UPSC Mains 2013 GS Paper 1 Q21: model answer

The Sahara, Arabian, Thar, Sonoran and Mojave deserts all lie roughly between 20° and 30°N. The Sahara receives in places less than 25 mm of rain a year. Their location is not chance but follows from the circulation of air and water.

Why these latitudes

• Air rising at the equator moves poleward and sinks at about 30° in the Hadley cell, creating the subtropical high-pressure belt.
• Sinking air is compressed and warmed, so it becomes dry and stable; clouds cannot form.
• Clear skies give intense heating by day and strong cooling at night.

Why the western side

• Trade winds blow from east to west. By the time they reach the western margins they have lost moisture over the land, or they blow offshore.
• Cold ocean currents flow along western coasts: the Canary current off the Sahara, the California current off the Sonoran. They cool the air from below, making it stable, so fog forms but no rain.
• On the eastern sides, onshore trade winds and warm currents (Gulf Stream, Kuroshio) bring rain, so Florida and southern China are wet at the same latitudes.

Other factors

• Rain-shadow effects (the Sierra Nevada for the Mojave).
• Distance from the sea and continental size (the Sahara's interior).

The southern hemisphere repeats the pattern (Atacama, Namib, Kalahari and the Great Australian deserts) alongside the Humboldt, Benguela and West Australian currents, which proves the global logic.