Deserts are areas of land that immediately evoke thoughts of vast sand dunes, unbearable heat, a baking sun and parched lands. They can be found on every continent and come in all shapes and sizes. Deserts are actually a subclassification of a more encompassing ecosystem called “drylands”. If you look at a globe, you may notice that deserts have a tendency to form in certain regions on the planet, usually around ± 30° latitude. Is there a particular reason for this? What makes these regions more susceptible to desert formation?

Why Are Deserts Common Around ± 30°?

In the Southern Hemisphere, there’s the Kalahari, the Atacama Desert, the Australian Outback (Great Victoria, Great Sandy), and the Namib. In the Northern Hemisphere, there’s the Sahara, the American Southwest, the Arabian Desert, the Lut Desert and the Thar Desert. These deserts all have something in common — they exist at or near 30° latitude. The reason for this similarity is due to a system of “air conveyor belts” which move vast amounts of air in the Earth’s upper atmosphere. The conveyor belt responsible for these deserts is known as the Hadley Cell.

The Earth receives massive amounts of sunlight every day, and the equator receives more sunlight than anywhere else on the planet. This vast amount of sunlight creates a lot of heat. Since hot air rises, the air at the equator will rise high up into the earth’s atmosphere after being heated by the sun. From there, it’s carried northward where it begins to cool. After sufficient cooling, it begins to fall back down towards the ground. This system of rising and falling air is the Hadley Cell.

How Does The Hadley Cell Create Deserts?



When the warm air rises at the equator, it’s filled with a lot of moisture and water vapor. As that vapor-rich air continues to rise, it’s free to expand in the lower pressure atmosphere. This expansion also slightly cools the warm air. This change causes the moisture within the air to condense which results in heavy rains. This process is the the reason why there tends to be a lot of rain near the equator, and is why the tropics are so wet.

The air hasn’t finished its journey, however. Now dry as a bone, the warm air continues its journey and follows the Hadley cell convection belt. Once that dry air has cooled enough, it descends. This descent happens at ± 30° latitude. If the region has no other source of moisture, either from local weather climates or interactions with other weather systems, the area will dry out and eventually become a desert. The state of Florida is one area that exists at 30° latitude and has beaten the odds. Thanks to evaporation off of the Gulf, the area has an abundance of moisture-rich air which keeps it from drying out and turning into a desert.

Side Effects Of The Hadley Cell

The descending air at 30° assists in the creation of an additional cell. As the air falls, it pulls with it air down from the north. This pulling creates another conveyor belt which creates a second cell called the Ferrel Cell. This second cell has its air falling at 30°, but where does its air come from? The Ferrel cell pulls air up from 60° latitude which is the reason the region is temperate and has lots of vegetation (e.g, Northern Europe, Forests in Russia, Canada).

How Does El Niño and La Niña Effect The Climate?

El Niño is a warming of a large section of the Pacific Ocean which usually takes place every 3-7 years. This warming throws a bit of a wrench into an otherwise stable system. The warming of the ocean produces a bit of a feedback loop causing the atmosphere circulation to slow down. This slowing causes winters to be wet in the American southwest, and causes droughts in Australia and Indonesia.

La Niña is basically the opposite of El Niño. It’s the cooling of the same section of ocean. La Niña is typically short lived compared to its counterpart, only lasting 6-9 months. The effects of the La Niña are less severe but do cause increased monsoons in Asia, droughts in South America and drier than normal weather in the American southwest. The lull between the two atmospheric anomalies is called “La Nada“. In Spanish, it means “the nothing”.

Fun Fact: Due to how little precipitation Antarctica receives, just 8 inches a year near the coast and even less inland, it’s actually considered a desert. In addition to being the coldest continent on the planet, it’s also the driest and windiest.

Citations:

Ecosystem Assessment (2005). Drylands Systems. Ecosystems and Human Wellbeing: Current State and Trends, Volume 1.

Heinrich; Siegmar-W. (2002). Walter’s Vegetation of the Earth: The Ecological Systems of the Geo-biosphere. Springer.

Briggs, Kenneth – Physical Geography: Process and System. Hodder & Stoughton. pp. 59–62.

Marshak. Essentials of Geology, 3rd ed.. W. W. Norton & Co. p. 452. (2009)

S.Ineson (2009). “The role of the stratosphere in the European climate response to El Niño“. Nature Geoscience.

