298 Erdkunde Band XXVII

Kawagoe und Shiba einem doppelten Spannungsver der Landflucht ist jedoch aus Kapazitatsgriinden si haltnis ausgesetzt. Als zentripetale Zentralorte und cher. Wanderungszentren sind sie fiir ein weiteres Hinter 2. Hauptanziehungsgebiet werden die grofistadtorien land kaum besonders attraktiv. Die Zentralisations tierten pazifischen Ballungsraume bleiben. Dabei spannungen fiihren iiber sie hinaus direkt zum Haupt wird sich eine weitere zunehmend grofiraumige zentrum. von Ihre Entwicklung wird zentrifugalen der in mit - Ausweitung Ballungszonen Verbindung Kraften der Sie Ballungskerne gesteuert. wachsen, dem Bau neuer Strecken des Schienenschnellver aus - jedoch nicht eigener Kraft sondern als Satelliten. kehrs und neuer Autobahnen abzeichnen. Demgegenuber steht die sich eigenstandig verstar 3. Eine weitere Verstarkung der zwischenstadtischen kende regionale Vormacht der Prafektur-Hauptorte, Wanderungen wird sich auch auf den Austausch die echte Regionalzentren geworden sind. Sobald es zwischen den Ballungsgebieten auswirken. Eine Stu ihnen gelang, konkurrierende Nachbarstadte funktio wird sich nur da vollziehen, wo kla nal zu iiberschichtenund zu iiberholen, fenwanderung grofienmafiig re zentralortliche Hierarchien vorliegen. wuchs und wachst ihr Vorsprung unaufhaltsam wei ter.Das istAomori gegeniiberHirosaki erst halb, Sap 4. Das Prinzip der Wanderungs-Zentralitat aufierhalb erster den poro gegeniiber Asahikawa und Otaru bereits voll ge der Ballungsgebiete wird in Linie grofie ren mit mehr als 300 000 Einwoh lungen. Mentalitat und Wanderungsverhalten der ja Regionalzentren nern wer panischen Bevolkerung unterstutzen den Prozefi der zugute kommen. Kleinere Landeszentren Selbstverstarkung, der das Modell des ?Grofien To den mit der Ausdunnung ihres landlichen Umlandes Prafektur kyo" auf verschiedene ?Klein-Tokyos" im ganzen weiter abnehmen. Die dominierenden werden am starksten weiterwachsen. Land ubertragt. Hauptstadte Es wird abzuwarten sein, in welchem Mafie die zur des neuen Minister D. Ansatze Abschatzung kiinfti grofiangelegten Plane japanischen Tanaka zu einem ger Wanderungstendenzen prasidenten grundsatzlichen ?Um bau der japanischen Inseln" in konkrete Raumord Versucht man abschliefiend ein Urteil zu den ein verwirklicht wer nungsmafinahmen umgesetzt und im mit den Thesen Toshio Ku gangs Zusammenhang den. Die iibermafiigeVerdichtung von Industrie, Be rodas nach der Ent aufgeworfenen Fragen kiinftigen volkerung und Verkehr in den Ballungsgebieten sowie der in so werden wicklung Binnenwanderung Japan, die weithin bereits unzumutbaren Umweltbelastungen bei aller Vorsicht und die fiir einen Zuriickhaltung, und Schadigungen lassen eine Wende der staatlichen westlichen Beobachter in ostasiatischen Entwicklungs Raumpolitik als langst iiberfallig erscheinen. Doch ist unerlafilich aus der intensiven Beschafti fragen sind, Skepsis geboten. Schon heute wehrt sich die Bevolke mit doch gung Wanderungsproblemen einige grund rung in den landlichen Randzonen gegen ein weiteres satzliche fiir die nachsten Vorausschatzungen Jahre Ausufern und Verlagern der Industriezonen. Auf je sein: moglich den Fall wird eine grofiraumige Strukturpolitik der 1. Fiir eine grundsatzliche Umkehrung der Tendenz Regierung die hier aufgezeigten Trends der Wande mit beriicksichti der Binnenwanderung bestehen bisher keine Anzei rungszentralitat bei ihren Planungen zu chen. Die Entleerung der landlichen Gebiete wird gen mussen, um zu realitatsgerechten Losungen ge weiter anhalten; eine zahlenmafiige Abschwachung langen.

