Causes of Aridity, and Geography of the
World’s Deserts
Study a physical map of the
world. As you follow the Tropics of Cancer and Capricorn, thirty degrees on
either side of the equator, you will see, distributed with suspicious
regularity, a brown band of drylands circling the
planet, a sere belt warding off greener climes: the deserts of the world. They
lie in the so-called Horse Latitudes, where constant high-pressure systems
drive away the rain clouds, and swirl above the earth to the music of global temperature
variations and the Coriolis Effect produced by the
earth’s rotation in space.
Most deserts are hot. Within
the latitudes of the Tropics of Cancer and Capricorn, the sun appears directly
overhead at
Deserts are not scattered
haphazardly over the earth; the pattern of their occurrence is shaped by the
factors that produced them. The major world deserts occur in these two
discontinuous belts ringing the globe. Because more land surface occurs in the
Northern Hemisphere in the latitudes described, more deserts occur in the
Northern than the Southern Hemisphere.
Not all deserts are hot. Some
of these drylands, like the Atacama of Chile, the Namib and Kalahari of southern
Others, like the
Still others, like the
Causes of Aridity
1. Atmospheric
high pressure zones (Hadley Cells)
Earth’s atmosphere moves in general, somewhat
predictable patterns that are largely driven by the sun’s rays and the earth’s
rotation.
At the Equator, the sun’s rays are perpendicular with
the Earth’s surface; solar heating is intense.
Air is heated at the equator, ascends, and is replaced
by inrushing air.
As the heated air moves upward, it is gradually
cooled.
Cool air, with a lower saturation point, is capable of
holding less moisture within than warm air. (Equatorial air is very moist).
(Oceans
cover most of the equator and the high equatorial surface temperature allows
large amounts of water to evaporate. So it is not simply warm air that rises at
the equator; it is warm, moist air.
As the air cools, it releases the excess moisture it
contains, helping to produce the moist tropics.
i.
As the air rises,
it cools; the water condenses and precipitation is common. Water returns to the
surface as rain.
ii.
(Deserts do not occur near the Equator, tropics occur there).
Higher in the atmosphere, the now
cold, dry air rises and moves away from the equator.
At about 30 degree latitudes in both hemispheres
(north and south), the air descends.
As it does, it warms.
As it warms, the air expands, condensation and
precipitation are infrequent. (Horse latitudes).
To the north and south of these desert latitudes, the
air once again ascends, producing moisture for the land; finally, over the
poles, the air descends again
Desert formation in these
particular latitudes is primarily due to complex global air-circulation
patterns caused by the rotation of the earth on its axis (earth moves at great
speed near the equator and slowly near the poles), the seasonal tilting of the
earth in relation to the sun, and other factors.
2. Continentality or (Distance from oceans) –
a.
Most water in
atmosphere is evaporated from the sea, and this water eventually precipitates
on land.
b.
Land closer to
the sea generally receives much of this moisture.
c.
As air moves
inland, it gets depleted of moisture and precipitation drops.
d.
Areas lying deep
within a continent may become desert simply because air currents reaching them
have already traversed vast land distances; by the time they arrive over the
deserts, these currents have already lost the moisture they once carried.
e.
This is true of
some of the Asian deserts., the
i.
j.
340miles inland,
k.
Coastal Cooling:
Deserts may result if air is cooled, and then rewarmed,
prior to reaching the region.
a.
Cool air holds
less moisture than warm air.
b.
When warm, moist
air is cooled, excess water condenses and falls as precipitation. If it is
subsequently re-warmed, it will be drier than it was previously.
1.
Air at 30 C (86
F) can hold 30.4 grams of water per cubic meter (m3).
2.
If saturated air
(100% relative humidity) was cooled from 30 C to 10 C (50 F), 21 grams of water
would condense and precipitate because this cold can only hold 9.4 grams of
water per cubic meter.
3.
If the air were
then re-warmed to 30 C, it would have just a fraction (31%) of the moisture it
did originally.
4.
(31% relative
humidity is fairly dry, and further precipitation is unlikely).
5.
Winds that blow
onshore tend to do so across cold currents produced by movement of water from
high latitudes (poles) to low latitudes (equator), and associated with the upwelling
of cold waters from the ocean's depth.
6.
Cold or cool
winds have relatively small moisture-bearing capacity and, when warmed during
their passage over the land, they become stable and, thereby, reinforce the
stability produced by the global stability of these latitudes. (Subtropical
highs).
c.
This occurs along
coastal areas where there are cold coastal seas (
d.
Air moving across
the frigid currents is cooled to a low temperature; thus the air holds little
moisture when it arrives over land, where it may provide fog or mist, but
rarely rain. (Namib and Atacama).
4. Rainshadow effects
a. Moisture-laden air encounters a mountain mass and is
moved upward.
b. The ascending air is cooled and releases moisture on the
windward side of the range.
c. Once over the summit, the air descends the lee side of
the range, warming as it does so, and hence increasing its evaporative power.
d. The windward side of a range may support a heavy
well-watered forest, while the leeward side and the area far below it, robbed
of moisture, is occupied by a desert or steppe plant
community.
