Majestic landscapes. Endless plains. Where a lack of rain combines with cold winters.
On the wrong side of mountains, or thousands of miles from the ocean.
One such place is the driest on our planet. The unlikely centre of the largest land empire
in history. And the only world climate to spawn a popular
genre of fiction. These are the cool deserts of Planet Earth.
As we continue our journey in this series from e quator to poles, we have reached as
far north as we can go for the time being with the Oceanic climate. So it’s now time
head into the interior of the continents at the mid-latitudes. And the first of the climates
we will look at here are the cool deserts. Like the hot deserts that we covered in Episode 4
the cool deserts are defined by a lack of rain throughout the year. But the mechanisms
that cause such drought are very different. Found at higher altitudes, deep with the northern
continents hundreds or even thousands of miles from the moisture of the ocean, or on the
leeward side of great mountain ranges, these causes are very different to the mass downward
movement of tropical air that causes the hot deserts such as the Sahara, Kalahari or Mojave.
The first of these mechanisms is called “Continentality” – literally being in the middle of a large
continent, so far from the ocean, that any wind that has penetrated thus far has been
sapped of any moisture that it started with. The great deserts of Central Asia are a good
example of this – these areas are factually the furthest from any ocean on the planet.
Another cause of cool deserts is what is known as “Rain Shadow”. Winds originating from
the ocean almost always carry moisture, but if these winds hit mountains, the air is pushed
upward and cooled, causing it to lose its moisture upon the windward slopes of these
mountains, such that when the winds have passed over, there is nothing but dry air left, and
so any land found on this leeward side receives little or no rain. Rain shadow is perfectly
illustrated by taking a look at a satellite image of lower South America. The wet and
dry regions can clearly be seen by the presence or absence of green. In the southern part
of the continent, the prevailing winds are westerly, as we showed in the last episode
on Oceanic climates. So southern Chile, on the windward side of the Andes is wet, while
Patagonia on the leeward side is dry. Travel further north, however, and the prevailing
winds are the easterly trade winds of the tropics that we covered in Episode 2. These
winds are sapped of moisture as they travel over the high Altiplano of Bolivia, such that
they are bone dry by the time they reach the Atacama desert, the driest place on the planet
outside of certain valleys in Antarctica. A final factor is where one would normally
find a hot desert, but high altitude typically in the form of high plains or plateaux, results
in cooler temperatures and frosty winters. This is the cause of much of the arid conditions
in the Western United States. The cool deserts differ in terms of recorded
climate in one key way, and that is winter temperatures. Cool deserts can get as warm
in summer as the hot deserts, but in winter, temperatures can plunge to minus 30 degrees
or lower, whereas hot deserts rarely touch freezing point.
In the Koppen climate classification system, cool deserts are subdivided into two climates,
depending upon how much rain falls. The truly dry cool deserts have the classification BWk
– this is the classification of the Atacama and Karakum deserts, where very little vegetation
grows, and where no major cities are located due to the harshness of the environment. Where
more rain falls, the classification BSk is ascribed, which is referred to as cool semi-arid,
or simply “steppe”, from the Russian word for short grass, since such grasses predominate
in this climate in a wide band across central Asia.
So, where in the world are the cool deserts? Starting in North America, we find that most
of the inland western half of the United States is dominated by arid or semi-arid conditions.
In the south, the arid conditions of Nevada, northern Arizona and New Mexico are extensions
of the hot deserts of the Mojave and Chihuahua, where higher altitudes lead to freezing winters
and a cool classification. Further north into the eastern parts of Oregon and Washington,
and most of the states of Utah, Colorado, Wyoming and Montana, we have continentality
and rain shadow both contributing to this huge area of dramatic landscapes and big skies
that gave birth to the books and movies of the Western genre. On the eastern slopes of
the Rockies, the plains are lower, but within the rain shadow of the Rocky Mountains. But
with occasional moist rains bubbling up from the south, dry prairies rich in wheat farmlands
are possible and dominate the landscape. In South America, we find the famous Atacama,
driest of all deserts, a product of the rain shadow of easterlies over the Andes, while
further south, the desert switches sides with the winds moving to the west, and the result
is the dry semi-desert of Patagonia. Southern Africa and Australia also have bands
of cool arid and semi-arid conditions that border on the hot deserts to their immediate
north. Much of Eastern and Central Spain is classified
as cool semi-arid, due to a combination of moderately high altitude, and weak penetration
of winterly westerlies into the interior of this wide peninsula at the Mediterranean latitude.
Last we come to Asia, where the sheer enormity of this continent produces a band of arid
landscapes so far from the ocean, comparing in size to the Sahara. On the western edge,
we have the country of Iran which is dominated by cool desert, largely due to its higher
altitude compared with the rest of the hot and dry Middle East. As we move north and
east we continue the high altitude and dry mountains into Afghanistan, and north again
we are into the flat Karakum desert of the Central Asian republics of Turkmenistan and
Uzbekistan, also home to the Aral Sea, once one of the biggest freshwater lakes in the
world, but now reduced to a salty remnant due to over-irrigation. Moving further north
and we are into the large semi-arid steppe of Kazakhstan, which then blends into the
richer grasslands of central Russia. We’ll take up these in the next episode. Hopping
over the Altai mountains, and we arrive in Western China, and the great sandy Taklamakan
desert, a product of both continentality and rain shadow, with mountains on all sides leading
to extremely arid dune covered plains. Our journey ends in perhaps the most famous cool
desert of them all, the Gobi of Mongolia, birthplace of the largest land empire in history,
the Mongols of Genghis Khan. Here the winters are bitterly cold, with the land so far north
and so far from the moderating ocean, with Mongolia’s capital, Ulaanbaatar, being the
coldest in the world. Starved of rain, the cool deserts, like their
hotter counterparts, have little in the way of vegetation, with the most arid parts such
as the Atacama seeing virtually lifeless conditions. In the semi-arid parts, stunted shrubs and
grasses are the most commonplace. With the exception of the western prairies of the United
States, where large scale wheat farming is possible, agriculture is difficult, and is
largely confined to grazing of sheep and cattle. These parched and inhospitable lands are also
home to few major cities on earth, but I’ll present the few that there are, as they are well known.
And that brings us to a close on the cool
deserts. I hoped you enjoyed the tour, and if you did,
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section, particularly if you’re one of the rare people who have experienced life in these
challenging areas around the world. Thanks again for watching, and I’ll see
you in the next episode, where we’ll be looking at the great grasslands of the Northern
Continents. Until then, stay warm!