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Rain falling on one side of a mountain range and nothing on the other A ridge of two mountain peaks stands in the middle of a ground line that runs the whole width of the picture. Above the slope on the left, and reaching right down onto that slope, is a dense group of short upright strokes showing rain falling. On the right of the ridge there are no strokes at all, and the ground there carries two low rounded humps of sand instead.

A Desert Is Not Made By Heat

About 11 minutes

Think of a desert for a moment before reading on.

Almost everybody pictures the same thing: yellow sand, a blazing sun, shimmering air, maybe a camel. Heat is doing most of the work in that picture.

Now here is the actual rule geographers use, and heat is not in it.

A desert is anywhere that gets less than about 250 millimetres of rain in a year.

That is a quarter of a metre of water over twelve whole months. In a rainy country a single wet month can beat it. Nothing in that sentence says anything about temperature at all.

Which makes Antarctica the biggest desert on Earth

Not the Sahara. Antarctica.

The Sahara covers around nine million square kilometres, which is roughly the size of the United States, and it is the largest hot desert. Antarctica covers about fourteen million, and over most of the interior the snowfall for a whole year comes to less than fifty millimetres of water. That is drier than the middle of the Sahara.

The confusing part is all that ice. But ice sitting on the ground is not the same as water falling from the sky. Antarctic snow piles up over hundreds of thousands of years for one reason: it is far too cold for any of it to melt. Almost nothing arrives, and almost nothing leaves.

There are even valleys there with no ice in them at all — bare rock and gravel, swept clean by wind, because so little snow falls and the wind takes away what does.

Eight places, and the yearly rainfall each one gets. Use the rule, not the picture in your head.

  • Inland Antarctica — under 50 mm of snow a year, measured as water
  • The Atacama in Chile — some places under 5 mm
  • Central Sahara — around 20 mm
  • Phoenix, in Arizona — about 200 mm
  • London — about 600 mm
  • Sydney — about 1,100 mm
  • The Amazon rainforest — about 2,300 mm
  • Bergen, in Norway — about 2,250 mm

Two of those probably felt wrong. Antarctica is a desert. Phoenix, which is hot, only just scrapes in — and it is a desert because it is dry, not because it is hot.

So what actually stops the rain

If heat is not the cause, something else has to be. There are a few, and the neatest one is a mountain.

Air arriving from the sea is carrying water. When a mountain range gets in the way, that air has nowhere to go but up — and air that rises cools. Cold air cannot hold as much water as warm air, so the water comes out as rain on the side of the range facing the sea. There is a whole article about that step in why clouds have flat bottoms.

By the time that air spills over the top and slides down the far side, its water is gone. It arrives dry, and it warms up again on the way down, which makes it thirstier still.

Air crossing a mountain range and arriving dry on the far side A mountain stands in the middle of a ground line that runs the width of the picture. On the left is the sea, drawn as three wavy lines. A long dashed arrow leaves the sea, climbs the near slope, crosses the summit and runs down clear of the far slope. A cloud sits against the near slope with many short strokes of rain falling from it onto that side. On the far side of the mountain there is no cloud and no rain at all, and the ground there carries two low rounded humps of sand. That side is labelled the rain shadow. the sea forced up, and cooling so it lets go of its water comes down with nothing left rain shadow wet side
One range of mountains, two completely different worlds. Everything the air was carrying falls on the near side. What comes down the far side has nothing left to give.

The dry strip behind a mountain range is called a rain shadow, and the name is a good one. The land is not in the shade — it is in the shadow of the rain.

Air crosses a mountain range and comes down the far side dry. Where did its water go?

Mountains are not the only way to starve a place of rain. Some deserts sit under a belt of permanently sinking air, roughly thirty degrees north and south of the equator — which is exactly where the Sahara, Arabia and the Australian deserts sit, all in a line. Some are simply so far from any ocean that the air has been wrung out long before it arrives, which is the Gobi. And the Atacama has a cold ocean current sitting offshore, chilling the air before it ever reaches land.

Different causes. Same result: nothing falls.

And that is why a desert looks like a desert

Once you know a desert is a dry place rather than a hot one, the way it looks stops being decoration and starts being evidence.

No rain means no plants worth speaking of. No plants means no roots, and roots are what hold soil together. So there is nothing anchoring the loose grains, and wind is free to pick them up, roll them along and pile them into dunes that move.

And when rain does come, it comes all at once onto ground with nothing to soak it up. It runs straight off the bare rock in a sheet, gouging out steep-sided channels in an afternoon and then vanishing.

Next time somebody describes a place as a desert, the useful question is not how hot it gets. It is how much falls on it in a year.