### THE CHANGING FREQUENCY OF DUST STORMS

THROUGH TIME

A. S. GOUDIE and N. J. MIDDLETON Environmental Change Unit,School ofGeography, UniversityofOxford,MansfieldRoad, Oxford OX1 3TB,England

Abstract. Dust storms are major, but under-studied actors in theworld's drylands.

Not only are they an important manifestationof desertification and land degradation, but they also have a whole suite of important environmentalimpacts, includingpossible rainfall suppression (Maley, 1982), fertilizationof offshore areas, and disturbance to satellite communications. It is therefore important to ascertain whether their frequency and extent is changing. An increasing dust-storm incidence could be both a manifestationof and a contributorto global change. By analysinglong-termmeteorological records for a large number of areas (the Great Plains of the USA, the USSR, Morocco, The Arabian Gulf, Australia, the Sahel-Sudan zone of Africa, China, Mongolia and Mexico) certain conclusions canbe drawn. The first of these is that there is no one globalpattern ofdust-storm frequency trend. Some stations (e.g. in the Sahel) show a clear upward trend of great severity, others show a downward trend (e.g. Mexico City), while others show a more cyclical pattern. In many cases it is evident that essentially natural processes (precipitation totals, snow cover, wind strength)determine the frequency of dust events in any one year. It has also been possible to show the importance ofruns ofdrought years (e.g. intheHigh Plains inthe 1930s, and inthe Sahel zone of Africa in the 1970s and 1980s). Elsewhere, however, various human activities have been significant in determining dust-storm frequency variations: the introduction of centre-pivotirrigation in the High Plains, the abstractionofwater from the Owens and Mono basins in California, the disruption of surfaces by construction activity and vehicle use (e.g. in Ulan Bator, Mongolia), and the deliberate stabilisationof susceptible surfaces (e.g. theLake Texcoco schemeinMexico).


### Introduction Dust storms have often been regarded as one of the most important manifestations of deserfification, and interest in this phenomenon has increased in recent years (see Pye, 1987, for a review). As McTainsh (1986) has pointed out, it is important to monitor dust-storm events if one is to appreciate the extent of desert degradation. As Table I indicates, dust storms have many environmental consequences, a large number ofwhich are adverse. These encompass possible changes atthe global scale, including suppression of rainfall and substantial nutrient addition to offshore areas. There are a variety of methods for monitoring dust-storm events, including satellite imagery (Middleton et al., 

### 1986), archival information (Bucher, 1986) and the analysis of meteorological data (e.g. Goudie, 1983). This last method forms the basis ofthis paper, since itprovides the most complete geographical coverage at the global scale. Locations are listed in Table II. Climatic Change 20: 197-225, 1992. 9 1992 KluwerAcademic Publishers. Printedin theNetherlands.198 A.S. Goudie and N.J. Middleton

TABLEI: Some environmental consequences and hazards to human populations caused by dust storms Consequence Example A.Environmental Soil erosion Soil nutrient gain Plant nutrient gain Salt deposition and groundwater salinisation Playa formation and relief inversion Sediment input to streams Case hardening ofrock Stone pavement formation Desert varnish formation Loess formation Silcrete development Calcrete development Climatic change Ocean sedimentation Glacier Mass budget alteration Ventifact sculpture Rockpolish Radiation B. Human-related Transport disruption Disease transmission (human) Disease transmission (plants) Radioactive-dust transport DDT transport Air pollution Radio communication problems Animal suffocation Animal madness Rainfall acid neutralisation Machinery problems Warfare Closing ofbusiness Reduction ofproperty values Reduction of solar power potential Car-ignition failure Drinking-water contamination Electrical-insulator failure Kalma et al. (1988) Syers et al. (1969) Das (1988) Logan (1974) Khalaf et al. (1982) Goudie (1978) Conca and Rossman (1982) McFadden et al. (1987) Dorn (1986) Liu et al. (1981) Summerfield (1983) Coud~-Gaussen and Rognon(1988) Maley (1982) Rea and Leinen (1988) Davitaya (1969) Whitney and Dietrich (1973) Lancaster (1984) Coakley and Cess (1985)

Houseman (1961) Leathers (1981) Clafin et aL (1973) Becket (1986) Riesborough et aL (1968) Hagen and Woodruff(1973) Martin (1937) Choun (1936) Saint-Amand et aL (1986) L6ye-Pilot et al. (1986) Hilling (1969) AgenceFrance Press (1985) Gillette (1981) Gillette (1981) Reinking et aL (1975) Clements et al. (1963) Clements et al. (1963) Kes (1983)
The following terms can be used to define the miscellaneous types of dust event caused by aeolian processes: a. Dust storms are the result of turbulent wind systems entraining particles of dust into the air, so that visibility is reduced to below 1000 m. Examination of humidity data may differentiate visibility reduction resulting from wind from that resulting from fog. Climatic ChangeMarch 1992The ChangingFrequencyofDustStorms ThroughTime 199
TABLE II: Locations of meteorological stations for which long-term dust-event records are presented Dodge City (Kansas, USA) 37~42'N 100~ 0'W Big Spring (Texas, USA) 32~ 10'N 101~ 25'W Lubbock (Texas, USA) 33~40'N 101~ 53'W Mexico City (Mexico) 19~ 20'N 99~ 10'W Ouarzazate (Morocco) 30~55'N 6 ~ 50'W Marrakech (Morocco) 31~ 9'N 8~ 0' W Potsikum (Nigeria) 11~ 39' N 11~ 2'E Maiduguri (Nigeria) 12~ 0"N 13~20'E Sokoto (Nigeria) 13~ 2'N 5~ 15"E Nouakchott (Mauritania) 18~ 9"N 15~ 58'W Dakar (Senegal) 14~ 34' N 17~ 29' W E1 Fasher (Sudan) 13~ 33'N 25~ 26'E E10beid (Sudan) 13~ 8'N 30~ 10'E Khartoum (Sudan) 15~ 31'N 32~ 35'E Kuwait 29~30"N 47~ 30'E Bahrain 26~ 0'N 50~ 35'E Doha 25~ 15'N 51~ 35'E Aqaba (Jordan) 29~ 13'N 35~ 0'E Ma'an (Jordan 30~ 12'N 35~44'E H4 (Jordan) 32~ 30'N 38~ 12'E H5 (Jordan) 32~ 12'N 37~ 8"E Shobak (Jordan) 30~ 32'N 35~ 33'E Hotien (China) 37~ 25'N 79~ 55'E Minqin (China) 38~41'N 103~ ll'E Ulan Bator (Mongolia) 47~54"N 106~ 52'E Zamiin-Und (Mongolia) 43~44' N 111~ 54' E Choibalsan (Mongolia) 48~04'N 114~ 30'E Onslow (Australia) 21~40' S 115~ 12'E Alice Springs (Australia) 23~40'S 133~ 50'E Mildura (Australia) 34~ 13'S 142~ 9'E
b. Dust haze consists of aeolian dust particles suspended in the air, which are not activelybeing entrained and whichhave been raised from the ground by a dust event that occurred prior to the time of observation or at a considerable distance away. Visibility may sometimes be reduced to less than 1000 m, but is frequently morethan that. c. Blowing dust is the state where dust is raised above the ground locally, but does not reducevisibilityat eyelevelto less than 1000 m. d. Dust devils or dust whirls are local, spatially limited, columns of dust that neithertravelfarnor lastlong. The Great Plains ofthe USA The Dust Bowl ofthe 1930s is perhaps thebest-known and most oft-quoted example oflarge-scale wind erosion and dust-storm activityanywherein the world. The core of the dust bowl area comprised the western third of Kansas, south-east Colorado, the Oklahoma Panhandle, the northern two-thirds of the Texas PanClimatic Change March 1992