The rain-shadow effect
produced by great mountains can create arid areas in the lee of the mountains
even when continentality is not particularly marked,
such as in
All these climatic, desert-producing factors –
descending, drying air currents; mountain-produced rainshadows;
distance from oceanic moisture sources; and cold ocean currents – are
instrumental, sometimes singly, more often in combination, as primary forces
producing arid lands.
Why are deserts hotter than tropics? (Specific heat capacity).
1.
Sand and rock heat
up much more rapidly than water, and they also lose heat more quickly.
a.
On a hot summer day in the
2.
When a substance
is exposed to heat, its surface temperature rises, but different substances
heat up at different rates.
3.
The amount of
heat that must be applied to a substance to make its temperature rise by 1o
is called the specific heat capacity (shc) of that
substance.
a.
the specific heat capacity of water at 59 F is 4.19.
b.
the specific heat capacity of sand and most solid rocks
between 68-212 F, is about 0.8.
c.
This mean it
requires more than 5 times more heat energy to raise the temperature of water 1o
than to raise the temperature of rock or sand by the same amount.
4.
As the sun climbs
higher in the sky, the dry ground heats much faster than water, and much faster
than the ground would if it were wet. (Its s.h.c.
would then be 1.48).
5.
Sand and rock
heat more quickly than water, and for the same reason, it cools more quickly,
also.
6.
The ground
temperature falls sharply, reaching a minimum shortly before dawn, but the
temperature of water falls much more slowly.
7.
Over the ocean in
subtropical latitudes, the difference in day and night air temperatures
(diurnal temperature range) is about 0.4 F (0.2C). Over the desert at the same
latitude the diurnal temperature range is about 72 F (40C).
Sand is a poor conductor of
heat (many air spaces), as is air.
Radiant heat raises the surface temperature, but heat is not conducted
very far below the surface. The upper, heated layer gets continually hotter,
while just below the surface the temperature changes very little.
Some heat is conducted below
the surface, but at a very slow rate. Heat reaches a certain depth before the
daily peak temperature is passed. Beyond this depth, the temperature does not
alter. This is called the damping depth.
In dry sand it is about 3” (7.6 cm) below the surface.
Because heat penetrates so slowly, the peak temperature is reached at the
damping depth several hours later than it is reached at the surface.
Microclimates
1.
Conditions
(climates) below ground are very different from those at the surface.
2.
A local climate
of this type is called a microclimate, within the macroclimate of the region as
a whole.
a.
Many living organisms exploit the advantages afforded by microclimates.
3.
The subsurface
climate is not the only desert microclimate.
a.
Hollows that are
shaded most of the time are cooler than exposed, sun-lit surfaces
b.
Shelter from (or
exposure to) prevailing winds can also create microclimates.
c.
Above ground air
is much cooler than surfaces of sand or rack. At a height of 6.5’ (2m) the
4.
Fertile islands
Desert Geography
Study a physical map of the
world. As you follow the Tropics of Cancer and Capricorn, thirty degrees on
either side of the equator, you will see, distributed with suspicious
regularity, a brown band of drylands circling the
planet, a sere belt warding off greener climes: the deserts of the world. They
lie in the so-called Horse Latitudes, where constant high-pressure systems
separate the westerlies and trade winds, driving away
the rain clouds, swirling above the earth to the music of global temperature
variations and the Coriolis effect
produced by the earth’s rotation in space.
Some of these drylands, like the Atacama of Chile, the Namib and Kalahari of southern
Others, like the Mojave and
Still others, like the
1.
The Arabic sahra, means “wilderness”, and Arabic speakers apply it
to the barren emptiness of the world’s largest desert, also known as the ”
2.
It occupies more
than 3.5 million square miles (9.1 sq. kilometers) in
3.
The
a.
in the west by
the
b.
in the north by
the
c.
in the east by
the
d.
in the south partly by the
4.
Much of the
5.
At various times
what is now desert lay beneath ancient seas. This led to a deposition of sand
and calcareous deposits that now form sandstone and limestone.
6.
a.
The high
temperature record, recorded at Al-Aziziya in
northern
b.
At night the
temperatures drop sharply, by as much as 50F: nights can be cold and
temperatures often fall below freezing in winter.
c.
Precipitation –
rainfall is sparse, <2” per year.
7.
There are 3 types of lands surfaces in the
a.
erg – “sand seas”, composed of dunes that have formed
basins and depressions from the sand carried into them by rivers of earlier
times.
i.
The large area of dunes are in the northern
ii.
Surface
temperatures can reach 180 F (85C) Air heated by contact with the surface
expands and rises. This produces an area of low pressure near ground level.
Denser air rushes in to compensate and its movement causes strong, gusty winds.
These are called thermal winds because they are caused by temperature
differences.
b.
Reg – name given to ground covered by boulders and
gravel. The reg is fairly level, and the areas are
generally bleak, windswept, and monotonous.
i.
Reg is formed by wind action. Small particles are blown
from around and between the larger, heavier stones, in a process called
“deflation”.
ii.
The wind keeps
the surface clean, and the finest particles are carried very long distances:
Saharan dust has been identified in the
c.
