Why Deserts Form in Different Parts of the World
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When you picture a desert, you may imagine hot sand under a blazing Sun. Some deserts fit that picture, but others are rocky, cold or even icy. What they share is not temperature, but a long-term shortage of precipitation.
Dry air descends around the subtropics
Strong sunlight near the equator warms moist air and makes it rise. As the air rises, it cools, water vapour condenses and heavy rain often falls. The now drier air moves away from the equator high in the atmosphere.
Around 30 degrees north and south, much of this air sinks. Sinking air becomes warmer and can hold more water vapour, which makes cloud formation less likely. These broad zones of descending dry air help explain why many large deserts sit in the subtropics.
This pattern is part of global atmospheric circulation. It does not draw perfect desert bands around the planet because oceans, mountains, winds and seasons complicate the picture. Still, it gives you a useful first clue when you compare world maps. The Sahara, Arabian Desert and deserts of central Australia all sit partly within these dry latitude zones, though local conditions shape each one.
Mountains can create rain shadows
Moist air may be forced upward when it reaches a mountain range. As it climbs, it cools and releases rain or snow on the side facing the incoming wind. By the time the air crosses the peak, it has lost much of its moisture.
The air then descends on the other side, warms and becomes drier. This sheltered area is called a rain shadow. A wet forest and a dry landscape can therefore exist on opposite sides of the same mountain range.
Rain shadows show why distance alone is not enough to explain climate. Two places at similar latitudes can receive very different rainfall because the path of moving air is different. When you study a dry region, look upwind and check whether mountains have already squeezed moisture from the air.
Cold currents and continental interiors add dryness
Cold ocean currents can cool the air above them and reduce evaporation. The cool, stable air may produce fog near a coast but little rain, helping coastal deserts form. Far inland, places may also be dry because moist ocean air loses water before travelling such a great distance. The Atacama Desert is a famous example where cold coastal water, stable air and nearby mountains combine to limit rainfall.
Use four questions to investigate a desert:
- •Is it near a zone of sinking air?
- •Do mountains create a rain shadow?
- •Does a cold ocean current pass nearby?
- •Is the region far from a major moisture source?
The takeaway
Deserts form wherever the atmosphere delivers very little precipitation over time. Subtropical sinking air, mountain rain shadows, cold currents and inland distance can all contribute. Focus on the movement of air and moisture, and dry landscapes begin to make geographic sense.