### 200 A.S. Goudie and N. J. Middleton



### handle and north-east NewMexico, although most ofthe Great Plains experienced dust-bowl conditions at some time during the 1930s. Indeed some of the worst conditions werefound as far north asWyoming, Nebraska and the Dakotas. The most severe dust storms ('black blizzards') occurred in the Dust Bowl between 1933 and 1938, with activitybeing at a maximum during the spring of these years. At Amarillo, Texas, at the height of the period, one month had 23 days with at least ten hours of airborne dust, and one in five storms had zero visibility (Choun, 1936). For comparison, the long-term average for this part ofTexas isjust sixdust storms a year (Changery, 1983). The reasons for this most dramatic of ecological disasters have been widely discussed, and blame has largely been laid at the feet of the pioneering farmers and 'sod busters' who ploughed up the plains for cultivation. For although dust storms are frequent in the area during dry years, and the 1930s was a period of drought, the scale and extent ofthe 1930s events wereunprecedented (Figure l(a)). The waves of settlers that arrived in the area from 1914 to 1930, in conjunction with the inceasing use of mechanised agriculture, catalysed by high wheat prices, led to exceptionally large-scale wind erosion when drought hit the plains in 1931. In 1937 the US Soil Conservation Service estimated that 43% of a 6.5 million ha area in the heart of the Dust Bowl had been seriously damaged by wind erosion. One event, which raised material in Montana and Wyoming on 9 May, 1934, carried an estimated 350 million tons of soil eastward, depositing dust in Boston and New York in the morning of 11 May, and on ships' decks 500 km out in the Atlantic during the next day or so (Worster, 1979). An approximate 22-yr drought cyclehas been identified in the Western United States (Mitchell et al., 

### 1979). Major droughts have occurred in the Great Plains in the 1890s, 1910s, 1930s, 1950s and 1970s, and these droughts are normally periods of exaggerated dust-storm activity (Figure l(b)). Soil loss in the 1970s was on a scale comparable to that ofthe 1930s (Lockeretz, 1978). This is significant for, as Gillette and Hanson (1989) observe, the early 1970s was not a period when cumulative departures from normal rainfall were as marked as theyhad been in the 1930s and 1950s in the Great Plains. Thus other factors such as the occurrence of strong, erosivewind storms maybe as important a causativefactor as simple lack of rainfall. Above all, in the 1970s the consequences of a new phase ofinappropriate cultivation practices became apparent. Perhaps the worst single dust storm occurred after two years of drought, in the Portales Valley area of eastern New Mexico, as a low pressure frontal system moved eastwards across the Great Plains on 23 February, 1977. In the Portales Valley dryland wheat farming had moved onto marginal land as a result of economic factors and high-technology land-use practices. In addition, certain government policies reduced the disincentives to cultivate marginal land - the Wheat Disaster Assistance Program compensated farmers for loss of crops to wind erosion. Thus, cultivation of wheat took place on marginal lands, some of which were highly susceptible relict Holocene or late Pleistocene dune
Climatic Change March 1992The ChangingFrequencyofDustStorms Through Time 201

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1950-51 1960-61 1970-71 1980-81 Fig. 1.(a) Frequency of dust-storm days at Dodge City,Kansas (1922-61), and for Big Spring, Texas (1953-70) (after Gillette and Hanson, 1989). (b) Yearly total hours of blowing dust for Lubbock, Texas(summing August throughJulyofthefollowingyear,afterWignerandPeterson, 1987).
fields and loess. This extension of the agricultural area was encouraged by the high prices that followed export sales of great quantities of wheat to the USSR in 1975. These crops were irrigated by center-pivot irrigation, which requires the removal of linear wind breaks made up of trees planted since the Dust Bowl era to help prevent wind erosion (McCauley et al., 1981). Some farmers ploughed their fields parallel to rather than transverse to the erosive wind direction, and some had failed Climatic Change March 1992

### 202 A.S. Goudie and N. J. Middleton



### to leave a protective cover of stubblemulch on their fields.The dust palls from Portales Valley and an area in eastern Colorado/western Kansas were tracked on SMS-2 and GOES-1 satellite imagery southeastwards, obscuring 400 000 km2 of ground surface in the south central USA and out over the mid-Atlantic Ocean (Breed and McCauley, 1986). Other Areas ofthe USA One area in the USA where allegedly increasing dust-storm activity is occurring and causing concern is in the Owens and Mono valleys of California. Here local consumption of water and longer distance inter-basin transfers are desiccating Owens and Mono lakes (Cahill, 1984). Dust storms deflate saline materials from the drying lakefloors. A variety offactors operate to cause local increases in dustiness in other parts of the American south west. Nakata et al. 

