Big question
Why are there deserts?
Deserts form where very little rain falls, usually because of dry, sinking air, mountains blocking rain-bearing winds, or being far from any ocean. Most of the world's biggest deserts sit in two dry bands around 30 degrees north and south of the equator, where descending air holds almost no moisture.
Rising and Falling Air
Near the equator, hot air rises, carrying huge amounts of moisture with it, and as it climbs and cools that moisture falls as heavy tropical rain. Having lost most of its water, that same air then spreads out and sinks back down to Earth around 30 degrees north and south of the equator, in what scientists call a Hadley cell. Sinking air warms up as it descends, which makes it even better at holding onto whatever tiny bit of moisture is left instead of releasing it as rain. That is why belts of desert, including the Sahara, the Arabian Desert and the deserts of Australia, line up along these latitudes across the globe.
Blocked by Mountains
Some deserts form for a completely different reason: a mountain range standing directly in the path of moist winds blowing in from the ocean. As that moist air is forced to rise over the mountains, it cools, and its water vapour condenses into clouds that drop their rain or snow on the windward side, facing the wind. By the time the air crosses over the peaks and descends the other side, it has already lost almost all its moisture, so it arrives dry and warms further as it sinks. This dry zone on the far side is called a rain shadow, and it explains deserts like the Atacama in South America, tucked behind the towering Andes mountains.
Far From the Ocean's Moisture
Most of the water vapour that turns into rain clouds starts out evaporating from the surface of oceans and seas. Land that sits deep inside a continent, thousands of kilometres from any coastline, is simply too far for much of that moist air to travel before it cools, falls as rain elsewhere, and never arrives. The Gobi Desert in central Asia is a clear example: it lies so far from any ocean, and is additionally shielded by mountain ranges, that very little rain-bearing air ever reaches it, leaving the region dry, dusty and prone to extreme temperature swings between day and night.
Try it yourself
Look at a world map and find the Sahara, the Arabian Desert and the Australian deserts. Notice how they line up in roughly the same two bands, north and south of the equator, rather than being scattered randomly.
Questions children ask
Are all deserts hot?
No, a desert is defined by how little rain it gets, not by temperature, so some deserts are extremely cold. Antarctica and parts of the Gobi Desert can have freezing temperatures while still counting as true deserts.
What is a rain shadow desert?
A rain shadow desert forms on the dry side of a mountain range, after moist ocean winds have already dropped their rain or snow on the wetter, windward side. The Atacama Desert in South America is a well-known example.
Why are there deserts near 30 degrees latitude?
Deserts cluster near 30 degrees north and south because that is where air that rose and lost its moisture near the equator sinks back down, warming and staying dry. This pattern, called a Hadley cell, creates deserts like the Sahara and the Australian outback.