Deserts 265

15 Deserts
Topics
A. 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 de?nition of aridity? What is the de?nition 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 ?rst, a few principles
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 moistureholding capacity than does cold air,
so as warm air is cooled it tends to lose
moisture (as rain or snow).

Q

15.1 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?

#4
Rising air is cooled (because of
decreasing atmospheric pressure)
Moisture condenses, forming
clouds and rain

#3a
Warm air is less
dense, so it rises
#2
Warm land adds heat to air

#3b
Cold air is more
dense, so it sinks

#1
Air over an ocean acquires
moisture through
evaporation; moves inland,
replacing rising land air

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

266 Deserts
B. Global Subtropical deserts
From the principles discussed and illustrated 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.

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 latitudes. Second, the greater angle of incidence of solar energy at low latitudes
results in less loss via re?ection of that
energy.

E qu a t

or

Cooler, more
dense air sinks

ey

ce

ll

dl

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.

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

Ha

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 Africa, the Rub-al-Khali of Saudi Arabia,
the Sind of Iran in the Northern Hemisphere, and the Great Australian Desert
in the Southern Hemisphere. But why
deserts within these particular latitude
belts?

ift

rd

dr

a

tw

es

W

Warmer,
less dense
air rises

Eastward
rotation

Equator

tw
d

ar
ift

dr

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 atmospheric and ocean currents owing to Earth’s
eastward rotation.

es

W

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 British physicist who ?rst 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).

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

Deserts 267
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.

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 adjacent cells.
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.

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.)
Dry cold air,
more dense,
sinks.

cell
lar
Po

North pole

Wet

Fe

rr e

lc
el

l

60° N latitude

Dry

W e s te r l i e s

30° N latitude

High

cell

High
Northeast trades

Hadley

Moist hot air,
less dense,
rises.

Equatorial low

Equator

Wet

y
Hadle

Southeast trades

cell

High

High
30° S latitude

Dry

Westerlies
Fe

rre

ell
lc

60° S latitude

Wet

Pol
ar c
e ll

South pole

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

268 Deserts
C. Regional causes of deserts
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

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

Q15.4 Given principles 1–4 on page 265, explain the occurrence of the two
contrasting climates illustrated in Figure 15.5.
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 accounts for the sur?ng industry and for
coastal trees leaning landward.

Where coastal waters ?ow 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).

Little moisture to form
clouds (dry conditions)

Cold, dry air

rt
Dese

itions

cond

Warmer land

Cold ocean

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

Deserts 269

Figure 15.7 shows two great deserts, the
Great American Desert and the PeruChilean 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 pro?les
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
pro?les so as to match it with one of
the ?gures on facing page 268 (i.e.,
include in each of your pro?les either
labels from Figure 15.5 or labels from
Figure 15.6).

B
Great American
Desert

A

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

Peru-Chilean
Desert
D

C

Animals know about air.
Soaring birds apply one of the
principles in Figure 15.1 on page 265.

Q15.6. When a hawk or eagle wishes 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 ventilation 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?
Air str

Airfoil
Air stream

eam

Air in

Wind

Air out

270 Deserts
D. Desert landscapes
The Great American Desert
The de?nition 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 precipitation, which includes some 30% of
Earth’s land surface.
A more scienti?c 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 American Desert of our western states is characterized by values in excess of 2.5 on
the E.I. scale. Desert landscapes occur
within this region.

Desert hazards
In addition to the issue of water, arid
regions pose special hazards:
Sudden dust storms—As blinding as
‘whiteout’ snow storms.

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

Evaporation index
2.5

3.0

3.5

5.0

Flash ?oods—Arid regions present special ?ash-?ood 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 ?at-?oored stream
channels with steep
walls. Occasional
water evaporates and
soaks into the dry
earth—depositing
sediments en route,
creating a ?at channel
?oor.

Surprisingly chilly nights—Arid regions typically exhibit surprising differences in daytime/nighttime temperatures. About ?ash ?oods…in Almería Province, Spain—the driest spot in all Europe—a
concrete-?oored and concrete-walled arroyo passes through the center of coastal
Almería town en route to the Mediterranean Sea (Fig. 15.10). Local geologists
15.8 Why are nights in arid lands
quip that there is a “delta of car bodies” offshore of Almería.
surprisingly cold? Hint: Think of a
feature of the atmosphere that holds
15.9 So how do you suppose this delta of car bodies developed? Hint: To
daytime heat during the night.
what use do you imagine drivers put this concrete arroyo? (It’s a universal
need in urban settings and on college campuses.)
Maze-like corridors—One can easily
become lost in the narrow passages of
badlands (i.e., rugged arid landscapes).
Cartagena
Ebenezer Bryce, from whom Bryce Canyon National Park takes its name, once
quipped that his canyon was “a hell of a
place to lose a cow.”
Granada

Q

Q

S P A I N

Almería

M
0
0

ed

ite

r

ea
ran

n Sea

N
50 km

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

50 mi

Deserts 271
Desert erosion
In those deserts where sediments are
effectively carried away by occasional
rainwater, imposing spires and spines
of rock rise above ?at valley ?oors. An
example is famous Monument Valley,
Arizona (Fig. 15.11). Countless western
movies have been ?lmed here.

Q15.10 Draw a topographic pro?le along the segmented white line

in Figure 15.11. Also, show, with a
geologic cross-section beneath your
topographic pro?le, three horizontal
rock layers rising from the ?at valley
?oor. Label the lowest and highest
layers (i.e., the cliff formers) sandstone 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.)

Desert erosion
Monument Valley, Arizona

N.J.