### (1981) documented numerous ways in which human activityhad destroyed the stability of desert surfaces, producing dust plumes when SantaAna 

### winds blewin January 1973. Such activities included road building, agriculture, urban development, stream channel modification and offroad-vehicle use. In Arizona, where dust storms cause numerous road accidents along interstate routes 8 and 10, cool-air downdrafts from summer thunderstorms raise loose material largely from abandoned, sparsely vegetated farm fields (Hyers and Marcus, 1981). Mexico A study by Jauregui (1960) of dust storms at Tacubaya in Mexico City over the period 1923-58 shows a marked interannual variability in frequency, but with no trend over the period. Jauregui points out in another paper (1969) that dust-storm intensities are generally lower at Tacubaya than at the airport on the northeast edge ofthe urban area, but as Jauregui's data (1960) for Tacubayahave no apparent visibilitylimit itis impossible to compare his frequencies with those taken from the airport. Nevertheless, he found no trend in dust-storm frequency from 1923 to 1958, whereas a marked downward trend in frequency is apparent for allvisibility classes used in the data from the airport over the period 1952-83 (Figure 2). This decreasein frequencycan be explained with reference to three major factors. 1. Rainfall 

### A gradual increase in rainfall in Mexico City over the data period (Figure 2) is shown for precipitation figures from Tacubaya. A similar steady increase has been noted for the station at San Juan Aragon, about 7 km north-north-west of the airport and on the same side ofthe urban area, annual rainfall totals here being ofthe order of 200 mm less than those at Tacubaya (Janregui and Klaus, 1982). Jauregui
Climatic Change March 1992The Changing Frequency of Dust Storms Through Time 203

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1952 1955 1960 1965 1970 1975 1980 1984 Fig. 2. Annualrainfalltotals and dust-dayfrequencies forMexicoInternational Airport (1952-84).
(1960) found little correlationbetween one or two year's rainfall and the following year's dust-storm frequency, so that a yearor two oflow rainfalldid not necessarily result in a high number of dust storms the following year. For the data period analysed at the airport correlations with antecedent annual rainfallwere also poor. Comparing annual dust-event frequency (visibility <5000 m) with the previous year's rainfall produced a Spearman's rank correlation coefficient of -0.14 (56% probability of a relationship)and a correlationwith the average annualrainfallover the preceeding three years was also poor: r= -0.22 with 23% probability. Nevertheless, a gradual increase in rainfall can reasonablybe expected to hffvehad some effecton a concomitantdeclinein dust-stormfrequencyoverthe sameperiod when consideredwiththe otherfactors outlinedbelow. 

### 2. Urban Development The growth and spread of the urban area around the airport may also have had some impact on dust-storm activity. Although perhaps in the short term urban sprawl may act to further destabilize dust sources during phases of construction, the encroachment ofthe urban area around the airport that has occurred since the 1950s will have acted to protect susceptible soil surfaces after the initial stages (Jauregui, 1989). 

### 3. Proyecto Lago Texcoco The identification of the dried bed of Lake Texcoco as a major source of dust storms resulted inthe initiationofa project in 1972 that aimedto stabilizethis area. Lake Texcoco, at lowlevels during dryyears,has been a source ofdust storms over Climatic Change March 1992

### 204 A.S. Goudie and N. J. Middleton



### Mexico City for at least 100 years (Jauregui, 1960), but drainage for industry, agriculture and human use resulted in the lake's complete desiccation in the early 1950s. During this decade the lake, situated immediately north-east of the present airport, became a major source of dust storms over the airport itself and the city. The project has planted pastures of zacate salado 

### (Distichlisspicata),irrigated with recycledurban waste water 

### (aguanegra)and constructed a number ofreservoirs on the former lake bed. In 1971 the area accounted for 40% of Mexico City's dust storms (SARH 1985); that percentage was reduced to zeroby 1984. It is interesting to note that the average annual number of dust-storm days occurring at Mexico City shown in the present study is very much less than that quoted byJauregui (1973) and subsequently referred to in a number of more general texts on dust storms (e.g. Goudie, 1978; Coudt-Gaussen, 1984a). The valueused here is 4.5 days a year. Jauregui's (1973) value of 68 is quoted from a previous paper (Jauregui, 1960), which, as noted above, uses no visibility limit in the dust-storm data. Further, Jauregui (1973) uses the value 68 as the average number of dust storms, whereas in the original paper (1960) he shows an average 68 dust-storm days. This example serves as a useful illustration of the problems and dangers of definition and datausage. Morocco 

### Discussion of dust data for this country is based on new analysis of information provided bythe Direction de la Mtttorologie Nationale, Royaume du Maroc. Data for Ouarzazate, situated on the headwaters ofthe Dra, south-east ofthe High Atlas in an area ofMiocene deposits, show an annual average of4.1 dust-storm dayswith annual average rainfall of 119 nun (Figure 3(a)). The annual variation in frequency of dust-storm days from 1951 to 1980 shows a distinct peak in activity from 1970 to 1975. Although rainfall wasbelow average in 1971, 1973, 1974 and 1975, other periods Ofbelow-average rainfall in the 1960s produced less-remarkable peaks in dust-storm activity. The reasons for the generally weak relationship between rainfall and dust-raising inthis area areunclear and need further investigation. North of the High Atlas, Marrakech is the only other Moroccan station for which recent data are available where significant dust-raising occurs (Figure 3(b)). The annual average number of days is 5.0. The interannual variation (1951-80) shows a distinct peak from 1952 to 1955, reflecting low rainfall in the period 1951-55. A minor peak in dust-storm activity in 1961/62 seems to be related to rainfall of about half the annual average in 1961, and a decreasing trend in rainfall from 1972 to 1980 is mirrored to an extent by a minor rise in dust storms over the same period after generally higher rainfall and minimal dust storm activity from 1963 to 1971.

