Deserts 265

15 Deserts

TopicsA. What are the principles that apply to the atmosphere as concerns the transfer of moisture? B. What are Hadley and Ferrel cells? How do they explain the occurrence of subtropical deserts?C. What are the explanations for deserts ascribed to rain shadows and cold-water coasts? What are two examples of how animals use principles of atmosphere dynamics? D. What is the practical definition of aridity? What is the definition of ‘evaporation index,’ and what E.I. value characterizes the Great American Desert? What are some of the erosional and depositional landforms that characterize deserts? What are four kinds of sand dunes, and how does each kind record the direction of wind that shaped it?E. What geologic history is recorded by features evident on the Lakeside, Nebraska quadrangle?

A. The what and where of deserts—but first, a few principles

Figure 15.1 This simple model illustrates principles #1 through #4.

The locations of deserts are largely controlled by atmospheric circulation. Why? Because the agent that moves moisture from its principal source—the world ocean—onto the land, is wind.

The capacity of air to obtain moisture from the ocean through evaporation, along with the effectiveness of air to deposit that moisture on land through condensation, involves the following four principles, which are illustrated in Figure 15.1.

#1: Air obtains moisture from the ocean through evaporation of sea water.

#2: Land absorbs solar energy more readily than does sea water, so air over a landmass is warmed to a greater degree than that over an adjacent ocean.

#3: Warm air is less dense than cold air, so warm air tends to rise, and cold tends to sink.

#4: Warm air has a greater moisture-holding capacity than does cold air, so as warm air is cooled it tends to lose moisture (as rain or snow).

Q15.1 Where moisture-laden air moves from sea to land, which is

almost everywhere the case, (a) it is heated by the warmer land, (b) made less dense, and (c) caused to rise. At higher elevations moisture within the air condenses, producing clouds and rain. Review: In one sentence, why the condensation?

#1Air over an ocean acquires

moisture throughevaporation; moves inland,

replacing rising land air

#2Warm land adds heat to air

#4Rising air is cooled (because of

decreasing atmospheric pressure)Moisture condenses, forming

clouds and rain

#3bCold air is more

dense, so it sinks

#3aWarm air is less

dense, so it rises

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B. Global Subtropical deserts

From the principles discussed and illus-trated on the previous page, it follows that rising air promotes rainfall, and descending air promotes dryness. That simple principle pretty much explains the geography of deserts.

The most extensive deserts on Earth are associated with two circumglobal belts of descending air between 20° and 30° north and south latitudes. Notable examples of these subtropical deserts include the Sahara and Kalahari of Af-rica, the Rub-al-Khali of Saudi Arabia, the Sind of Iran in the Northern Hemi-sphere, and the Great Australian Desert in the Southern Hemisphere. But why deserts within these particular latitude belts?

As we have already seen, the heating of air causes it to expand, thereby making it less dense, which causes it to rise. This phenomenon, known as convection, lifts hot-air balloons.

As everyone knows, heat from the sun is greater in equatorial regions than in polar regions. But why? Two reasons, both of which are illustrated in Figure 15.2: First, solar energy per unit area of Earth’s surface is greater at low lati-tudes. Second, the greater angle of in-cidence of solar energy at low latitudes results in less loss via reflection of that energy.

The Hadley cell—From the preceding paragraph, it follows that warmer, less dense air in equatorial regions tends to rise, and cooler, more dense air in polar regions tends to sink (Fig. 15.3). This simplistic model of global circulation is known as the Hadley cell, named for George Hadley (1685–1768), the Brit-ish physicist who first envisioned the mechanism. But the plot thickens. The Hadley cell as it is illustrated in Figure 15.3 would be the case were Earth not rotating. But Earth is rotating, which causes atmospheric and ocean currents to drift westward (see bold arrows in Figure 15.3).

Figure 15.2 At lower latitudes (a) solar energy is more concentrated (i.e., there is more heat energy per unit area), and (b) reflection of that energy into space is less.

Q15.2. On page 277 of the Answer Page, sketch Hadley cells for the other three quadrants on the two-dimensional view of Earth.

Figure 15.3 A Hadley cell would be the simple consequence of solar heating, were it not for the fact that Earth is rotating. The bold arrows schematically show the tendency for westward drift of atmo-spheric and ocean currents owing to Earth’s eastward rotation.

E qua tor

Warmer,less dense

air rises

Cooler, moredense air sinks

Had

ley cell

EquatorEastward

Westward drift

rotation

Wes

twar

d drif

t

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National Oceanic and Atmospheric Administration home page is at http://www.ncdc.noaa.gov

The Ferrel cell—An elaboration of Hadley’s model, which takes Earth’s rotation into account, was proposed by U.S. meteorologist William Ferrel in 1865 (Fig. 15.4). Ferrel’s model explains the world’s great subtropical deserts. Earth’s rotation can be viewed as breaking up an otherwise single Hadley cell into three cells: Hadley, Ferrel, and Polar. Wind directions with east west components (e.g., Northeast trades, Southeast trades, and Westerlies) occur between ad-jacent cells.