Hammada – is the rocky desert.
i.
No sand or small
particles on the surface – just rock, either as large boulders or bedrock.
ii.
Hammadas are found in northern
8. Vegetative
cover very sparse.
a.
Susceptibility to
wind erosion is very high,
b.
Water
infiltration rates are good, depressions fill with saline water.
c.
Herbs, small shrubs,with larger shrubs and
trees where moisture is abundant (oases).
d.
Succulents not common.
a.
An empty quarter
in the southern part of the desert is too hot and dry even for desert nomads
(similar to the
i.
Includes the
largest sand desert in the world (south of
ii.
Despite its name,
it includes many watering holes and, is therefore, “crossable” to those
familiar with the desert.
b.
Arid or extremely
arid, < 4” per year.
a.
Hot desert,
averaging between 100-107 F from May-Sept.
b.
Most of the plants are annuals; most of the
perennials are halophytes.
The Saharan and Arabian deserts lie mainly within the
Tropics. They are hot deserts produced by descending air on the poleward side of Hadley cells, producing a belt of fairly
permanent high pressure. Farther north, the deserts of
1.
There are no
towns in the
2.
Precipitation
falls mainly in the summer, the center of the desert receiving 1-2” per year.
3.
Vegetation is
sparse, consisting of grass, shrubs, and thornbushes.
Takla Makan –
1. Rainshadow desert and continentality from
2. West of the
3. Much more barren than the
4. Climate is extremely dry
because air must cross high mountains to reach the desert and in so doing loses
much of the moisture it carries.
5. There is some vegetation
on the borders of the desert, where there are permanent rivers, but no
vegetation in the interior.
1.
Coastal strip
desert along the west coast of
2.
Very dry (much
drier than its inland neighboring desert, the Kalahari), but not too hot
because of coastal fog.
3.
High humidity and
fog (like Atacama), with less than 2” (51 mm) annual precipitation.
4.
Very little vegetation:
Lichens on the leeward side of rocks, leafless, stem-succulent plants, and
halophytes.
5.
The Benguela Current, is a cold
current that flows northward along the coast. It flows from the edge of the
6.
Despite having
crossed the ocean, winds and air bring no rain because it was chilled as it
crossed the cold water of the Benguela Current and
its water condensed. It condenses as low clouds and fog, rather than rain and
fog.
1.
Inland desert
from the Namib; southwestern
2.
Notable for its
deep subsurface sands.
3.
Landscape
dominated by gentle dunes.
4.
Precipitation in
the northern Kalahari reaches 25” annually; precipitation in the southern
Kalahari, 10” annually.
5.
The rate of
evaporation is high enough to remove much of the surface water before it can be
absorbed by the ground.
6.
Temperatures
range from 90F (32C) in summer to 70 F (21C) in winter, but frosts are common
in winter nights.
1.
Narrow strip, 600
miles along (parallel to) coast of
2.
World's driest
coastal desert; 0.04" (10 mm) precipitation; most of it from coastal
condensation (fog). Rain can be expected no more than 2-4 times per century.
Rain is rare, but the relative humidity is 75% - bare iron rust rapidly there.
3.
Coastal cooling:
Air reaching the coast from the ocean loses most of its moisture at sea, where
it is chilled. It is dry when arrives on land. Any remaining moisture is lost
as the air rises to cross the coastal ranges, forming clouds that rarely bring
rain, although may be low enough to be called
fog.
4.
Air from the
southwest crosses South American continent, then loses the moisture it carries
during its ascent over the
5.
Average elevation
is about 2,000’; its height contributing to its moderate temperature.
6.
A cool (little
seasonal variation – about 65 F), very dry climate in a coastal region suggests
the presence of a cool ocean current.
i.
Both the Benguela and Peru Currents originate in the
7.
Almost no
vegetation save for lichen.
8.
Very little
vegetation, except along streams.
9.
On slopes
moistened by mist or drizzle during the winter a sparse stand of Tillandsia may exist with a few lichens in association.
10. Despite its bleakness, in the last century
i.
The desert has
rich deposits of sodium nitrate and copper.
ii.
Sodium nitrate is
the raw material for commercial fertilizer and explosive manufacture.
1.
Cold, rainshadow desert, average annual temperature is 7C.
2.
Average rainfall
< 10” annually.
3.
There is no other
desert in the world lying on the eastern side of the continent in a latitude
north of 40d S.
4.
Highest
precipitation in the winter, April - Aug.
5.
Bleak desert, but
in the north there are tough grasses and shrubs; the grasses providing pasture
for sheep.
6.
Little vegetation
farther south, where climate is colder and drier.
1. The Tropic of Capricorn
passes thru the center of
2. Inland lies the Great
Dividing Range, a mountain range running the length of the country parallel to
the east coast.
3. Lands to the west of the
mountains lie in a rain shadow.
4. 40% of
5. No succulents, very few
spiny plants. Prevalent desert plants are the perenial
evergreen tussock grasses (spinifex) and small trees
or shrubs belonging to the genus Acacia.
6.
a.
b. Great
c. Great
d.
e.
1.
2.
3.
4.
Chihuahuan Desert- No.