Why Can Deserts Form on Coastal Edges?

When imagining deserts, many people immediately think of vast expanses of sand far from water sources. However, in reality, some of the driest deserts on Earth are located right along coastal edges.

Examples include the Atacama Desert in Chile and the Namib Desert in southern Africa. Both of these deserts stretch along ocean coastlines. This naturally raises the question: if located so close to the sea full of water, why do these regions remain so arid?

According to David Kreamer, a hydrologist from the University of Nevada, Las Vegas, the formation of deserts at coastal edges is influenced by three main factors: vertical air movement, horizontal air movement, and the effect of mountain ranges on air moisture.

In equatorial regions, sunlight strikes the Earth’s surface directly, causing air to heat up and rise. When warm air rises, a low-pressure system forms.

Kreamer explains that under these conditions, moisture in the air cools and condenses into clouds and rain. This is why regions near the equator are filled with dense tropical forests like the Amazon rainforest.

However, after rising, this air disperses towards the north and south. When it reaches subtropical regions (approximately 20–40 degrees from the equator), the air descends back to the Earth’s surface.

This descending air suppresses cloud and rain formation. As a result, these regions become extremely arid. This is why many large deserts are located in the subtropical belt, such as the Sahara and Kalahari deserts.

Around the equator, trade winds generally blow from east to west. These winds carry water vapour from the oceans and typically bring rain to the eastern sides of continents.

Consequently, when air reaches the western side of a continent, most of its moisture has already been lost.

In the case of the Namib Desert, rain actually falls more frequently in the mountainous regions to the east of the desert. Abi Stone, a physical geographer from the University of Manchester, explains that this rainwater does not fall in the desert area itself.

This means that despite being close to the sea, the western coastal region remains extremely arid because the air reaching it has already lost most of its water vapour.

When air passes over cold ocean currents, it cools as well. This condition makes the air stable and difficult to rise into the atmosphere.