Ferrel’s three-cell model for each hemisphere derives in large part from Earth’s 24-hour period of rotation. Other planets have different angular velocities. For example,Jupiter turns on its axis once every 11 hours.

Q15.3 How many cells do you suppose Jupiter has in each hemisphere relative to those of Earth—fewer, equal, or more? Hint: If cells are broken into smaller cells by rotation, what’s your intuitive guess as to the result of faster and faster rotation? (Disregard the relative sizes of Earth and Jupiter.)Figure 15.4 The three-cell model

of William Ferrel—which includes Hadley, Ferrel, and Polar cells in each hemisphere—best explains the paths of winds nearest the ground.

Moist hot air,less dense,

rises.

Dry cold air,more dense,sinks.

Equator

30° N latitude

60° N latitude

30° S latitude

60° S latitude

North pole

South pole

High High

Southeast trades

High High

Northeast trades

Equatorial low

Weste r l ies

Westerlies

Wet

Dry

Dry

Wet

Wet

Ferre

l cell

Ferrel cell

Polar cell

Polar cell

Had

ley

cell

Hadley

cell

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Q15.4 Given principles 1–4 on page 265, explain the occurrence of the two contrasting climates illustrated in Figure 15.5.

Figure 15.6 A cold-water coast illustrating the reason for arid conditions.

Cold-water coasts—Coastal land is almost invariably warmer than adjacent ocean water; so, the warmer air over a coast usually rises, and cooler ocean air moves in to replace it (again, see Figure 15.1). The resulting onshore breeze ac-counts for the surfing industry and for coastal trees leaning landward.

Where coastal waters flow from either Arctic or Antarctic regions, air above the water is exceedingly cold, to the extent that it cannot hold appreciable moisture. So, even though the air moves onto warmer land, gains heat, and therefore rises, precipitation seldom occurs. Deserts result (Fig. 15.6).

C. Regional causes of deserts

Figure 15.5 Where prevailing winds move across a mountain range, humid conditions oc-cur on the windward side, and dry conditions occur on the leeward side.

Rain shadows—Where atmospheric circulation drives prevailing winds across a mountainous region, arid or semi-arid conditions commonly occur on the leeward side in what is called a ‘rain shadow’ (Fig. 15.5).

Windward side (wet) Leeward side rain shadow (dry)

Mountains

Lowlands

Cold, dry air

Little moisture to formclouds (dry conditions)

Cold ocean

Warmer landDesert conditions

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Figure 15.7 shows two great deserts, the Great American Desert and the Peru-Chilean Desert, along with prevailing westerly wind in North America and the exceptionally cold Humbolt current offshore of Peru and Chile.

Q15.5. On page 277 of the Answer Page construct topographic profiles along lines A-B and C-D in Figure 15.7. For each, indicate with arrows the air circulation that accounts for the related desert. Label each of your profiles so as to match it with one of the figures on facing page 268 (i.e., include in each of your profiles either labels from Figure 15.5 or labels from Figure 15.6).

Figure 15.7. Map of two West-ern Hemisphere deserts showing relevant westerly wind in North America and relevant Humbolt ocean current offshore of South America.

Soaring birds apply one of the principles in Figure 15.1 on page 265.

Q15.6. When a hawk or eagle wish-es to gain altitude after a tiring day of hunting, that bird heads for a spot beneath a cumulus cloud. Why? Hint: What is the motion of air beneath a cumulus cloud? What accounts for the cloud development?

Prairie dog architects mound dirt around their entrances/exits to provide a watch tower and to turn away surface water. But they build some mounds much higher than others. One theory is that the higher mounds assure ventila-tion within the village, thanks to the Bernoulli principle.

Q15.7. How does the Bernoulli principle apply to the architecture of a prairie dog village? Hint: Ever hear of an airfoil—the shape of an airplane wing that gives it lift?

Animals know about air.

A

B

C

D

Great AmericanDesert

Peru-ChileanDesert

Air streamAirfoil

Air stream

Air outAir in Wind

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D. Desert landscapes

Figure 15.8 Desert landscapes of our conter-minous 48 states occur within areas where the evaporation index is in excess of 2.5.

The Great American Desert

Desert hazards

In addition to the issue of water, arid regions pose special hazards:

Sudden dust storms—As blinding as ‘whiteout’ snow storms.

