Tukuk-OSA Desert Is a Water Account, Not a Landscape
The definition of a desert is purely about precipitation: a place that receives roughly less than 250 mm (about 10 inches) of rain a year counts as arid, because in such places water evaporates faster than it is replaced. That is the entire criterion. Nothing about sand, heat or cactus. It follows that:
- Antarctica and the high Arctic are deserts — brutally cold, yes, but bone dry, since frigid air holds almost no moisture and the interior gets only a few centimetres of snow-melt-equivalent a year. Antarctica is often called the world's largest desert.
- Sandy dunes are a minority. Most deserts are rock, gravel, gravel plains or hardpan; the great ergs of blowing sand (like the Rub' al Khali) are only a fraction of all desert area.
- A desert can bloom. An arid plain that rarely rains will, after an unusual downpour, green almost overnight from dormant seeds — proving that the limiting ingredient was water, not sunlight or soil.
So the real question is not "why is it hot" but "why does the moisture never arrive." Four mechanisms answer that.
1. The Subtropical High: The Big One
The largest hot deserts — the Sahara, Arabian, Kalahari, Australian — sit near 20–30° latitude for a shared reason tied to global circulation. Air warmed at the equator rises, dumps its rain on the tropics, then travels poleward aloft and sinks around the 30° belt. Sinking air warms and dries as it compresses, and a dry, descending blanket is precisely what suppresses clouds and rain. These are the "trade-wind / subtropical high" deserts, and they wrap the globe in a near-continuous arid band.
2. Rain Shadow: The Dry Side of Mountains
Moist air forced up a mountain range cools, expands and rains out on the windward side. By the time that same air descends the leeward slope, it has little water left and warms as it falls, soaking up moisture from the land. The result is a dry shadow in the lee of the range. The Great Basin and Mojave deserts lie in the rain shadow of the Sierra Nevada and Cascades; Patagonia is arid because the Andes strip the moisture out of the westerlies before it reaches Argentina.
3. Cold Ocean Currents: Fog Without Rain
Where a cold current runs along a coast, it chills the air above it so that the air is stable and holds its moisture as fog rather than rising to make rain. Coastal deserts like the Atacama (fed by the cold Humboldt Current) and the Namib (Benguela Current) are hyper-arid yet often blanketed in fog — the driest place on Earth gets its hint of water as mist, not downpour.
4. Continentality: Too Far From the Sea
Air that has crossed a huge continent has already dropped most of its moisture near the coast by the time it reaches the interior. The Central Asian deserts — the Gobi, the Taklamakan — are arid partly because they are simply the farthest places from an ocean, thousands of kilometres inland.
Most real deserts are a combination — the Atacama, for instance, is rain-shadowed by the Andes and fed by a cold current and under the subtropical high, which is why it is the extreme it is.
Why Deserts Are Not Static
Deserts expand and contract with climate cycles. Over human history, shifts in the monsoon and subtropical high have "greened" and then re-aridified regions like the Sahara; today, overgrazing and land misuse drive desertification at the margins, and the great deserts are among the places climate change is expected to push further. A desert is not a permanent sentence on a landscape — it is an ongoing balance between how much moisture arrives and how much is lost.
You can browse the world's deserts — size, climate type and country — on our deserts hub, and see where they sit among the planet's other extremes on the records index, from the driest places on Earth to the hottest.