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Where the rain never arrives

Why Are Deserts Dry?

Deserts sit where air sinks, or where mountains and distance strip the rain out first.

Rain needs rising air Air cools, watercondensesDeserts have sinking air Air warms, cloudsevaporate
No rising, no rain.

  1. Step 1 of 5

    Rain needs air to rise.

    1 Air rises andcools2 Water condenses3 It rains

    Rising air cools, and cool air cannot hold its water, so clouds and rain form. Wherever air sinks instead, the opposite happens.

  2. Step 2 of 5

    At about 30 degrees latitude, air sinks.

    Air rises at the equator Travelspoleward highup Sinks around 30° Warm, dry,cloudlessWhydeserts

    Air rising at the equator travels poleward and comes back down around 30 degrees north and south. Sinking air warms and dries — no clouds.

  3. Step 3 of 5

    Mountains wring the rain out first.

    Windward side Forced up — heavy rainLeeward side Descending dry air —rain shadow

    Air forced over a range rains on the way up. Coming down the far side it is dry and warming, leaving a rain shadow desert behind the mountains.

  4. Step 4 of 5

    Some deserts are just too far from any sea.

    Coastmoist airarrives500 km inlanddrierDeep interioralmost nomoisture left

    Air travelling deep into a continent has already dropped its moisture. The Gobi is dry mainly because the ocean is very far away.

  5. Step 5 of 5

    Cold ones count too.

    Sahara About 100 mm a yearAntarctica About 50 mm — thelargest desert

    The definition is low precipitation, not heat. Antarctica gets less than the Sahara, which makes it the largest desert on Earth.

    The Atacama has weather stations that have never recorded rain in their entire operating history.

The short version

Deserts are dry because sinking air, mountain rain shadows or sheer distance from the sea prevent rain.

Try it yourself

Find the world's deserts on a map. Most sit in two bands around 30° north and south — the sinking air belts.

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