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 noon on at least one day of the year. To the north or south of them, the sun is never directly overhead, and not as concentrated as the beam that reaches the tropical areas.

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 Africa , and the western Australian desert, are the result of cold oceanic currents that divert rain-laden air away from coastlines.

Others, like the Great Basin of western United States , and the deserts of central and eastern Australia are caused by the “rainshadow effect”, through which coastal mountains milk rain from the air before it passes inland.

Still others, like the Gobi and Takla Makan deserts of Mongolia and China , are simply so far away from the ocean that the winds lose any moisture they may hold long before reaching the far off continental interior, even what little moisture remains in the Indian-born clouds after they have scraped over the jagged Himalayas .

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 Gobi and Takla-Makan Deserts .

i. Galveston , TX , on the Gulf Coast , gets 42” of annual precipitation.

j. 340miles inland, Sonora , TX , receives 22” annual precipitation.

k. Pecos , TX , 500 miles inland, receives still less, 11” annually.

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 ( Baja , CA ), and in rain shadows (adiabatic heating and cooling).

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 Patagonia where the Western Ghats and the Andes intercede. Rainshadows include adiabatic heating and cooling.

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 Sahara , sand and rock can heat up to 170o F).

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 midday temperature may be 55 F (30 C) lower than the temperature at ground level.

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 Africa , and the western Australian desert, are the result of cold oceanic currents that divert rain-laden air away from coastlines.

Others, like the Mojave and Sonoran Deserts of the American southwest, and Mexico are caused by the “rainshadow effect”, through which coastal mountains milk rain from the air before it passes inland.

Still others, like the Gobi and Takla Makan deserts of Mongolia and China , are simply so far away from the ocean that the winds lose any moisture they may hold long before reaching the far off continental interior, even what little moisture remains in the Indian-born clouds after they have scraped over the jagged Himalayas .

Sahara Desert

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 ” Great Desert ”.

2. It occupies more than 3.5 million square miles (9.1 sq. kilometers) in North Africa , not far short of the area of the United States (3.7 million sq miles/9.5 million sq. kilometers).

3. The Sahara is bounded:

a. in the west by the Atlantic Ocean

b. in the north by the Atlas Mtns. and the Mediterranean Sea

c. in the east by the Red Sea

d. in the south partly by the Niger River , and eventually the desert gives way to the semiarid lands known as the Sahel .

4. Much of the Sahara is a plateau with an elevation varying between 1300-1600’ (395-490m) above sea level. Altho parts of the desert lie below sea level, and the tallest mountains in the Sahara , Mount Touside of the Tibesti Mountains , an extinct volcano, rises to 10,712’ (3,265m).

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. Sahara is a hot desert, with summer daytime temperatures exceeding 90F for 8 months of the year, and often exceeding 100F.

a. The high temperature record, recorded at Al-Aziziya in northern Libya , reached 1236.4F (58C).

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 Sahara : erg, reg, and hammada.

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 Sahara , extending across the Libyan Desert and into Egypt .

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 United States .

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 Libya , and there resembles a vast and uneven parking lot. Probably the most inhospitable type of desert surface. Very little can live on it.

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.

Arabian Desert – includes the whole peninsula of Arabia ( Jordan , Iraq , Israel , Syria , and part of Iran );

a. An empty quarter in the southern part of the desert is too hot and dry even for desert nomads (similar to the Sahara ).

i. Includes the largest sand desert in the world (south of Najd ), called the Rub’ al-Khali (the empty quarter), covering 230,000 square miles (595,700 sq kilometers).

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 Central Asia are also caused by persistent high pressure, but they are well-clear of the Tropics and much cooler.

Gobi Desert : China , Mongolia , Mountain and basin desert, Northwest of Takla-Makan, Sand dunes and gravels (gobi), shrubs and semi-shrubs.

1. There are no towns in the Gobi Desert , but the capital of Mongolia , Ulan Bator , lies not far from its northern margin.

2. Precipitation falls mainly in the summer, the center of the desert receiving 1-2” per year. Ulan Bator ahs an annual precip of 8”/year.

3. Vegetation is sparse, consisting of grass, shrubs, and thornbushes.

Takla Makan –

1. Rainshadow desert and continentality from Tien Shan and Kunlun Shan Mountains in northwestern China , and the Tibetan plateau (12,000’).

2. West of the Gobi .

3. Much more barren than the Gobi .

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.

Namib Desert – Coastal cooling

1. Coastal strip desert along the west coast of Africa , from southern Angola to South Africa .

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 Antarctic Ocean as a stream of water blown in an easterly direction by the West Wind Drift and turning north just south of the southern African continent.

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.

Kalahari Desert – covers an area of about 275,000 sq. miles (712,250 sq. kilometers).

1. Inland desert from the Namib; southwestern Botswana .

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.

Atacama Desert – 140,000 sq miles (363,000 sq kilometers) in northern Chile . The Tropic of Capricorn passes thru its center.

1. Narrow strip, 600 miles along (parallel to) coast of Chile .

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 Andes Mountains . The combination of coastal cooling from the west, and rainshadow from the southwest accounts for the extreme aridity of the Atacama.

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. South America has its equivalent of Africa ’s Benguela Current, called the Peru Current.

i. Both the Benguela and Peru Currents originate in the Antarctic Ocean , and bring cold water thru a series of upwellings that carry cold water to the surface.

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 Chile and Bolivia fought over part of the Atacama Desert . The War of the pacific lasted from 1879-1884, and ended with Chile in control of the area, and made Bolivia into a landlocked nation.

i. The desert has rich deposits of sodium nitrate and copper.

ii. Sodium nitrate is the raw material for commercial fertilizer and explosive manufacture.

Patagonia – occupying the whole of Argentina ; 300,000 sq miles (770,000 sq kilomteres)

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.

Australian Desert

1. The Tropic of Capricorn passes thru the center of Australia . This means the country lies close enough to trade wind latitudes for the prevailing winds to be from the southeast. These bring maritime conditions, with abundant rain, to the coast of New South Wales and Queensland .

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 Australia ’s land mass is classified as desert (3.4 million square miles). Of all the continents, Australia is the driest. ( Australia is alarge island with a continental climate).

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. Australia has 5 deserts: 4 on the west side, one in the center.

a. Simpson Desert – southeast corner of the Northern Territories ; < 500’ asl; stony desert

b. Great Sandy – north part of western Australia ; < 600’ above sea level.

c. Great Victoria – western Australia

d. Gibson Desert –

e. Nullarbor Plain – south of the Great Victoria Desert .

North America

1. Great Basin Desert - rainshadow desert

2. Mojave Desert - rainshadow desert

3. Sonoran Desert - rainshadow desert