THE URBAN OF CITY

With 15 figures and 13 tables

Ernesto Jauregui

naturlichen Aerosole in den Monaten am Ende der Zusammenfassung: Eine Darstellung der Klimaelemente der von zu dem in denen die vulkanischen Aschen fiir das Stadtgebiet Mexiko-Stadt fiihrt Ergeb Trockenzeit, feinkornigen im Hochbecken von Mexiko dem Wind nis, dafi vorwiegend antizyklonale Wetterlagen, mangelnde vegetationslos eine Rolle. Es kann Ventilation und die Ausbildung einer innerstadtischen War angriff ausgesetzt sind, grofie gezeigt dafi Staubsturme im Stadtbereich von Mexiko meinsel die Auswirkungen der Luftverunreinigung inMexi werden, im Monat in maximaler auftreten ko-Stadt erheblich vergrofiern. Eine negative Beziehung Stadt April Haufigkeit durch Vor konnte aufierdem zwischen der Intensitat des Warmeinsel und sowohl durch lokale Konvektion als auch im 200 mb-Niveau werden. effektes und der Windgeschwindigkeit der ?Grofiraumwin ticityadvektion ausgelost der Luftverunreini Fiir die Monate der kann wer de" nachgewiesen werden. Das Ausmafi Regenzeit nachgewiesen die das die raumliche der in Me gung zeigt tages- und jahreszeitliche Schwankungen, den, Verteilung Niederschlage der innerstadtischen einerseits auf die in den Nachtstunden besonders intensiv xiko-Stadt erheblich durch den Einflufi und auf den Warmeinsel bestimmt wird. In werden in ausgebildete Warmeinsel andererseits jahres grofier Haufigkeit Teilen der Stadt die hochsten Nie zeitlichen Wechsel von Regen- und Trockenzeit zuriickzu den zentralen taglichen fiihren sind. In diesem Zusammenhang spielt die Dispersion derschlagssummen registriert. Ernesto Jauregui: The urban climate of 299

Mexico City was founded on a small island on in 1344. At the time of the conquest the city was destroyed and later reconstructed by the Span iards in 1522. Since the valley ofMexico is an interior basin, flooding of the urban area was frequent and in order to control the drainage of excess waters of Lake Texcoco an outlet towards the was constructed in 1789. As the city grew, thewater areas ^^^^^^^^^^^^^^^^^^^^^^^P^e^to^^cocoj became smaller due to sedimentation and reclamation. Today, about 4/5 of the large (about 450 km2) urban area stand on the ancient lake sediment plains; the rest lies on hilly terrain to theWest and South. Being surrounded by mountains that stand some 800 m or more high above the bottom of the elevated (2250 m above sea level) valley, the city is poorly ventilated by winds; this and other circumstances favour the sharpening of some of the elements of the urban climate of the capital. 1. Urban Characteristics. A rough indication of the nature and density of urban development inMexico City is given in fig. 1. Greater building density is located along two main thouroughfares: Paseo de la - Reforma Avenida Juarez and Avenida Insurgentes E^1 M2 S3 which make a cross point some 4 km West of the old i: areas and station location downtown district where the streets are narrower. Fzg. Built-up 1 industrial areas; 2 of 3 climatic The rest of the urban area is characterized by rather high density buildings; station; 4 observatory wide streets particularly to the South and West where housing densities are low and open spaces A study of the frequency of surface weather types more frequent. inMexico made by the author (Jauregui, 1971) for 2. Air Climatological Records. temperature and the period 1919-38, revealed that cold fronts passing measures rainfall have been taken in Mexico City over Veracruz on the southern Gulf of Mexico coast, since about the of the nineteen but as can beginning twenties; are most frequent from October until March, be on. regular observations began from 1940 There are seen in table I. However, not all these cold fronts about 30 stations within or near the climatological affectMexico Cky, cold air masses being frequently month records to the capital, sending every daily shallow, only 80?/oof winter fronts sweeping over the Service in Meteorological Tacubaya Observatory Gulf of Mexico reach the elevated valley of Mexico Other weather elements like (fig. 1). fog, cloudiness, (Hill, 1969). are thunderstorms and prevailing wind also observ not over ed. Although ideally distributed the urban Table I: Average frequency of surface coldfronts crossing are area, the stations sufficient for studying the varia overthe Southern Gulf ofMexico (Period: 1919-38) tions of climatic elements in the city and the sur (Jauregui,1971b) country. rounding J F MAMJ JAS OND YEAR 6.2 5.5 5.6 3.8 2.5 0.5 0.1 0.0 0.9 6.3 7.4 6.8 45.6 3. Weather Types. Anticyclonic weather types pre vail during the dry (October toApril) inMexi co City; they are associated with dry conditions and Dominguez (1940) describes this cold weather type lightwinds. The major fluctuations of temperature in as producing a uniform, often thick cloud cover, which are winter inMexico produced by the cold polar out on occasions lasts for many days (see also Klaus, breaks. The frontal passage is sometimes a rather 1971). At other times polar continental air masses sharp temperature discontinuity due to cold air ad penetrating far into the plateau area bring cold but In a vection. study of the winter climate of Mexico clear weather with very little associated cloud. Fig. 2 covering a five year period record,Hill (1969) points illustrates a characteristic cold frontal situation. out that in 75?/o of cold spells the period forMexico During the wet season cloudiness and City lasted for only 12 hours and during this time are associated with the Easterly Trade current; much 96?/o of all temperature drop associated with the front of the rain falls during thunderstorms connected with occurred. The average temperature drop recorded by wave distrubances or with the existence of a hurricane the passage of each front is 3 ?C, according to this or other tropical depression in the vecinity of either author. (or both) the Pacific or Gulf coasts; in themiddle of 300 Erdkunde Band XXVII

ly down, (see table IV) and a decrease in the diurnal variation results as minimum temperatures rise in part due to highermoisture in the air.