MD.

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

N
1 mi

N. 36° 55’ 21’’, W. 110° 08’ 39’’

1 km

D
A

B

C

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

272 Deserts
Desert deposition
Deposition by water—Deserts are
characterized by a lack of vegetation
because (a) there is insuf?cient water
to sustain plants, and (b) there is insuf?cient water for chemical weathering
of bedrock to produce nutrient-rich soil.
Without plants to obstruct the ?ow of
surface water, and without porous soil
to absorb that water, runoff of rainwater
is at a maximum—resulting in ?ash
?oods. However, ?ood waters in deserts
quickly evaporate and/or soak into what

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 ?gure 15.12 is quite
different from that in ?gure 15.11. The
bedrock in ?gure 15.11 consists of
layered sedimentary rocks, so the ?at
valley ?oor might very well owe its occurrence to a horizontal layer of rock.
But in ?gure 15.12 the bedrock appears

Desert deposition
California

to consist of massive plutonic igneous rock, without a layer or layers that
might explain the ?at valley ?oors.

Q15.13. Draw a cross-section along
line A–B in ?gure 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.

N.J.

MD.

Frink NW, California, 7 1/2’ quadrangle

1/4 km

N. 33° 25’ 03’’, W. 115° 37’ 39’’

A

B

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

N
1/4 mi

Deserts 273
Deposition by wind—The word desert
brings to mind sand dunes, of which
there are several types. The most common is the barchan dune (Fig. 15.13),
in which the horns of this crescentshaped dune point downwind.

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

A

B

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

Q15.15 Do the dunes in Figure 15.14
re?ect 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 vegetation 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 particular dune and a cultural feature. Name
that dune.

Q15.16 Notice in Figure 15.14 that

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

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

N
1/2 mi
1/2 km

D

MD.

C

A
Sand dunes
Salton Sea, California

E

B

Kane Spring NE, California,
7 1/2’ quadrangle
N. 33° 11’ 05’’, W. 115° 50’ 58’’

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 re?ect changing wind directions?

274 Deserts
Types of sand dunes—Sand dunes
come in a variety of shapes (Fig.
15.15). Three shapes—barchan, parabolic, and transverse—indicate wind
direction. Barchan and parabolic dunes
appear in map view to have contradictory shapes, but there is one universality 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 de?nitive
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.
WIND
Windward
(up-wind side)
Map view

Leeward
(down-wind side)
Cross-sections

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 ?lled
by water, become natural lakes.

Q15.17 On page 278 of the Answer
Page draw a topographic pro?le 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 illusE. An ancient desert:
Lakeside, Nebraska
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 idealized contour map of a dune.

trated in Figure 15.15 do you believe
you traversed with your topographic
pro?le?

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 topographic pro?le that indicates that this is an
ancient dune, rather than an active
dune on the move?

Barchan

Parabolic

50
40

30

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.

20

Longitudinal

10

0

Transverse

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.

Deserts 275

276 Deserts

Intentionally blank

Deserts 277
(Student’s name)

(Lab instructor’s name)

(Day)

(Hour)

ANSWER PAGE
15.5

15.1

A

C
15.2

15.6

Ha

dl

c

?

ey

l
el

15.7
Equator

?

?
15.3

15.8
15.4

B

D

278 Deserts
15.16
15.9
15.10
A

B

D

C

15.17 (Lakeside, Nebraska, Quadrangle—page 275)
F-6.7
Feet
4100

15.11
15.12

4000

15.13
3900

A

B
15.18
15.19

15.14

15.20

15.15 (A)

15.21

(B)

H.5-10

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Deserts Worksheet | 1
GLG/101 Version 4

Associate Level Material
Deserts Worksheet
Answer the lab questions for this week and summarize the lab experience using this
form.
Carefully read Ch. 15 of Geoscience Laboratory.
Complete this week’s lab by filling in your responses to the questions from Geoscience
Laboratory. Although you are only required to respond to the questions in this
worksheet, you are encouraged to answer others from the text on your own.
Questions and charts are from Geoscience Laboratory, 5th ed. (p. 265-275), by T.
Freeman, 2009, New York, NY: John Wiley & Sons. Reprinted with permission.

Lab Questions:
15.4 Given principles 1–4 on page 265, explain the occurrence of the two contrasting
climates illustrated in Figure 15.5.
The two contrasting climates are wet on the windward side and dry on the leeward side.
From the principles I learned that in high elevations the evaporated moisture produces
clouds and rain, which is why the windward side is wet. At the lower elevations the
evaporated moisture as it rises it dissipates due to the humidity and does not produce
clouds like the windward side. As the air s

15.8 Why are nights in arid lands surprisingly cold? Hint: Think of a feature of the
atmosphere that holds daytime heat during the night. This happens because the air is
dry and can’t retain the heat that is prevalent during the daytime.

15.16 Notice in Figure 15.14 that some of the dunes are not perfectly symmetrical like
that shown in Figure 15.13. (A) Describe this asymmetry, and (B) try to explain it. Hint:
Study the caption to Figure 15.13A.

Deserts Worksheet | 2
GLG/101 Version 4

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

15.18 Which is the steeper side of this dune—the northwest side or the southeast side?

15.19 Which kind of dune illustrated in Figure 15.15 do you believe you traversed with
your topographic profile?

Lab Summary:
Address the following in a 200-300 word summary:
?
?

Summarize the general principles and purpose of the lab.
Explain how this lab helped you better understand the topics and concepts
addressed this week.

?

Describe what you found challenging about this lab.

?

Describe what you found interesting about this lab.

Write your summary here:

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