Climatology of Arid Lands

James E. McDonald
Institute of Atmospheric Physics
The University of Arizona

Introduction

Viewed broadly, all of the research objectives of The University of Arizona Arid Lands Program can be regarded as ecologic in nature. Hence it is pertinent to draw attention in this first of our colloquium series to a very fundamental, and indeed almost self-evident, principle of ecology--the law of limits. The law of limits asserts that the physical environment sets limits to the areas within which any living organism can live and reproduce, the limits being either limits of excess or limits of deficiency. Thus, considering man himself, and his agricultural activities in particular, we find that of the 56 million square miles of the world's total land area, about 13 million square miles lie in regions too cold to permit significant agricultural activity, and another 16 million square miles are too dry for conventional agriculture. Thus we find the law of limits operating in a simple and straightforward manner to reduce, to some what under one half, the land area within which man can live in a locally self-sustaining economy. In these colloquia, our interest will center around the arid limit; and in the present discussion, in particular, our concern will be with those physical and climatological factors that impose this arid limit.

Aridity and the Hydrologic Cycle

Residents of arid lands looking out upon the more humid parts of the world are prone to complain that their own lands are not getting their proper share of world rainfall, as if there were some self-evident reason for believing that precipitation is a process to be taken for granted. As a meteorologist, I am forced to the opposite view; to me it seems rather more remarkable that our world is not entirely arid.

As a first point in defense of this thesis we may note that neither of our neighbor planets, Venus nor Mars, possesses spectroscopically observable traces of water in its atmosphere--hence, the very presence of this substance is not guaranteed even within the confines of the solar system. Furthermore, it has been made clear through geochemical studies that, early in its history, the earth lost all but perhaps one part in 1010 of its initial stock of substances with molecular weight as low as that of water, so our present hydrologic cycle now operates on the relatively small amounts of water derived either from breakdown of hydrates in the earth's crust or from volcanically released juvenile water. Astronomical factors also enter the argument through the fact that the earth would have to be moved only about 25 per cent farther from the sun to lower the global mean temperature from the present value of 15°C to a value of about -15°C, thereby locking up as relatively nonvolatile ice nearly all of the world's surface waters and hence powerfully suppressing the intensity of the hydrologic cycle.

But even given the earth's present abundance of ocean waters and its present mean temperature, it is not immediately obvious why the steady-state solution to the disposition of the available water substance did not turn out to be merely an evaporation--condensation equilibrium maintained across the air-ocean interface without any continental precipitation at all. Indeed, when one considers low-latitude west-coast deserts such as the Namib of South Africa or the Vizcaino of Baja California with their near rainlessness in spite of prevailing humilities near 100 per cent, this suggestion does not seem at all unreasonable. Certainly such an equilibrium is a physically possible state of affairs, and were such an evaporation-condensation equilibrium the actual nature of the terrestrial hydrologic cycle, continental areas would receive only such meager amounts of water as they could extract by dew deposition at night. Under such circumstances general aridity would prevail over all land areas.

The factor which enters to change radically the picture just outlined is the adiabatic ascent of moist air, which can result from forced lifting at mountain barriers, from buoyant ascent in areas of strong surface heating, or from forced ascent due to large-scale convergence of air in cyclonic storms. When air ascends adiabatically, it cools and this process, if carried far enough, finally brings the vapor to its point of condensation. But before taking this very important process for granted, it must be noted that most vapors must be compressed to cause them to condense. Were water one of these substances, our clouds would be found not at the tops of updrafts, but at the bottoms of downdrafts: The latter case would yield a greatly altered hydrologic cycle, since then the air feeding into clouds would originate in higher, drier strata; but still worse, the released latent heat of condensation would inhibit further growth (descent) of the cloud rather than enhance growth as is true under actual conditions. This major difference is produced simply by the circumstance that water has an anomalously large latent heat of condensation (though the thermodynamics of the point cannot be examined here).