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### 206 A.S. Goudie and~N. J. Middleton

Sahel-Sudan Zone 

### Bertrand et al. 

### (1979) show the variation in annual numbers ofdays with dust haze from 1947 to 1978 atBilma, Zinder and Niamey, although no visibilitylimit is indicated for their data. At Bilma the number ofhazy days rose steadily from 20-30 in 1955 to more than 200 in 1973. At Zinder and Niamey a rising trend is discernable from the mid-1960s. A't all three stations a significant rise is indicated in 1968/69. Investigation of rainfall at Niamey, Zinder and Agadez shows that the period of significantly increased dust haze accompanies a period of pluviometric shortage. The linear correlation coefficient between rainfall at Agadez and days with dust haze at Bilma is -0.65; this becomes -0.88 ifdust haze is correlated with the rainfall ofthe preceding three years. These stations are in the area affected by the Harmattan 

### dust haze. Further south and west the evolution of dust haze at Bobo-Dioulasso (Burkina) and Bouak6 (Ivory Coast) is estimated by the number of hours of visibility reduced to less than 5 km and 10 km for two periods: 1957-69 and 1970-74. At BoboDioulasso the average number of hours of dust haze at these visibilities increased by a factor of between 3.0 and 4.5 between the two periods, and between 2.0 and 2.5 at Bouakt. The seasonality of dust haze was also modified. Dust haze tended to appear earlier in the season and finish later, and the months of maximum activity changed from February at Bobo-Dioulassofor 1957-69 to Decemberin 1970-74. Further evidence from Niger is presented by Heusch (1980). At Tahoua, in the AderDutchi massif, the mean annual number ofhazy days for the period 1965-74 was 54, but the frequencyvaried from nine days in 1965 to an average of 82 days a yearfrom 1971 to 1973. In northern Nigeria, hazy days with visibility <3000 m at Samaru generally increased as rainfall declined from 1962 to 1973 (Adetunji et aL, 

### 1979), and observations of severe dust haze (visibility < 1000 m) at three-hourly intervals at four stations during the period 1970-76 showed a marked increase in frequency in 1972 and 1973 when the drought was particularly severe (Kalu, 1982). Also in Nigeria, Adefolalu (1984) shows that the highest numbers of cases of respiratory problems associated with dust were recorded during the drought spell 1970-75, and the effects of Harmattan dust on human health in Nigeria has been on the increase since 1976, and may become serious in the immediate future in southern parts ofthe countrywhichhave hitherto been little affected. The unusual strength of the Harmattan in recent years is also suggested by reports that in 1983 dust reached Libreville, the capital of Gabon, a fewkilometres north ofthe Equator, for the first time in 25 years (Morrison 1983). More detailed analysis ofthe transport and Harmattan wind strength needs to beundertaken. The above evidencefrom the literature on the Sahel drought refers to the part of the region affectedbythe dust source ofthe Chad Basin. Data for Nigeria were obtained from the Nigerian Meteorological Department for eight stations mainly in the north ofthe country. These data, which are shown in
ClimaticChange March 1992I



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### 208 A.S. GoudieandN.J.Middleton

Figure 4, indicate the variation in annual totals of severe dust haze days (visibility < 1000 m) and rainfall for the period 1955-79 and the average monthly dust day totals for two periods: 1955-65 and 1969-79, with the change in average annual rainfall between the two periods. Several stations are lacking data for 

### 1966, 1967 and 1968, when civilwar disrupted normal lifein Nigeria (Figure 4). All stations show a period of decreased rainfall at some time during the late 1960s/early 1970s, a relative recovery in rainfall totals in 1974 and 1975, with some stations experiencing a second low rainfall period in 1976 and 1977. At Sokoto this rainfall pattern is mirrored by an upward phase in the number of dust days recorded, that falls off again as rainfall rose in 1974/75. At this station the average monthly totals of dust days for the two periods do not greatly differ, although average rainfall totals were significantly lower during the second, or drought, period (1969-79). The pattern is somewhat different at Maiduguri to the east of the country. An increase in the number of dust days during the early 1970s is clear, and the frequency of dust haze remains generally high after this time despite the relative recovery in rainfall totals during the mid-1970s. Thus, for Maiduguri the average number of dust days during 1969-79 is significantly greater than in 

### 1955-65,by a factor oftwo for the maximum month. At Potiskum, high dust-haze day frequencies for 1970-76 do not show any obvious correlations with corresponding annual rainfall, and dust frequency falls sharply in 1978/79. Thus, average monthly dust-haze days are just slightly increased during 1969-79 as compared to 1955-65. It should be noted that since the dust haze at Nigerian stations is caused by material entrained to the north-east and blownby the Harmattan, the local rainfall totals are not directly related to dust haze, except by the amount of wash-out they represent. Thus, the relationship between dust haze and local rainfall is not a simple one, and although the general deficiency in rainfall totals from about 1968 occurred throughout the Sahel, the station in Nigeria furthest from the dust source

- Potiskum - does not show such obvious pluviometric control on the number of dust-haze days. At Sokoto and Maiduguri the year-to-year variations in rainfall are more closelyrelated to the variation in days with dust haze. Although the relationship between rainfall deficiency and dust-haze frequency is not always obvious or consistent at stations in Nigeria, all stations analysed show some degree of increased dust-haze frequency during the period of the first ten years or so of the 1970s Sahel drought. Another interesting aspect ofNigerian dust-storm activity at both Potiskum and Sokoto is that in the later period dust storms tend to occur earlier inthe season. Data from Mauritania were supplied by the Service M~tdorologique, Nouakchott, and for Senegal bythe Agence pour la Sdcurit6 de la Navigation Aerienne en Afrique et h Madagascar (ASECNA), Dakar. The variation in frequency of annual dust-storm days and annual rainfall totals for Nouakchott is shown in Figure 5. The increase in dust-storm days after 1968 is dramatic. Low rainfall totals of 48.1 mm ClimaticChangeMarch1992The Changing Frequency ofDust Storms Through Time 