Surprisingly chilly nights—Arid re-gions typically exhibit surprising differ-ences in daytime/nighttime temperatures.

Q15.8 Why are nights in arid lands surprisingly cold? Hint: Think of a feature of the atmosphere that holds daytime heat during the night.

Maze-like corridors—One can easily become lost in the narrow passages of badlands (i.e., rugged arid landscapes). Ebenezer Bryce, from whom Bryce Can-yon National Park takes its name, once quipped that his canyon was “a hell of a place to lose a cow.”

The definition of aridity is not based on precipitation alone (e.g., 10 cm or less per year). Aridity also depends on evaporation. Practically speaking, a region is viewed as arid where potential evaporation exceeds annual precipi-tation, which includes some 30% of Earth’s land surface.

A more scientific measure of dryness is the evaporation index (Fig. 15.17), which is inches of potential annual evaporation divided by inches of actual annual precipitation. The Great Ameri-can Desert of our western states is char-acterized by values in excess of 2.5 on the E.I. scale. Desert landscapes occur within this region.

Flash floods—Arid regions present special flash-flood hazards. An arid-lands hiker strolling along an arroyo (Fig. 15.9) can be unaware of a distant rainstorm before being confronted by a wall of water.

Figure 15.9. Arroyos are flat-floored stream channels with steep walls. Occasional water evaporates and soaks into the dry earth—depositing sediments en route, creating a flat channel floor.

About flash floods…in Almería Province, Spain—the driest spot in all Europe—a concrete-floored and concrete-walled arroyo passes through the center of coastal Almería town en route to the Mediterranean Sea (Fig. 15.10). Local geologists quip that there is a “delta of car bodies” offshore of Almería.

Q15.9 So how do you suppose this delta of car bodies developed? Hint: To what use do you imagine drivers put this concrete arroyo? (It’s a universal need in urban settings and on college campuses.)

Figure 15.10 Car bodies rest in their watery grave offshore of Almería, Spain.

Evaporation index

2.5 3.0 3.5 5.0

Granada

Almería

Cartagena

Mediterranean Sea

0

50 km0

50 mi

S P A I N

N

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Desert erosionMonument Valley, Arizona

Figure 15.11 Removal of sediments from this desert area by occasional rainwater has produced ‘islands’ and ‘peninsulas’ of flat-lying layers of sedimentary rocks.

Desert erosionIn those deserts where sediments are effectively carried away by occasional rainwater, imposing spires and spines of rock rise above flat valley floors. An example is famous Monument Valley, Arizona (Fig. 15.11). Countless western movies have been filmed here.

Q15.10 Draw a topographic pro-file along the segmented white line

in Figure 15.11. Also, show, with a geologic cross-section beneath your topographic profile, three horizontal rock layers rising from the flat valley floor. Label the lowest and highest layers (i.e., the cliff formers) sand-stone and the one red sloping layer between them shale.

Q15.11 Draw a circle around an abrupt spire standing like a ‘monu-

ment,’ an example of the features that give this area its name.

Q15.12 In Monument Valley erosion is along parallel fractures (called joints), which imparts a topographic fabric. Three or four ‘peninsular’ spines exhibit a mutual orientation. What is their approximate azimuth in a northerly direction? (See ‘azimuths’ on page 35.)

Mystery Valley, Arizona-Utah 7 1/2’ quadrangle

N. 36° 55’ 21’’, W. 110° 08’ 39’’

C

AD

B

N.J.

MD.

N

1 km

1 mi

272 Deserts

Deposition by water—Deserts are characterized by a lack of vegetation because (a) there is insufficient water to sustain plants, and (b) there is insuf-ficient water for chemical weathering of bedrock to produce nutrient-rich soil. Without plants to obstruct the flow of surface water, and without porous soil to absorb that water, runoff of rainwater is at a maximum—resulting in flash floods. However, flood waters in deserts quickly evaporate and/or soak into what

Desert deposition

Figure 15.12. Desert mountains become buried in their own debris.

little dry soil there is, so sediments are dumped along stream courses, burying hills and, in some cases, mountains as well (Fig. 15.12).

The landscape in figure 15.12 is quite different from that in figure 15.11. The bedrock in figure 15.11 consists of layered sedimentary rocks, so the flat valley floor might very well owe its oc-currence to a horizontal layer of rock. But in figure 15.12 the bedrock appears

to consist of massive plutonic igne-ous rock, without a layer or layers that might explain the flat valley floors.

Q15.13. Draw a cross-section along line A–B in figure 15.12. Show both the surface topography and your guess as to the boundary between the partially-buried mountains and the sediments. Hint: View the sediments as a lake that has partially drowned a hilly landscape.

