molecule's two hydrogen atoms formed in the universe's first second; they are about 13.8 billion years old. The oxygen atom was born inside stars. Where Earth's water came from is debated: some is thought to have come with the rocks that built the planet, and some with icy bodies that struck it later.

2. 2

   How did nature prepare it?

   The Sun warms the oceans, water evaporates and rises into the air, and plants release water vapour from their leaves. Rising air cools, and the vapour condenses on specks about 0.1 micrometres across, such as dust, sea salt or smoke, forming cloud droplets. About a million cloud droplets come together to make one 2-millimetre raindrop, which falls.

3. 3

   What did people do?

   People don't make rain, but they store it: dams and ponds collect rainwater, and treated water reaches homes through pipes. City roofs and pavements, however, stop rain soaking into the ground; the water runs off the surface quickly, which is a major cause of urban flooding.

4. 4

   What happens when we use it?

   The water we drink, the water that irrigates fields and the water plants draw up through their roots all come from rain in the end. In photosynthesis, plants bind water's hydrogen into sugar and release its oxygen into the air: some of the oxygen we breathe was once a raindrop.

5. 5

   Where does it go next?

   How long water stays in one place on average: 9 days in the air, 2–6 months in rivers, 50–100 years in lakes, about 3,200 years in the ocean, 10,000 years in deep groundwater. In the end it evaporates and falls again. The molecules in today's rain may once have passed through water that dinosaurs drank.