When you imagine a desert, you probably imagine a vast, empty landscape far from water. But surprisingly, some of the driest places on Earth are right next to the ocean. The Atacama River in Chile and the Namib River in southern Africa both run along coastlines. So how did this extreme desert form in a place surrounded by so much water?
David Creamer, a hydrologist at the University of Nevada, Las Vegas, told Live Science that there are three main factors that lead to the formation of deserts next to the ocean. It’s how air moves vertically, how air moves horizontally, and how mountain ranges interact with the moisture in the air.
If you look at a world map, you’ll see that most deserts are located above or below the equator. That’s because the equator receives the most direct sunlight, causing the air to warm and rise. As warm air rises, it creates a low-pressure area, an area where the atmospheric pressure is lower than the surrounding area, Creamer explained. Moisture in the air cools and condenses, forming clouds and rain. This is why areas along the equator, such as the Amazon, have lush forests.
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This updraft spreads outward and sinks between 20 and 40 degrees north and south of the equator, suppressing cloud formation. This explains why there are so many deserts in the subtropics, such as the Sahara and Kalahari.
Next, there is the horizontal movement of air over the Earth. Near the equator, trade winds blow from east to west. These winds tend to drop moisture on the eastern side of the continent and dry out the western side. For example, in the case of the Namib, when it rains, it doesn’t fall on the desert itself, but rather on the mountains to the east, said Abi Stone, a physical geographer at the University of Manchester in the UK.
Cold ocean currents also have an effect. The air blown along the cold current cools when it comes in contact with the cold current and picks up some moisture, making the air very stable. “We’re envisioning a package of air, in a sense, like a balloon, because it doesn’t mix completely, but the balloon envelope is very flexible and can stretch and contract,” Stone told Live Science. “Cold air won’t have much of an effect on its expansion.” Without convection, air parcels would become trapped and unable to rise. “But all it does is retain low levels of moisture, which can be blown overland, creating a very foggy environment in the desert west along the coast,” Stone said.
The presence of mountains also affects the dryness of these deserts. As moist air moves over the mountains, it cools and rain falls on the windward side, Creamer explained. By the time the air falls to the leeward side, much of its moisture is lost and a rain shadow is formed, an area by the mountains where the rain decreases. For example, Seattle, located on the west side of the Cascade Mountains, averages 39.3 inches (99.8 cm) per year, while Yakima, located on the east side of the Cascade Mountains, averages 8 inches (20.3 cm) per year.
In the case of Atacama, Creamer said Atacama would be unusually dry because “winds blowing from South America dump a lot of rain on the east side of the Amazon River, and then it hits the Andes. The Andes takes even more water from the wind, and then it falls along the South American coast to the west, where Chile is.”
These factors give coastal deserts unique characteristics not found in other deserts. These tend to have cooler, more stable climates than inland deserts, and are home to plants and animals that have evolved special traits to trap moisture. For example, in the Namib, some beetles draw water by pointing their butts into the misty air.
“People have been studying what that surface looks like to make more effective fog nets,” Stone said. “There are amazing creatures.”
Like much of Antarctica and the northernmost regions of the Arctic, the formation of polar deserts is driven by many of the same factors as temperate coastal deserts. Temperature also plays a role, as the air in these parts of the world is very cold and cannot hold moisture. “In the case of Antarctica, the strong winds and ocean currents that circle the continent have the effect of blocking weather systems from reaching the continent,” Stone said.
Equator Quiz: Can you name the 13 countries located on the center line of the Earth?
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