### 209

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### in 1970 and 17.9 mm in 1971 represented just 32 and 12%, respectively, of the 1949-67 average, and can be seen as the main onset ofthe drought. The number of dust-storm days increased markedly from 6 in 1970 to 65 in 1974 before a reasonably high annual rainfall of 190.6 mm in 1975; dust-storm activity declined to 25 days in 1976 and 27 days in 1977. In 1977, however, the rainy season brought just 2.7 mm of precipitation, making it the driest year since records began in 1931, and dust-storm activity rose to 55 and 61 days in 1978 and 1979, respectively.The total dropped to 33 dust-storm days in 1980 after a relatively heavy rainfall in 1979, but rose to an unprecendented 85 days in 1983 and remained at around 80 daysper yearuntil 1986. At Nouakchott, further investigation of the dust storm/rainfall relationship has been undertaken using linear-correlation techniques (Middleton, 1987). The linear-correlation coefficientbetween dust-storm frequency and the previous year's rainfall (note that the dust-storm season at Nouakchott is primarily in the first six months of the year, before the onset of the rainy season) is -0.53 (28% explanation). The relationship between dust storms and antecedent rainfall is stronger, however, when annual dust-storm days are compared to the average annual rainfall over the previous three years (linear-correlation coefficient = -0.75, with 56% explanation). This fairly strong relationship is similar to that found by Bertrand et al. 

### (1979) for Agadez and Bilma in Niger. In Senegal, dust haze from the more arid areas to the north and east is more common than dust storms generated locally. Data for annual frequencies of dust haze and rainfall at Dakar are shown in Figure 6. Fairly closely related trends in these two parameters are apparent. Low rainfall in 1968 marks the beginning of an increasing trend in the numbers of dust-haze days, peaking in 1970 and staying
Climatic Change March 1992210 A.S. Goudie and N. J. Middleton
60



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z I I I I I I I I i I I 0 1966 1968 1970 1972 1974 1976 19781979 Fig. 6. Annual rainfalltotals and frequency ofdust-haze days at Dakar, Senegal (1966-79). high while rainfallwas low, until a relative recoveryin rainfallin the period 1973- 75, during which time dust-haze days declined in frequency. After 1975 rainfall againdeclined,with a concomitantincreasein dusthaze. In Mali, where the River Niger sank to its lowest recorded level in 1984, data were unavailable, but reports in the newsmedia attest to the increase in dust haze and stormsthathas occurred sincethe onset ofthe recentdrought:

...everyhour of everyday ofthe week I spent in Mall... a giant dust cloud hung in the air, coveringthe country. Itwas Mali'sprecious topsoil, turninginto its shroud. (Lean, 1985, p. 14) Data from the Sudan were taken from the Sudan Meteorological Department Annual Meteorological Reports, with some additional material supplied direct from the department itself. The data refer to five stations across the Sudanese Sahel. Data in the yearbooks are given as monthly frequencies of dust storms recorded at 0600, 1200 and 1800 GMT, so that a record ofan event at 0600 and at 1200 may refer to a single storm that lasted for six hours or more, or to two separate events. Thus, the graphs in Figure 7 show annual rainfall totals with annual totals of recorded dust events (i.e. the summation of frequencies for 0600, 1200 and 1800 GMT). From 1975 most Sudanese stations started makingan extra visibilityreading at 0000 GMT, and some of the data presented below have been published without taking this additional observation into account (see Middleton,
Climatic Change March 1992The ChangingFrequencyofDustStorms ThroughTime 211

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........ 1956 1954 1958 1962 1966 1970 1974 1978 1982 Fig. 7. Annual rainfall totals and dust-storm frequencies for Sudan: (a) E1 Fasher (1950-83); Obeid (1950-78); and (c) Khartoum (1950-83). 29 1 Ii 24 II I I 22

/ 16

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. ~g
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1985a, Hulme 1985; Middleton, 1985b). Some of these data, therefore, were inflated - those for E1 Fasher, E10beid and Khartoum - but those for Aroma and Tokar were unaffected by the extra reading. Nevertheless, the general trends indicated in Middleton (1985a) are still evident in the adjusted data used in Figure 7
Climatic Change March 1992

### 212 A.S. Goudie and N. J. Middleton



### (omitting the 0000 GMT reading, thus maintaining consistency before and after 1975). The annual dust-storm frequency and annual rainfall totals for E1 Fasher, E1 Obeid and Khartoum (Figure 7) show a marked rise in dust-storm activity dating from the late 1960s/early 1970s. Particularly low rainfall in 1972 and 1973 at E1 Fasher, for example, was followed by a distinct rise in dust-storm frequency, peaking in 1974, falling in 1975 and 1976 after high rainfall in 1974, but remaining at increasing levels after that year as annual rainfall remained for the most part below 200 ram. The zero dust-storm reading for 1979 followed the wettest year in the central Sudan (1978) in the last 20-25 yr (Trilsbach and Hulme, 1984), although particularly high rainfall was not evident at E1Fasher itself. The data that have been presented for the Sahel-Sudan belt of Africa show a considerable degree of homogeneity. In particular there is evidence for clear increases in frequency at many stations, and there is a relationship between this trend and a regional rainfall decline since the mid- and late 1960s. This increase in dust frequency led to increases in dust loadings in the Caribbean, as noted byProspero and Nees (1977 and 1986). The Middle East 

### Some data covering a sufficiently long period were available for three stations located in the area largely affected by the Shamal 

