Where does rain come from?

The water that never runs out

The Earth is about 4.5 billion years old, and in all that time it has never run out of water. That is because water does not get used up. It goes round and round in a loop called the water cycle. The water in your drink bottle has been rain, cloud, river and sea many times over.

Most of Earth's water, more than 96 per cent, is salty water in the oceans. Only about 3 per cent is fresh, and most of that is frozen in ice caps and glaciers or hidden underground. Only a tiny sliver, less than 1 per cent of all the fresh water, is in the rivers and lakes we can see.

So where does rain come from? Mostly from the sea. Here is how water gets from the ocean to the cloud above your head, and back again.

Up: the Sun does the lifting

The Sun heats the surface of the sea, lakes and rivers, and even puddles and wet soil. Some of the water turns into water vapour, an invisible gas, and drifts up into the air. This is called evaporation.

Plants help too. They suck water up through their roots and let it out as vapour through tiny pores on the underside of their leaves. This is called transpiration. Together, evaporation and transpiration are the main way water gets from the ground into the sky.

In Australia the Sun is strong, so a lot of our rain goes straight back up. CSIRO scientists worked out that, averaged across the whole country, about 82 per cent of the rain that falls evaporates from the soil or passes back out through plants. Only about 14 per cent runs off into creeks and rivers.

Making a cloud

Warm, damp air rises. As it goes higher it cools down, and cool air cannot hold as much water vapour. So the vapour turns back into liquid water. This is condensation. You see the same thing when a cold drink bottle gets wet on the outside on a hot day.

The vapour does not turn into drops out of nowhere. It needs something to cling to. The air is full of tiny specks of dust, pollen and other particles, and the water condenses around them. Each droplet is so small and light that it floats. Billions of floating droplets together make a cloud.

Inside the cloud the droplets keep growing. When a drop gets too heavy to float, down it comes as precipitation: rain, or hail or snow if the air is cold enough.

Down and around

Rain that reaches the ground goes one of three ways. Some evaporates straight back up. Some runs downhill into creeks, rivers and lakes, and eventually back to the sea. And some soaks into the soil and trickles down into the cracks and spaces in the rock below, becoming groundwater.

Groundwater does not stay still. It creeps slowly through the rock and comes back to the surface in springs, wetlands and rivers. Then the Sun lifts it up again, and the cycle continues. The whole loop is powered by two things: the Sun's energy, which lifts the water, and gravity, which pulls it back down.

Where Australia's rain comes from

Australia is a dry place. Geoscience Australia says our continent and Antarctica are the two driest in the world, and about 18 per cent of the mainland is desert. Averaged across the country, about 504 millimetres of rain falls in a year, which is only 62 per cent of the world average.

Our rain comes from the oceans around us, but it arrives in different ways. In the tropical north, the wet season runs from November to April. As the land heats up in summer, it draws in moist air from the Indian Ocean and the seas to our north. This is the monsoon, and it brings bursts of dark clouds, heavy rain and thunderstorms to the Top End, each lasting a few days to a week or more.

In the south, cold fronts bring a lot of the rain, especially in winter. A cold front is the leading edge of a mass of cold air. Cold air is heavier than warm air, so it slides underneath and lifts the warm air up. The rising air cools, its vapour condenses, and clouds and rain follow. Fronts usually reach south-west Western Australia first, then sweep east across the southern states over a few days.

The wettest part of Australia is the coast of Far North Queensland. Near Babinda, south of Cairns, the usual yearly rainfall is more than 4,000 millimetres, about eight times the national average. The driest part is around Kati Thanda–Lake Eyre in South Australia, where only about 100 millimetres falls in a typical year.

📚 Words to know

Evaporation

When liquid water turns into water vapour, a gas, and rises into the air.

Water vapour

Water as an invisible gas in the air.

Condensation

When water vapour cools and turns back into tiny drops of liquid water.

Precipitation

Any water that falls from clouds: rain, drizzle, hail, sleet or snow.

Transpiration

Water vapour that plants release through tiny pores in their leaves.

Groundwater

Water held in the cracks and spaces in soil and rock under the ground.

🧪 Try this: make it rain in a bag

1. Pour about a cup of water into a clear zip-lock bag and add a drop of blue food colouring so you can see it. Seal the bag tightly.
2. With a marker, draw a sun and a cloud near the top of the bag. Tape the bag to a sunny window, with the water at the bottom.
3. Check back every few hours. Drops will appear on the inside of the bag near the top. The Sun evaporated some water, it rose as vapour, then it cooled on the plastic and condensed. When the drops get heavy they run back down, like rain.
4. Challenge: on a warm, sunny morning, slip a clear plastic bag over a few leaves on a living plant outside (ask first), tie it loosely and come back after lunch. The mist inside is transpiration. Take the bag off when you are done.