But even if we take for granted the occurrence of adiabatic ascent and take for granted the consequences of the odd thermodynamic properties of water vapor, we still face an obstacle to cloud formation that ought not be put aside lightly. This is the nucleation barrier standing in the way of the phase transition from vapor to liquid that must precede the appearance of visible cloud material. Were it not for the fact that our atmosphere always contains tiny hygroscopic particles ("condensation nuclei") having diameters of the order of microns down to tenths of microns, clouds could form only on the ubiquitous ions created continually by cosmic ray bombardment of our atmosphere. Ionic condensation, however, is known to require about four-fold supersaturations (x+00 per cent relative humidity), from which requirement one can show that summer "cloud bases" over Arizona, .for example, would lie at about 25,000 feet altitude were there no hygroscopic debris in our atmosphere. It seems quite unlikely that dynamical processes would frequently produce updrafts of such depth as to reach so high a condensation level; but even if they did there remains the fact that the large numbers of cosmic ray ions would almost certainly give rise to a cloud composed of such tiny droplets that these could not quickly aggregate into raindrop-sized particles, so precipitation would be inhibited.

Finally, even if we take for granted the observed abundance of hygroscopic condensation nuclei, and hence assume existence of clouds of familiar kind, there remains an intricate series of improbable steps by which these clouds of tiny droplets (order of 10 micron diameter) are converted into much larger raindrops (order of 1 millimeter diameter). Since knowledge of precipitation physics is still in a rudimentary state, it is not possible to point to the most crucial requirements Nature must meet to complete the precipitation process. Nevertheless, from the easily observed fact that only a small percentage of all clouds (a reasonable guess would be 1 to 5 per cent) succeed in reaching the precipitation stage, we know that much more often than not the last requirements are not fulfilled and the cloud simply evaporates back into invisible vapor.

All of the above considerations are offered in support of the contention made earlier here: It is actually quite remarkable that our world is not entirely arid. Thus, when we note that 25-30 per cent of the continental areas are too arid to support agriculture we must not be surprised; rather we must be grateful that the percentage is actually that low:

What Is A Desert?

Speaking etymologically, the term "desert" implies simply a deserted place; hence it is not improper to speak of the "desert wastes of Antarctica." Speaking climatologically, however, the term invariably connotes precipitation deficiency. A rather commonly used rule-of-thumb holds that deserts are areas with less than 10 inches of precipitation per year. This rule serves surprisingly well as a first approximation, especially in the thermally rather homogeneous lower latitudes (e. g Africa, Australia). The rule overlooks, however, the basic ecologic fact that the determinant of plant, growth and hence of the overall carrying power of a given region is not simply the amount of precipitation, but rather the fraction of the precipitation that survives immediate evaporation from the soil. The latter fraction is strongly influenced by temperature. Hence one is forced to define desert climatic boundaries in terms of some function of both precipitation and temperature. This is done, for instance, in the familiar definition of a desert as an area where potential evapotranspiration exceeds precipitation. Similarly, dependence of desert climatic boundaries upon both precipitation and temperature is built into the well-known Koeppen system of climatic classification. In the present discussion, the Koeppen definitions will be understood to dictate locations of deserts and their surrounding semiarid steppes. Neither the algebraic nor the graphical form of these definitions will be given here, however, since they are readily found in many sources in the literature.

Factors Governing the Geographic Location of Deserts

Desert geography is controlled ultimately by precipitation physics. Of all of the factors governing the series of steps culminating in precipitation, the two that exhibit most marked geographic variability are: (1) frequency of occurrence of adiabatic ascent of large bodies of air, and (2) availability of an oceanic moisture source in the prevailingly upwind direction. The principal deserts of the world will be found where one or both of these factors operate negatively.

