Why Are There Deserts in Africa?
Why are there deserts in Africa — in two different places, no less?
Someone asked this excellent question on Quora, and I thought I would offer my answer.
First of all, let’s start by looking at a map of the world’s deserts. I think this one will do:
So, here we see that Africa has not one but two distinct zones of desert, one in the north, the Sahara, and the other in the south and west of the continent, the Kalahari and Namib deserts.
In fact, Africa is the only continent with two distinct hot desert zones like this. To understand why, let’s take a look at the reason deserts are where they are.
The major deserts of the world lie about thirty degrees north and south of the equator. What accounts for this distinct pattern is the Hadley Cells, atmospheric patterns which describe the movement of heated air and moisture. In essence, the Hadley Cells are a good shortcut to figuring out why different parts of the world are warm/cool/cold and wet/intermediate or dry.
If we start at the equator, the fact that the sun’s rays strike this area at a perpendicular, 90-degree angle means that this region is going to be very warm, which is going to tend to make the air rise upwards. Much of this area is ocean — if you take a globe and trace the equator with your finger, you will find that most of the time you are tracing over water.
Because the equator is very warm and also has a lot of water, the heated air that rises upward is going to generally be very moist and humid. As air rises higher into the atmosphere, it cools, and as air cools, it loses the ability to hold onto moisture — and as a result, the equator is known for its rains. All that rising air makes the equator and the tropical region generally a zone of low atmospheric pressure, which means the air is generally rising up rather than pushing down.
So, warm area with a lot of water means rising moist air, which means lots of rain. How in the world do we get to deserts?
Well, remember all that warm, moist, rising air at the tropics? A lot of that air loses its moisture when it gets high enough to cool off, but that doesn’t mean the air descends with the moisture — after all, for every rising sub-section of air, there’s another sub-section coming in below it.
What happens as a result is that a lot of very dry air plumes out from the tropics. This air is cold because it is high, and it is dry because, as we have seen, it has discarded its moisture.
Now, what goes up generally has to go down, and lo and behold, at the thirty-degree mark in both the northern and southern hemispheres our cold, dry air descends. As the air goes down, it warms up, which makes it more capable of holding moisture — not that there is much moisture to be had.
As a result, we get our hot, dry deserts where precipitation is rare.
If you go north of these zones, known as the Horse Latitudes, the air begins to rise, pick up some moisture, and produce precipitation, albeit on a more seasonal basis and to a much lesser degree than in the tropics. This produces the Mediterranean zones, including California, a small zone in south-central Chile, the West Cape of South Africa, southern and southwestern Australia, and the big one, the Mediterranean itself.
Fun fact: the story about the Horse Latitudes is that during the age of exploration, European ships would become becalmed in the Horse Latitudes — a real liability in the Age of Sail, before ships were powered by steam. Since some of these ships carried horses, and since drinking water was scarce — well, you do the math.
There are two other kinds of deserts I should touch on briefly as well. If you look at the world’s deserts, you will notice that deep, very deep in the hinterlands of Asia, there are deserts to the north and west of historic China, the Gobi and Taklamakan.
These deserts exist, fundamentally, because they are so far inland that the moisture is gone by the time air masses reach them from the sea. Some of the desert regions in the hinterland of North America are also at least partially the result of this same effect.
Mountains can also create so-called rain-shadows: moisture-laden air from the coast must ascend before it can go over a mountain range, and as it does so it will tend to deposit most precipitation in the mountains. This is why North America is much drier east of the Sierra Nevada and Cascades ranges — compare the state of California with Nevada, or Oregon and Washington west of the Cascades with Oregon and Washington east of the Cascades.
There are also cold deserts in the Antarctic and Arctic, characterized by cold, dry air and little precipitation. Finally, I’ll note that this is a fairly colloquial term, and you may find deserts like the Gobi and Utah deserts, which, yes, can get quite cold, described as cold deserts even though they are obviously nowhere near the polar regions.
So, Africa’s size and location, along with the patterns of the Earth’s climate, are the reason Africa has two distinct hot desert zones.
