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them off from the sea. Moisture-laden air currents moving inland from oceans cool down when they rise up the slopes of these mountain barriers, and release all their moisture as rain. The dry air warms up as it flows down the opposite side of the mountains. It evaporates any local moisture, forming deserts such as America’s Mojave Desert, east of the Sierra Nevada mountains. Coastal deserts form near oceans with cool ocean currents. The air above these currents is also cool, and loses its moisture as rain at sea. The cool, dry air moves inland and creates deserts such as the Namib Desert of southwest Africa along- side the cool Benguela Current, and the Atacama Desert of south Peru and northern Chile, caused by the cool Humboldt Current. The World’s Major Deserts The world’s largest deserts are not areas that are traditionally thought of as being desert, but they do fit the definition. Antarctica, which receives less than eight inches (20 cm) of precipitation annually at the coasts, and far less inland, is home to the world’s largest and driest desert, covering most of the continent. It is also the coldest desert, reaching tempera- tures of –81°F (–63°C) and below. The Antarctic desert covers roughly 5.5 million square miles (14 million square kilome- ters). There are also polar deserts in the northern Arctic regions, including parts of Alaska, as well as Canada, Greenland, Iceland, Norway, Sweden, Finland, and Russia. The Arctic deserts cover about the same area as the Antarctic desert.

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Deserts