The great low-latitude deserts, such as the Sahara, the Arabian Desert, and the Australian Desert, are dry primarily because of chronic lack of lifting processes. This is revealed to us in one simple way by the fact that isolated mountain areas lying within such deserts usually tend to be "humid islands" by virtue of their ability to cause orographic lifting. Thus the Ahaggar Mountains and the Atlas of North Africa, the Macdonnell Range in the middle of the Australian Desert, and all of the many faultblock mountains of the Arizona desert areas have markedly more rainfall than their surrounding plains. Were the aridity of these surrounding plains due solely to low humidity, no amount of mountain lifting would create the humid montane islands that are so strikingly present. This generalization needs qualification in the sense that deep within the interiors of the larger low-latitude deserts, sheer remoteness from oceanic moisture sources can dominate over even orographic lifting. Thus in the Sahara, the dryness of the Tibesti Range (as contrasted with the more humid Ahaggar Range) is probably an instance of this, since the Tibesti mass lies further from the Atlantic moisture source. The generalization also requires the qualification that presence of a mountain range upwind of a given lowlatitude desert may so effectively lower the humidity due to rain-out on the upwind slopes of the range that the desert is to some extent a "rainshadow desert." Thus, even the relatively low Eastern Highlands of Australia so block the southeast trades impinging on the Queensland coast that interior Australia is denied much moisture now falling on this barrier's east slopes. The arid limit is thereby drawn much closer to the coast than would otherwise be true.

When we look for the reasons for the chronic shortage of dynamical lifting processes in these low-latitude west coast deserts, we find that they are: (a) too far equatorward to be strongly influenced by the extratropical wave cyclones that so enhance the precipitation of the lands lying poleward from these deserts, (b) too far poleward to be benefitted by the doldrum (intertropical convergence) belt in its poleward excursions during the summer half-year, and (c) too far westward to be influenced by typical hurricane activity of low-latitude east coast littorals.

If we turn from the low-latitude, west coast deserts to the highlatitude continental arid regions (most of these, incidentally, are steppes and not deserts), we find that the principal factor imposing aridity is the second of the two main factors cited above--remoteness from an oceanic moisture source. This remoteness may be simply geographic, as in the case of the Kirghiz Steppe and the Gobi of Asia, or it may be "hydrometeorological remoteness" created by mountain barriers blocking the inflow of moisturebearing winds, as in the case of the steppes of North American (High Plains area) or as in the case of the peculiar Patagonia desert lying in the lee of the mid-latitude Andes. Mountain barriers not only reduce, by simple rainout, the absolute moisture content of the air currents traversing them, but, still worse, they lead to increase in potential temperatures of the traversing air currents by virtue of addition of the latent heat of condensation of the vapor released as precipitation on the upwind slopes of the barrier. What makes these effects seem to us so striking in the case of so-called "rainshadow deserts" is the fact that the entire desiccation process often occurs in horizontal distances of the order of only 100 miles or less. The desertic southwestern portions of all of the mountainous Hawaiian Islands, when contrasted with the lush tropically vegetated northeast portions, offer a particularly impressive instance of this process. The moist trade winds strike the northeast flanks of these island ranges and yield annual rainfall totals that exceed 100 inches over most of the upwind slopes (and reach about 450 inches in the world's wettest spot, northeast Kauai, in the Hawaiian chain), whereas amounts below 20 inches per year are typical of the cactusand algaroba-covered leeward coasts of the Hawaiian archipelago.

The above-outlined meteorological factors operate to hold precipitation amounts to below the Koeppen desert limits in a total area amounting to about 14 per cent of the entire land area of the earth. This figure has been determined here by measuring an equal-area map of the Koeppen climatic system, the accuracy of the planimetry process being of the order of a few per cent. The Koeppen steppes comprise an additional 14 per cent of the continents, so the combined desert-steppe areas account for 28 per cent of the global land area. Because the literature does not seem to contain any recently published figures on these areas, the complete list of the Koeppen deserts (BW*areas in the Koeppen system) derived from planimetry measurements is given below:

Some Climatological Highlights of the Principal Deserts of the World

The Sahara Desert

The 3.3 million square miles of Saharan desert area amount to over eight times the total desert area of the North American continent (Koeppen definitions assumed here and below), and, in fact, exceeds the entire United States area of 3.0 million square miles. Curiously, the Sahara does not, as far as records indicate, contain the driest spot on earth (a distinction reserved to the Atacama Desert of South America) but it holds the record for the world's highest temperature, 136.4°F, observed near Azizia in Libya. Nevertheless, most of the interior Sahara is believed to receive less than one inch average yearly rainfall (Kendrew, 1942). The Libyan desert is significantly drier than the Algerian Sahara, almost certainly a matter of remoteness from the Atlantic. This area must depend chiefly upon the occasional invasion of a cold front trailing from a cyclone traversing the.Mediterranean; and such fronts, by the time they reach Libya, are no longer lifting air masses containing appreciable moisture. In

*In the Koeppen system of climatic classification, BW stands for desert; BS stands for steppe, and BWk stands for cold-high-latitude desert.