### dust flow. Figure 8 shows the variation in annual numbers ofdays with visibility obscured to < 1000 m by dust at Kuwait (1962-84), Bahrain (1946-83) and Doha (1962-83). The graphs seem to indicate a gradual decline in frequency at Bahrain and Doha, though the length of record at Doha is inadequate for drawing firm conclusions. It should be remembered, however, that in these readings there is no distinction made between dust raised locally and that advected from other areas (largely from northern Saudi Arabia and southern Iraq in these two cases), so that it is impossible, from these data alone, to draw any conclusions as to possibledecreasing dust-storm frequency on either the west coast of the Arabian Gulf or the lower Mesopotamian Plain. Indeed, at Kuwait, nearer to the northern sources, but also affected by local dustraising, no trend is discernable. Nevertheless, it is worth noting that in south-east Iraq some large areas on the Iranian border were flooded during the Iran/Iraq war, which may reduce Shamal dust loads in future. Data from five stations in the Hashemite Kingdom of Jordan (not previously published) showed few discernable trends for individual sites, but when annual totals ofprecipitation and dust or sand storms were summed for all five stations for the period 1965-1988 Figure 9 was produced. Deflation is strongest during spring and autumn months in this part ofthe Middle East, when frontal cyclonesfrom the Mediterranean and North Africa track eastwards across the country (Middleton, 1986). These systems are also the most important for bringing precipitation to Jordan, and this isborne outbythe increasing trend ofprecipitation and deflational
Climatic Change March 1992The ChangingFrequencyofDustStorms Through Time 213

vt

x:

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0 i r i i , i i i i i e i i i i i i i [ i i , i i i i r i 17 1962 1970 1980 1983 ( ~" = Average numl)er of days per annum with visibility ~< lO00m in dust) Fig. 8. Annual frequencies of days with visibility less than 1000 m because of dust at Kuwait, Bahrain and Doha.

Climatic Change March 1992

### 214 A.S. Goudie and N. J. Middleton

1000 -

800 -

600 -

400- / E 

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I 0 z 90 65 70 75 80 85 Year Fig. 9. Annual frequency ofdust-storm days and mean annual rainfall forJordan (1965-1983), based on data from Aqaba airport, Ma'an, H4, H5, and Shonbak. 

### activity, as indicated bydust or sand storms from the mid-1970s. Another factor that has had an effect on atmospheric dust loads, particularly on the Arabian Peninsula, is the industrial development that has taken place since the mid-1970s. Development of industry and the concomitant expansion of urban areas destabilizes desert surfaces, producing increased local dust-raising during the construction phases, and once established, industrial pollutants contribute to atmospheric dust loadings locally (Behairy et aL, 

### 1985). Although such activities are highly localised and often on a small scale with respect to the vast areas of desert unaffected by human activities, they become more significant when it is remembered that meteorological stations are, for the most part, situated within or on the outskirts of settlements. Thus, industrial and construction activities maytake on a disproportional significance in their local effects on dust events as recorded by meteorological observers. 

### The USSR Two examples of changing dust-storm frequencies can be given for the USSR, and both demonstrate the important role played by human activities. The first of these relates to the effects of the vegetation removal and ploughing in the 1950s as part of the Virgin Lands Scheme. As Table III shows, in the Omsk region dust-storm frequency went up on average bytwo and a half times when one compares the data for the period 1936-50 with those for 1951-62. More recently, great concern has been expressed about the increasing deflation of dust from the desiccating Aral Sea. As the inflow of water into the lake is reduced, so the lake floor is exposed and dries out, exposing areas of susceptible
Climatic Change March 1992The ChangingFrequencyofDustStorms Through Time 215

TABLE III: The effects ofthe Virgin Lands schemes on frequency ofdust-storm daysin the Omsk regionoftheUSSR(after Sapozhnikova, 1973) Station Mean annual number ofdust-storm days Increase 1936-1950 1951-1962
Omsk steppe 7.0 16.0 x2.3 Isil'-Kul' 8.0 15.0 x 1.9 Pokrov-Irtyshsk 4.0 22.0 x5.5 Poltavka 9.0 12.0 x 1.3 Cherlak 6.0 19.0 x3.2 Mean 6.8 16.8 x2.5

materials to deflational attack. These materials include an annual blowoff of 43 million tonnes of salt (Sdasyuk, 1989). China Some of the earliest written records of dust-storm activity anywhere in the world are recorded in the ancient Chinese literature. They refer to dusffalls in northern China, and are variously known as 'dust rain', 'dust fog' or 'yellow fog', usually occurring in the spring months. The earliest known record of 'dust rain' was in 1150 BC, and is found in a historical book: Zhu Shu Ji Nian (Chronicles Recorded on Bamboo Slips, quoted in Liu et al., 1981). Zhang (1985) used 1156 historical records to show the decadal frequency of dust-rain years in China for the period since 300 AD (Figure 10). The periods of frequent occurrence are 1060-1090 AD, 1160-1270AD, 1470-1560 AD, 1610- 1700 AD, and 1820-1890 AD. Comparison of dust-rain years frequency with a winter-temperature index for the period 1470-1969 shows that they are in oppog 10
8

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~ o ~ o ~ Year Fig. 10. Thefrequencycurveofdustfallsince300 ADin China (afterZhang, 1985). I

ClimaticChangeMarch 1992216 A.S. Goudic and N. J. Middleton

HOTIEN 60

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Climatic Changc March 1992The ChangingFrequencyofDustStorms Through Time 217

