Why does Australia have so many deserts?

A very dry continent

Apart from icy Antarctica, Australia is the driest continent on Earth. Around 70 per cent of it is arid or semi-arid, which means it gets very little rain. About half of the continent averages less than 300 millimetres of rain a year. The driest part, around Kati Thanda–Lake Eyre in South Australia, gets less than 150 millimetres.

Geoscience Australia lists ten named deserts on the mainland. Together they cover about 18 per cent of it. The biggest is the Great Victoria Desert, in Western Australia and South Australia, which covers about 348,750 square kilometres. That is larger than the whole state of Victoria.

So why is so much of Australia so dry? There is no single answer. Climate scientists point to three big reasons working together.

Reason 1: a belt of sinking air

Around the world, at about 30 degrees south and 30 degrees north of the equator, there is a belt of high air pressure. Much of Australia sits in the southern one. Over Australia, this belt is called the subtropical ridge.

In a high-pressure system, air slowly sinks from high up towards the ground. Sinking air gets squashed and warms up, which makes it drier, so clouds struggle to form. The Bureau of Meteorology says dry air sinking from the high-pressure systems along the ridge is why central Australia is generally the driest part of the country.

The ridge is there all year, but it shifts. In summer it usually sits south of the continent, blocking rain-bearing cold fronts from southern Australia. In autumn it moves north over the continent, which lets cold fronts bring winter rain to the south. The middle of Australia, though, sits under the ridge for most of the year.

Reason 2: mountains that catch the rain

Winds blowing in from the Pacific Ocean carry moisture towards eastern Australia. When they meet the Great Dividing Range, the air is pushed upwards. Rising air cools, and cool air cannot hold as much water vapour, so clouds form and rain falls on the eastern side of the mountains.

By the time the air has crossed the range, it has dropped much of its moisture. The Bureau of Meteorology says the Great Dividing Range and nearby ranges boost rainfall near the east coast and partly block moisture from reaching further inland. A drier area on the far side of mountains is called a rain shadow. You can see one in Tasmania too, where the west is wet and the east is much drier.

Reason 3: a long way from the sea

Australia is huge. The middle of the country is a long way from the ocean in every direction, so moist air has a long trip to get there. There are also few big mountain ranges inland to squeeze rain out of the air.

When rain does come to the deserts, it is very unpredictable. Years with less than half the usual rainfall are common. Then, now and again, a whole year's worth of rain can fall in a single month, or even in a single day.

Surprisingly, Australia's deserts are not the driest in the world. The Atacama Desert in South America has places where rain has not fallen for hundreds of years, and parts of the Sahara get less than 10 millimetres a year.

Full of life

Dry does not mean empty. Australia's deserts are home to a rich mix of plants and animals, and many of them live nowhere else on Earth. Prickly, hummock-forming grasses called spinifex grow across as much as a quarter of the Australian landscape.

Much of the desert is covered by windblown sand, and the long red dunes of places like the Simpson Desert line up with the winds of about 20,000 years ago. Small wells, rock holes and soakages hold precious water. People have lived in central Australia for at least 35,000 years, and knowing exactly where to find water like this is part of the deep knowledge First Nations peoples hold about desert Country.

📚 Words to know

Arid

Very dry. Arid places get so little rain that few plants can grow in most years.

Semi-arid

Dry, but not quite as dry as a desert. Semi-arid places get a little more rain.

Subtropical ridge

A belt of high air pressure over Australia, where sinking air makes the weather dry and settled.

Rain shadow

A dry area on the side of a mountain range away from the wind, because the air dropped its rain on the other side.

Evaporation

When liquid water turns into invisible water vapour and rises into the air. Heat speeds it up.

🧪 Try this: be a rain detective

1. Make a rain gauge: take a straight-sided clear jar or container and stick a ruler to the outside, with 0 at the bottom. Put it outside in an open spot, away from trees and roofs.
2. Check it at the same time each day for a month. Write down how many millimetres of rain fell, then empty it.
3. Add up your month's total. Multiply it by 12 for a rough yearly guess. Is your town wetter or drier than 150 millimetres a year, the average near Kati Thanda–Lake Eyre?
4. Bonus: put the same amount of water in two identical saucers, one in the sun and one in the shade. Check them each day. Which dries out first? In the desert, strong sun means water disappears quickly.