In addition, it is Libya's misfortune that the deep indentation of the equatorial Atlantic coast does not extend far enough eastward to place a warm water surface directly south of Libya, while the Algerian Sahara, at least on its southern margins, enjoys occasional summer precipitation derived from vapor evaporated into the southwest monsoon of the Guinea coast area. The greater aridity of Libya as compared with the western Sahara has apparently characterized most of post-Pleistocene time, for the abundant evidence of neolithic occupancy in the western Sahara is not duplicated in Libya.

The Australian Desert

Surely the most outstanding feature of this desert area is the unusually large fraction of its parent continent that it occupies: 4per cent of all Australia is a desert by Koeppen standards. This may be contrasted with the 31 per cent of all Africa which is desert (Sahara plus Namib), the 6 per cent of South America, and the 5 per cent of North America that is desert (all values derived from planimeter data presented above).

Why does Australia exhibit so unusually large a desert-fraction? Clearly, one part of the answer lies in the simple observation that Australia's latitudinal extent precludes its possessing either extensive tropical rainforests such as suppress the desert-fraction in both Africa and South America, or extensive high-latitude tree climates to dilute the desert influence. In addition, Australian winter synoptic patterns of weather are prevailingly dominated by anticyclones to such an extent that the steady succession of cyclones passing by in the "Roaring Forties" south of Australia exerts only weak winter influence on the precipitation of the interior and of the north. Furthermore, for synoptic climatologic reasons that are not clear to me, the subtropical anticyclone of the winter halfyear over the Indian Ocean (upwind from Australia), produces low-level divergence (Mintz and Dean, 1952), unparalleled in extent and intensity elsewhere in the north or south subtropics, and such divergence enhances dryness due to subsidence. Finally, as has been noted before, the Eastern Highlands add a rainshadow influence that is surprisingly effective considering the low altitude of this barrier (mostly under 3000 feet).

The driest portions of the Australian Desert, in the Lake Eyre area, average about 5 inches rainfall per year. Thus we see that aridity is here not nearly as intense as in the central Sahara, on which only an inch falls annually. Even the annual average of about 3 inches on Yuma, Arizona is not matched for dryness in Australia, despite the latter's desert area being threefold greater than that surrounding Yuma. We can correctly say that desert climate is very extensively but not intensively developed in Australia.

Grazing activities are pushed much farther beyond the dry side of the 10-inch isohyet in Australia than, for example, in North America. Taylor (199) states that the grazing density in the "sparse-lands" of western Australia is about one cow per 640 acres, whereas in Arizona there are few active ranges where the carrying power is not at least one cow per 75 acres. This difference is a difference in availability, outside the arid zone, in good grazing lands in these two geographic areas.

The Arabian Desert

It would be difficult to decide whether it is the vast Sahara with its Foreign Legion or the smaller Arabian Desert with its bedouin horsemen that the layman pictures when he thinks of "the desert." Since we can scarcely doubt that the harsh climatic stress of Arabia Deserta shaped the life-views of the horsemen who swept out of this wasteland during the Dark Ages and established a Moslem culture across all of North Africa and into Iberia, we must admit that here is a region whose climate has influenced history.

Covering almost a million square miles (0.93 million according to the table above), the Arabian Desert seems to enjoy the distinction of being the sandiest of all deserts in terms of total coverage. Nevertheless, even in Arabia, the sand-covered portion is estimated as covering only about one-third of the total desert area--illustrating the rule that most of the world's deserts are far less sandy than is popularly believed to be the case. (Only about 10 per cent of the Sahara is sand-covered, the zest being "reg" or "serir," a desert pavement, or else bare rock called "hammada." Of the Gobi Desert, which is actually a steppe under the Koeppen system, about 5 per cent is sandy.)