site phase. Although the data set is extensive it is not evident how homogeneous it is through time. Modern meteorological records for the latter half of this century show that Hotien (or Khotan) on the southern margin of the Taklimakan Desert records an average of 32.9 dust-storm days a year, with a mean annual rainfall of 35 ram. Large-scale dust-raising is caused by frontal passages moving westward, and local storms may be produced by katabatic wind flow. The dustiest months are from March to June, when 67% of storms occur. The diurnal variation in dust-storm frequency shows that activityoccurs throughoutthe 24-hr period - manyturn-of-thecenturytravellers crossing the region describe storms lastingfor several days - with a definitemaximum around middayduring the most active months. The annual variation in dust-storm frequency at Hotien from 1954 to 1980 is shown in Figure ll. The interannuai variation is not great (CV= 24%). A gradual decline in frequencyover this period is discernable, but no increasingtrend in rainfall is apparent, and correlations between dust-storm days and annual rainfallwere found tobe poor (linear-correlationcoefficient= -0.03). The Gansu Corridor (sometimes referred to as the Kansu or Hexi Corridor) is a narrow belt of land extending along the northern piedmont of the Qilian Shan mountains and bordered to the north by the Longshou Shan mountains.The orientation of the corridor, WNW-ESE, is such that outbreaks of cold polar air, often associated with the passage of south-westerlymovingfrontal systems,are funnelled through the corridor causing large-scale dust storms. These 'black storms', which occur principally in the spring, cause widespread damage to property in the area. Xu et al. (1979) define the black storm in the Gansu Corridor as a gale with wind speed > 25 m/s, together with a visibility < 50 m. Dust storms caused by thunderstorm downdraftsmayalso occur in the corridor (G. L.Wells, pers. comm., 1985). The corridor is dry, receiving less than 300 mm of rainfall a year, and the surficial sediments comprise sandy loess in the belt along the south ofthe corridor and alluvial sediments to the north brought from the Longshou ranges. The station of Minqin is situated at the eastern end of the corridor and averages 37.3 dust-storm days a year,with an annual rainfallof 116 ram. Dust storms largelyoccur duringthe spring months (63% ofall storms are recorded from March to July),whilerainfallis largelyconcentrated later in theyear, duringthe summer months.The annual variation in frequencyfrom 1954 to 1980 (Figure 11) shows no particular trend, and the interannual variation is not large (CV= 27%). There seems to be a vague relationship between dust-storm frequency and the rainfall of the previous year, and this relationship was investigated in more detail but was found to be statisticallyinsignificant.A test ofannual dust-storm frequencyagainst average rainfallover the previous three years also showed littlerelationship (r= -0.03). Further investigationofmonthlydata on dust-storm frequency,temperature and rainfall for 15 Chinese stations across the north ofthe countryhas been carried out by Littmann (in press) using spectral analysis. Spectral energy for these time series was mostly concentrated in the wavelength of seasonal variations, but supraClimatic Changc March 1992

### 218 A.S. Goudie and N. J. Middleton



### seasonal signals were also identified indicating fluctuations in the range of ENSO variabilitybetween 3.6 and 5.5 yrwithvarious phase dislocations. Littmann speculates that this interannual variability maybe linked to shifts in circumpolar vortex dynamics. 

### Mongolia Time-series data from three stations in the Mongolian People's Republic illustrate differingpatterns ofannualdust-stormfrequencyvariation (Figure 12). A rising trend in frequencyat Ulan Bator during thelate 1970s and early 1980s mirrors a declining trend in precipitation totals. Human activity has also had an effect,for off-roadvehicleuse and intense constructionactivityaroundthemeteorological station on the city's outskirts have had the effect of removing vegetation and destabilizingsoil surfaces. The influenceofprecipitation totals is also clear at Zamiin-Uud on the Chinese border. Low totals in the early 1960s produced intense dust-storm activity culminating in 114 dust-storm events in 1966. The period since the early 1970s, by contrast, when precipitation has been relativelyhigh, has reduced dust-storm frequencies to an average of around 20 events. The precipitation signature on annual dust-stormfrequencyat Choibalsaninthenorth-eastis lessmarked. 

### Australia The increase in dust-storm activity during dry years in Australia was noted by Loewe (1943) for the country as a whole and byThompson (1982) at Broken Hill. This observation is partly confirmedby long-term dust-event frequencydata from a number of stations from differentparts ofthe country (e.g., Figures 13(a), (b) and 

## (c). Heathcote (1983), however, suggests that the 1940s was a period ofhigh duststorm activity, indicating that dust from the wind eroded wheatfields of southern Australia, coloured the sunsets ofthe eastern capitals and even reached the snowfieldsofNewZealand. Gibbs (i975) notes that 1973 and 1974 were years of extremelyheavy rainfall over much ofthe country, and these years show minimal dust-storm frequencies at all stations. Bigg and Turvey (1978) studied the sources of atmospheric particles over Australia, sampling from aircraft in October 1974, November 1975 (years of high rainfall and low dust-storm activity) and May 1977 (although a drought year, during the season of generally low dust-storm activity). Largely as a result of the sampling periods, therefore, they concluded that 'the land surface itself is almost ineffectivein generatingparticles' (Bigg and Turvey, 1978, p. 1649). The drought of 1977 is reflectedby minor peaks in dust storms at Alice Springs and most ofthe stations to the south,but notto thenorth. The effect of antecedent rainfall on wet deposition dustfaUs has been studied
Climatic Change March 1992F,"

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l,o220 a.s. Goudie and N..I. Middleton

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......... ~-w~. 01947 1950 1960 1970 *','?~0 1"352 Fig. 13. Frequency of dust claysin Australia: (a) Onslow (1941-74); (b) Alice Springs (1941-82); and (c) Mildur a (1947-82).

Climatic Change March 19927"heChangingFrequencyofDust Storms Through Time TABLE IV: Rainfall, wet-dust deposition and numbers of dust events at Merbein, Victoria (modified after Ht,tton 1980) 

### 221

1954 1955 1956 1957 1958 1959 1960
Rainfall (mm) 280 350 420 240 441) 190 310 CaCO3(kg/ha) 24.5 23.8 i6.5 20.5 24.5 26.8 41.0 Dust-event days 7 9 I 4 4 13 29



### over a period of six summers by Hutton (1980) at Merbein,just north of Mildura, Victoria. Calcium carbonate suspended in the atmosphere is derived from the fine calcite particles present in the soils of the area, and the amount deposited per hectare in rain during the summer Was found to be inversely related to the rainfall recorded during the two previous springs and winters. Hutton observes that the total annual weight of carbonate deposited appears to be independent of the amount of rain. However, when the annual totals of dust-event days are examined in conjtmction with Hutton's figures (Table IV) the situation becomes clearer. In 1958, for example, when annual rainfall was at a maximum for the six-year period, deposited calcium was below average for the period, which seems to be related to the small number ofdust-event days (4). In 1960, bycontrast, although annual rainfall was over 100 mm lower, the number of dust-event days was much greater (29), and consequently the deposition rate was at a maximum for the study period. In Australia no simple trend in dust-storm frequencies is evident, and this may be the result of there being no clear trend in annual precipitation totals (see, for example, Hobbs, 1988). Conclusion 