Desert depositionCalifornia

Frink NW, California, 7 1/2’ quadrangleN. 33° 25’ 03’’, W. 115° 37’ 39’’

A

B

N.J.

MD.

1/4 km

1/4 miN

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Deposition by wind—The word desert brings to mind sand dunes, of which there are several types. The most com-mon is the barchan dune (Fig. 15.13), in which the horns of this crescent-shaped dune point downwind.

Figure 15.14 is a Google Earth image of a part of the Salton Sea of California, (Not the most of definitive photo, but such is desert imagery.) Occasional rain reduces bedrock to sand, which is then fashioned by wind into dunes.

Q15.14 According to figure 15.13, what is the prevailing wind direction indicated by dune A in figure 15.14?

Q15.15 Do the dunes in Figure 15.14 reflect recent movement (i.e., within historical time), or have they been stationary for, say, a thousand years? Give two lines of evidence for your answer. Hint: (A) One line of evidence is indicated by the distribution of veg-etation in front of a dune compared with that behind it. (Give the letter label of such a dune and explain.) (B) The other line of evidence is indicated by the relationship between a particu-lar dune and a cultural feature. Name that dune.

Q15.16 Notice in Figure 15.14 that some of the dunes are not perfectly symmetrical like that shown in figure 15.13 (e.g., dune A). (A) Describe this asymmetry, and (B) try to explain it. Hint: See caption to figure 15.13A.

Figure 15.13 Barchan dune aerodynamics. A Map view with arrows indicating air flow. A constant wind direction produces a dune that is symmetrical in plan view. B Oblique view.

Figure 15.14 Sand dunes near Kane Spring, California, are typically of the barchan variety. But horns point in a variety of directions. Does this reflect changing wind directions?

Sand dunesSalton Sea, California

Kane Spring NE, California, 7 1/2’ quadrangle

N. 33° 11’ 05’’, W. 115° 50’ 58’’

B

A C

D

E

A

B

MD.

N

1/2 km

1/2 mi

Types of sand dunes—Sand dunes come in a variety of shapes (Fig. 15.15). Three shapes—barchan, para-bolic, and transverse—indicate wind direction. Barchan and parabolic dunes appear in map view to have contradic-tory shapes, but there is one univer-sality among the three types of dune shapes that indicate wind direction: The windward side is the gentler slope, and the leeward side is the steeper slope. So their cross-sections are more definitive of wind direction than are their shapes in map view. A longitudinal dune is ambiguous; wind direction can be either of two directions parallel to the dune’s long axis.

Another product of wind erosion is a depression called a blowout. Unlike running water, both glacial ice and wind can scoop out soil and rock creating closed depressions, which, where filled by water, become natural lakes.

Figure 15.15. Four types of dunes. Three of the shapes—barchan, parabolic, and transverse—indicate wind direction by their steeper slopes being on their leeward sides.

Figure 15.16. Idealized contour map of a sand dune 50+ feet high. The base of the dune is arbitrarily assigned the value of 0 feet.

The Lakeside, Nebraska, quadrangle on facing page 275 covers part of an area of Pleistocene (aka The Ice Age) sand dunes that have long since been overgrown and stabilized by grass. But these fossil dunes still exhibit much of their original shapes. As a guide to the study of sand dunes on the Lakeside quadrangle, Figure 15.16 shows an ide-alized contour map of a dune.

Q15.17 On page 278 of the Answer Page draw a topographic profile from coordinates F-6.7 to H.5-10 on the Lakeside quadrangle.

Q15.18 Which is the steeper side of this dune—the northwest side or the southeast side?

Q15.19 Which kind of dune illus-trated in Figure 15.15 do you believe you traversed with your topographic profile?

Q15.20. From which direction did the wind that fashioned the dunes of the Lakeside region blow (NE, NW, SE, or SW)?

Q15.21 What is there about the shape of the dune in your topograph-ic profile that indicates that this is an ancient dune, rather than an active dune on the move?

E. An ancient desert: Lakeside, Nebraska

10 20 30 40 500

Map view Cross-sections

WIND

Windward(up-wind side)

Barchan

Parabolic

Transverse

Longitudinal

Leeward(down-wind side)

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Deserts 275

276 Deserts

Intentionally blank

Deserts 277

15.6

15.7

15.8

(Student’s name) (Day) (Hour)

(Lab instructor’s name)H

adley cell

Equator

?

??

15.5

BA

DC

ANSWER PAGE

15.1

15.2

15.3

15.4

278 Deserts

15.9

15.10

15.11

15.12

15.13

15.14

15.15 (A)

(B)

15.16

15.18

15.19

15.20

15.21

15.17 (Lakeside, Nebraska, Quadrangle—page 275)

A B C D

A B

4100Feet

F-6.7 H.5-10

4000

3900