Arabia has as another interesting distinction the complete absence of permanent rivers originating within it, or flowing across it. The latter absence stems from the accidental disposition of highlands in the vicinity of Arabia. No well-watered mountain sources send across this desert any analogs of the Nile, the Colorado, or the Indus.

The Turkestan Desert

The truly desert areas of Turkestan cover about 0.76 million square miles, but these areas are small compared with the great steppes that border the true deserts of the region. The oasis towns of Turkestan bear names steeped in history--Tashkent, Samarkand, Bokhara--but viewed more prosaically they appear as communities wherein man has struggled with aridity for centuries. The melting snowfields of the Hindu Kush and the Pamirs send out annual succor to the oases along the Amu Darya and Sir Darya, but agriculture is most precariously balanced here. As much as 10 acre-feet of irrigation water per acre of tilled land are required annually in most of this region. The peculiar kanatz-type of subterranean irrigation channels, now employed throughout widely scattered parts of the Afro-Asian arid zone, are utilized in all of the Turkestan farm villages, though modern irrigation methods are slowly appearing under Soviet development of this area.

The Caspian Sea bounds the Turkestan desert region on the west. Evidences of repeated strandline fluctuations in this now undrained sea have long been studied by paleoclimatologists seeking clues to the rainfall variations of this region. During the Pleistocene period the Caspian overflowed into the Black Sea through a channel lying 150 feet above the present surface of the Caspian, and this present level is about 85 feet below the present Black Sea level. It is generally believed that the Caspian, like the basin lakes of western North America, was completely dried up during the Climatic Optimum some five to eight thousand years ago, and that it was reestablished as a modern sea when the Near East began to receive more rainfall in the millenium or two before the Christian era. Historical records place its A.D. 1300 stand at about 45 feet above the present level, yet even today one can look down through ten feet of water and see foundation-work revealing locations of a community built on its shores in even drier Dark Ages. The Caspian stands two feet higher today than it did in 1845--an upswing that is strikingly opposed to the concurrent sharp downswing in the Great Salt Lake of the western United States. The latter has lost almost fifty per cent of its total volume in the last century. Since these two quite different trends are not understood in meteorological (or hydrologic) terms, they stand as danger-signs in the way of easy globalscale extrapolation of climatic inferences from one desert to another.

The North American Desert

In the southwestern United States and northwestern Mexico, 0.44 million square miles of land fit the Koeppen desert specification. Because those of us who live in or near this desert area naturally tend to regard it as "quite extensive," it is well to note here that it is only about an eighth as large as the Sahara Desert and only about a third as large as the Australian Desert.

It is interesting to speculate on what changes we might have asked of the geologic processes that shaped North America in order to yield much greater areas of desert than we actually find on our continent. Surely the chief requirement would be obliteration of the deep embayment of the Gulf of Mexico. Were the Gulf all land area, desertic conditions would quite certainly extend eastward from the present Sonoran and Chihuahuan deserts across all of Texas; and Louisiana would probably be in the steppe fringe. The now well-watered Mississippi Valley and marginally-watered Great Plains would suffer heavily from such an imaginary change of land-sea distribution. In addition, very much greater area of Koeppen BWk climates (high-latitude cool deserts) would exist if the Gulf were filled, at least as long as we did not also:permit, in our speculations, the removal of the Cordilleran barrier to influx of Pacific moisture.

The southwestern deserts illustrate quite well the climatic dictum, cited earlier here, that low-latitude deserts are dry due to three main causes, which for the North American case run as follows: a) Our southwestern desert region is too far equatorward to be appreciably influenced by cyclonic storms coming in off the Pacific, except in winter when fringe effects give, say, southern Arizona its very modest winter rainfall. b) On the other hand, the region is too far poleward to be sensibly influenced by any precipitation that can be correctly attributed to the summer-migrating intertropical convergence belt. c) Finally, the region is too far westward from the east coast to receive any direct rainfall contributions from trade winds or hurricanes.