### The data which we have assembled in this paper demonstrate that there is no one global pattern of dust-storm frequency trend. Some stations show a clear upward trend (e.g. those in West Africa), others show a downward trend (e.g. Mexico City), while others show a more cyclical pattern. In many cases it is evident that essentially natural processes (precipitation totals, snow cover, wind strength) determine the frequency of dust events in any one year. It has also been possible to show the importance of runs of drought years (e.g. in the High Plains in the 1930s and in the Sahel zone ofAfrica in the 1970s and 1980s). Elsewhere, however, various human activities have been significant in determining dust-storm frequency variations: the introduction of centre-pivot irrigation in the High Plains, the abstraction of water from the Owens and Mono basins, the disruption of surfaces by construction activity and vehicle use (e.g. in Ulan Bator), and the deliberate stabilisation of susceptible surfaces (e.g. the Lake Texcoco scheme in Mexico). Further progress in understanding trends in dust-storm activity requires additional analyses to those presented here. In particular knowledge will be furthered
Climatic Change March 1992222 A.S. Goudie and N. J. Middleton



### by a morethorough study ofwind-strengtheffects and ofthe sources and paths of dusttransport. 

### Acknowledgements The authors would like to thank the followingnational meteorological agencies, whokindlyprovided data:Australia, Bahrain, China, Kuwait,Mauritania, Mexico, Mongolia,Morocco,Nigeria,Qatar and Senegal. 

### References Adefolalu,D. O.: 1984, 'OnBioclimatologicalAspects ofHarmattanDust Haze inNigeria',Archivfiir Meteorologie, GeophysikundBioclimatologieSer.B 33, 387-404. Adetunji,J.,McGregor,J., andOng,C. K.: 1979, 'HarmattanHaze', Weather34, 430-436. AgenceFrancePresse: 1985, Tchad- divers;fogdesablesurNdjamena,Ndjamena, 16 February 1985. Becker, J.: 1986, 'GobiN-Tests BlamedforHigh CancerToll', The Guardian,January28, 11. Behairy,A. K.A.,E1-Sayed, M. K.,and Rao,N. V.N. D.: 1985, 'EolianDustinthe CoastalAreaNorth ofJeddah, SaudiArabia',Journalof AridEnvironments 8, 89-98. Bertrand, J., Cerf, A., andDomergue, J. L: 1979, 'Repartitionin Space and Time ofDust Haze South ofthe Sahara', WMO 538,409-415. Bigg, E. K. and Turvey, D. E.: 1978, 'Sources of Atmospheric Particles over Australia',Atmospheric Environment 12, 1643-1655. Breed, C. S. and McCauley,J. E: 1986, 'Use ofDust Storm Observations on SatelliteImages to IdentifyAreasVulnerableto SevereWindErosion', Climatic Change9, 243-258. Breed, C. S., Fryberger, S. G., Andrews, S., McCauley, C., Lennartz,F., Gebel, D., and Horstman, K.: 1979, 'Regional Studies of Sand Seas Using Landsat (ERTS) Imagery', in E. D. McKee (ed.), A Study ofGlobal Sand Seas.US Geological Survey Professional Paper No. 1052, Washington,DC, pp. 305-393. Bucher, A.: 1986, 'R6cherches sur les poussi~res minerales d'oringineSaharienne',unpublishedPhD thesis,Universit6de Rheims-Champagne-Ardenne. Cahill, T. A.: 1984, Studies ofparticulate episodes at Mono Lake, final report to the California Air ResourcesBoard onContractNo. A 1-144-32, Davis,Ca, 16 pp. Changery,M. J.: 1983,A Dust Climatologyofthe UnitedStates,NOAA Asheville,NC, pp. 25. Choun,H. E: 1936, 'DustStormsinSouthwesternPlainsArea',Monthly Weather Review64, 195-199. Claflin, L. E., Stuteville, D. L., and Armburst, D. V.: 1973, 'WindblownSoil in the Epidemiology of BacterialLeafSpotofAlfalfaandCommonBlightofBeans',Phytopathology63, 1417-1419. Clements,T., Stone,R. O.,Mann,J. E, andEymann,J. L.: 1963,A Study ofWindborneSand andDust in DesertAreas, US Army Natik Laboratories, Earth Sciences Division, Technical Report ES-8, Natik,MA. Coakley, J.A. and Cess, R.D.: 1985, 'Response of the NCAR Community Climate Model to the Radiative'Forcingby the Naturally Occurring Tropospheric Aerosol', Journal oftheAtmospheric Sciences42, 1677-1692. Conca,J. L. andRossman,G. R.: 1982, 'CaseHardeningofSandstone', Geology10, 520-523. Coudr-Gaussen, G.: 1984, 'Lecycledes poussibres 6oliennesdrsertiques actuelleset la srdimentation des loess peridrsertiques quaternaires', Bulletin CentreR~cherche et Exploration-Production ElfAquitaine 8, 167-182. Coudr-Gaussen, G. and Rognon, E: 1988, 'Origine 6oliennede certalns encrrutements calcaires sur l'iledeFuerteventura(CanariesOrientales)" Geoderma42, 271-293. Das, T.M.: 1988, 'Effects of Deposition of Dust Particles on Leaves of Crop Plants on Screeningof SolarIlluminationandAssociatedPhysiologicalProcesses" Environmental Pollution 53,421-422. Davitaya, E E: 1969, ~Atmospheric Dust Content as a Factor Affecting Glaciation and Climatic Change',AnnalsoftheAssociation of American Geographers59, 552-560. Dorn, R. I.: 1986, 'Rock Varnishas anIndicator ofAeolianEnvironmentalChange',inW. G. Nickling ClimaticChangeMarch 1992The ChangingFrequencyofDust Storms Through Time 223

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