The creation of deserts
Everyone knows that water is life. There is a lot of water on Earth, but it is not everywhere. Today, we will tell you about the driest places on the planet—deserts. Find out how they appear, where they are located, and how dry they really are.
What is a desert?
Deserts can be found anywhere where there is less than 250 mm of precipitation per year. This is about three times less than London. Water in deserts is a limited resource, which makes animal and plant life scarce and makes life hard for people in deserts. And deserts do not have to be hot. If there is no rain and no fresh water, there will be no life at any temperature.
There are a lot of arid places on Earth, and they are scattered around different corners of the planet. There are rocky deserts and sandy deserts. There are even "snow deserts"—full of water, but all of it is snow and ice.
The Antarctic desert. Photo: NOAA / Unsplash
But do the deserts have anything in common? Let’s look where they are, and the reasons why they occur.
Map of the world’s deserts. Windy.app
Where there’s no monsoon
The greatest number of deserts are, of course, in the tropics. In the entire tropical zone (about 10-25° latitude), downward air movements prevail—the conditions are similar to the anticyclones of temperate latitudes: slight cloudiness may appear, but find «tropical deserts» in the Sahara and the Arabian Peninsula, and «tropical rainforests» in Vietnam and China? It’s all about the monsoon!
Monsoons are seasonal rains that fall in tropical regions every summer, enough to get plants and animals through the dry winter. But monsoons are not found throughout the tropics, so there are both lifeless deserts and verdant forests at the same latitude.
Satellite image of the tropical zone of Africa and Eurasia. The red rectangle shows the zone of the African monsoon, yellow — the East Asian monsoon zone
You can see in the map that the entire tropical zone outside the rectangles is a desert. For example, the African monsoon comes to southwest Africa every summer and rains on Côte d’Ivoire, Nigeria and other countries in the region. Moving from south to north, it gradually loses moisture. The Sahara begins exactly at the latitude the monsoon never reaches.
Western coasts of the continents
In subtropical latitudes (about 25-40° latitude), huge anticyclones swirl over the oceans. And in tropical latitudes, they change into trade winds that blow toward the equator. Both subtropical anticyclones and trade winds exist all year round.
Trade winds and subtropical anticyclones in the Atlantic Ocean. Windy.app
In both hemispheres, the wind blows from the pole to the equator, on the west coast of a continent (in this case, Africa), and it is stable throughout the year. This wind brings cold water from the poles, forming cold ocean currents, which cool the air near the water’s surface. The combination of anticyclones and stable stratification (cold air under warm air) throughout the year prevents rain clouds from forming.
On the western coasts of continents, some of the driest and lifeless deserts of the planet are located, including Atacama in South America, and Namib in southern Africa. Precipitation in these deserts is less than 50 mm (12 times less than in Paris). Often, there is no precipitation for a whole year, or even longer. The Atacama has not seen a single rainfall recorded from the sixteenth to the twentieth century!
In some parts of the Atacama, there is nothing but rocks and sky. Photo: Thiago Sanchez / Unsplash
Rain shadow
Terrain plays an important role in the absence of rain. When air rises along the side of a mountain, it cools, moisture condenses, and precipitation falls. And when the same air comes down the other side, there is no more moisture. This is why valleys surrounded by mountains are often dry. The dry «plume» behind a mountain is called a rain shadow.
An example of a rain shadow is Death Valley in the USA. It is one of the driest places in the world. And the main reason are the Pacific Coast Ranges and the Sierra Nevada Range, which shield it from the ocean.
A rain shadow has created one of the largest sand deserts on the planet—the Takla-Makan in China. It is encircled by mountains starting in the west (therefore blocking the west winds) and from the south (blocking the East Asian monsoon).
The driest place in the world
But all these deserts are gardens of paradise compared to the driest place on Earth: the McMurdo Dry Valleys in Antarctica.
On the ice plateaus around the McMurdo Dry Valleys, very cold, dry air accumulates. And as it rolls down the valley slopes, it becomes even drier. There hasn’t been any rain in the Dry Valleys for two million years! There is not even any trace of snow on them, and the wind speed sometimes reaches 100 m/s. In the driest part of these valleys, scientists have not been able to detect any life at all, not even bacteria.
By the way, the rest of Antarctica isn’t much better! For example, the south pole gets only 18 mm of rainfall per year, and there is almost no life there either—despite the fact that you are standing on several kilometers of water. After discoveries like these, it seems incredible that most of our planet is so comfortable for life!
Text: Eugenio Monti, a meteorologist and a climatologist
Cover photo: Dominik Mecko / Unsplash
Read more:
What are local winds and how do they work
Why do cyclones rotate counterclockwise