The last point requires some qualification. The peculiar summer rainy season of the Sonoran Desert is influenced by the trade circulation in the sense that the injection of moisture into the deep easterly currents that swirl in around the high-level anticyclone over the North American continent in midsummer occurs within the trade wind belt. Also, there appear to be weak but unmistakable indications that dissipating easterly waves originating in the Gulf or Caribbean area drift into the Sonoran desert occasionally and cause temporary increase in the convective activity of the July-August rainy spell. Finally, hurricane influences of a different sort than envisaged in the climatic dictum cited above do appear in the Sonoran Desert. Hurricanes forming not in the Caribbean but off the west coast of Mexico occasionally drift up into our desert area, and though they no longer have the surface circulations characteristic of true hurricanes, they do provide deep moist air masses that have given the Southwest and northern Mexico some of the heaviest rainfalls on record in the late summer period. (This situation is only parallelled climatologically, I believe, by the southward drifting hurricanes, locally called "willie-willies," that move down into the northern fringes of the Australian desert from the Timor Sea.)

I should like to stress that investigators dealing with any ecologic aspect of the arid Southwest need to bear constantly in mind the danger of loose extrapolation of climatological arguments (above all paleoclimatological arguments) from the western to the eastern limits of this desert region. The danger stems from the fact that the western portions (Pacific coastal areas) receive almost all of their precipitation in winter, but are almost rainless in summer due to the inhibiting influence of subsidence in the Pacific anticyclone, while the eastern portions (Texas high plains, Coahuila, etc.) receive almost all of their precipitation in summer. World-wide circulation changes which favor increased precipitation in one of these two subregions will tend to leave the other drier; so one must be constantly on guard against overlooking this opposition in seasonal phase of the rainfall of the east and west extrema. Tucson lies near the half-way point, with just about half its precipitation coming from each season. This two-season feature of the precipitation of southeastern Arizona is quite important to the sustenance of grazing plants and to the existence of the oddly arboreal character of the natural desert vegetation of the area. In this respect, I believe that the part of the world most likely to resemble southeastern Arizona with respect to its seasonal precipitation pattern is the Thar Desert of Pakistan. There the winter rains come as scanty leftovers in the cyclonic storms that move in from the Mediterranean basin, while the summer rains are due, broadly speaking, to convectional release from the western margin of the monsoonal current of moist air sweeping in from the south.

The monsoonal nature of the summer rainfall of Arizona and New Mexico is a final climatological feature of this arid region worth noting here. In southern Arizona, July typically receives about six or seven times as much rainfall as June. The rather sudden arrival of the rainy season is associated with a midtropospheric wind shift from southwesterly flow out of the dry-subsiding source regions of the Pacific to southeasterly flow from the deeply-moist Gulf of Mexico region. The shift results ultimately from continental heating over the central United States, but details are not yet well understood. It is paradoxical that this monsoonal current of moist air flows into the western United States in July and August across the country's most arid region on its way to supplying the vapor for the precipitation over the comparatively humid Rockies area of Utah and Colorado.

The Patagonian Desert

In the narrow southern tip of South America, some 0.26 million square miles of desert area lie in a region where mere land-sea geography ought to preclude any but a very cool moist climate. The Patagonia area is dry for one reason--it is the world's outstanding example of a "rainshadow" desert.

The Andes barrier immediately to the west effectively shut off the moist westerlies blowing in from the South Pacific. Perhaps half or more of this desert lies within only 200 miles of either the Atlantic or the Pacific oceans. One is tempted to term Patagonia a "maritime desert."

The Thar Desert

This desert lies along both sides of the Indus near the tatter's mouth, a seemingly odd location when one thinks of the general pattern of India's Southwest Monsoon. That the Thar is dry is due to the rather sharp northwestward boundary to the moist current of the monsoon. The analogy between, say, Karachi in the Thar and, say, Yuma in the Sonoran Desert is very close. Both lie just a bit too far west of the main monsoonal upperair flow to receive much monsoonal rainfall, save in exceptional years.

The Indus Valley, like the Nile and the Mesopotamian Valleys, was the site of very early civilizations. The cities of Mohenjo-Daro and Harappa were large communities four thousand years ago. Whether their decline was due to climatological changes is controversial. Archaeological evidence such as oversized street-drains, baths in most houses, and various pictorial representations of humid-zone fauna have been adduced by Piggott (1950) to support the view that the Southwest Monsoon must have lain farther west four thousand years ago than it does today and that the decline of these cities was dictated by an eastward shift to the modern pattern. Here, as in all other paleoclimatic arguments, caution is indicated.

The Kalahari Desert

Although the Kalahari bears the name of "desert," most of the Kalahari is only a Koeppen steppe (BS area). The portion truly constituting a Koeppen BW area covers 0.22 million square miles, exactly half the area of the BW region of North America. The Kalahari is to be regarded as chiefly a rainshadow arid region. The southeast trades of the Indian Ocean strike the east coast of Africa and lose so much moisture in crossing the Drakensberg Range that they descend as a warm, dry current into the Kalahari. The Kalahari extends out to the coastal desert known as the Namib, a close analog of the Atacama-Peruvian Desert.

The Taklamakan Desert

This desert occupies about 0.20 million square miles in the middle of the Tarim Basin of Sinkiang. Like Turkestan, its history is intimately entwined with that of the caravan routes from China to the Near East. It is difficult to decide whether to assert that the Taklamakan is a rainshadow desert or just a land-locked region too far from any moisture source to receii much rainfall. The area is agriculturally unimportant, but has some fascinating regional hydrologic problems that have been most confusing to students of climatic fluctuation.

The Iranian Desert

This is a very small desert--only 0.15 million square miles in area. The presence of many Neolithic occupation sites plus the ruins of capitols of powerful empires in or near this desert area raise questions of climatic change. The bleak landscape surrounding Persepolis today is probably enough to encourage such speculation on the part of anyone viewing it, but the evidence-for marked desiccation in this area is not conclusive.

The sand dunes of the southeastern parts of the Iranian Desert are reportedly the largest (highest) of any desert in the world, rivalled only by those of the Taklamakan, according to desert explorers.

The Atacama-Peruvian Desert

The smallest desert area of all these here considered (0.14 million square miles), the Atacama enjoys the reputation of being the driest area on earth. The town of Arica averaged only 0.02 inches per year over a 17year period of record, and all of this came in just three measurable showers. Iquique, another town in the area, has a long-term average annual precipitation of only 0.05 inches. This may be compared, for example, with the rough estimate of one inch per year in the heart of the Sahara, about three inches per year near Yuma, Arizona, and about five inches per year in interior Australia. The great dryness of the Atacama Desert is chiefly attributable to the persistent subsidence of air in the South Pacific anticyclone, but upwelling of cold bottom water along the coast is a secondary factor. As in the Namib, or as in the Vizcaino of Baja California, the coastal strip (perhaps ten miles deep) of this desert is extremely foggy despite its rainlessness. The cold offshore water is responsible for this feature of the climates of such low-latitude west coast littoral deserts. It is, perhaps, one of the most ironical aspects of the entire disposition of climatic liabilities to the world's arid region, that in the Atacama coastal region the native suffers under the joint climatic handicaps of frequent, heavy fogs and of almost utter lack of precipitation:

Bibliography

Kendrew, W.G. The Climates of the Continents, 3rd Edition, New York, Oxford University Press, 1937.

Mintz, G., and Dean, G. "The Observed Mean Field of Motion of the Atmosphere," (Geophysics Research Paper No. 17), Air Force Cambridge Research Center, Cambridge, Massachusetts, 1952.

Piggott, S. 1950. Prehistoric India. Harmondsworth, Penguin Books, 1950.

Taylor, G. Australia.5th Edition, London, Methuen and Co., 1949.

From:
Arid Lands Colloquia, 1958-1959. Tucson, Ariz.: The University of Arizona Press, 1959. p.3-13.