Fig. 2: Typical cold front crossing over the Gulf of Mexico

zone the rainy season the intertropical convergence (IT2) may be displaced as far North as Acapulco, producing squally weather inMexico City. At other times rain is the result of strong convec tion and relief. A secondary minimum of rainfall is Fig. 3: Average annual number of cloudy days observed in July or August due to a temporary return of anticyclonic conditions when the semi-permanent - Bermuda Azores High is split in two cells by an elongated trough located along theUnited States At lantic coast (seeMosino, and Garcia, 1968).

4. Precipitation. Table II shows the average distri bution of precipitation for Tacubaya Observatory located on hilly terrain to theWest of downtown.

Table II: Mean Monthly precipitation inMexico City (1921-45) (inmm)

JFMAMJJASOND YEAR 13 5 10 20 53 120 170 152 130 51 18 8 750

But precipitation amounts vary within the urban area. Orographic effect is evident in the cloud and pre cipitation isoline configuration as can be seen in figures 3 and 4. Cloudy skies and rain are more frequent over the hills to the South and West; on the central plains scarce to near Lake Texcoco however, precipitation is the point that the climate may be classed as semi-arid (BS inKoppen's classification). Fig. 4: Mean annual precipitation (mm) 5. Temperature. Mexico City exhibits many of the features of tropical mountain having a small Table III: Mean monthlytemperature (DEG C) for annual as can be seen in Table III. temperature range Mexico City (Tacubaya) 1951-60 However, a high diurnal temperature range is observ YEAR ed, particularly during the warm period (March J FMAMJ JASOND 13 14 17 18 18 17 16 16 16 15 14 13 15 May) ; once the rains start, the temperatures go slight Ernesto Jauregui: The urban climate of Mexico City 301

Table IV: Mean monthly maximum and minimum per year in the rural areas, to 40 in the suburbs,with Mexico 1945-65 no Temperatures for City (Tacubaya) frosts occurring in the center of town. JFMAMJJASOND 6. Atmospheric Pollution. In the 1920's Mexico Ci 24.4 26.9 30.1 30.4 30.3 28.5 26.0 25.9 25.6 25.4 25.0 23.9 see ty's residents could very often the snow-capped 0.1 1.5 3.5 5.1 7.0 8.4 8.1 8.2 7.0 4.0 1.9 0.8 volcanoes that surround the valley to the East, shim mering majestically 50 km away. This degree of visibi The central areas of Mexico are warmer than City lityused to be common, but not any more; as the capi the districts results have also been surrounding (similar tal and its neighbours grew, the industry grew with observed Lauer and Gab for the by (1970) (1970) them and the air became more and more polluted. nearby city of Puebla); average annual tempera Uncontrolled emissions from factories and the ever in tures are 2? than those of the suburbs as greater by creasing number of vehicles, all contributed to the loss can be seen in 5. In table V a of figure comparison of transparency. minimum is made between a average temperatures Visibility has been reduced on the average from central and a rural station. The influence is city 10-15 km in the 1930's to 2-4 km and poor visibilities on weather the stronger radiation-type during dry are now much more frequent (30%> or more) than and weaker the wet season period during (May-Sep 30 years ago as shown elsewhere (Jauregui, 1958, tember). 1969) (see figs. 6 und 7).

Dezember km T 15-20- y\

10-15--slr-j>^om<^

" 1 2 1937 1966

_August_

15-20- / Vo^Sa-?q 10-15-\ f \y-y\> j?

1 - 2 1937 1966

Fig. 6: Mean visibility variation in Tacubaya Observatory at 2 p.m. for the period 1937-66

% km 0_5_10S _&_S_I Fig. 5: Mean annual temperature (Deg. C)

Table V: Average differences in minimum Temperatures an a between urban (C. F. E.) and rural (Los Reyes) Station inMexico City (DEG. C) JFMAMJJASOND 7.6 7.6 7.7 6.5 6.5 4.9 3.5 3.4 5.1 7.3 6.9 9.9 50-4r--W-<

Intense heat islands inMexico City develop during 40?\~4-7-ir-V season as a the dry result of strong temperature inver sions; under these conditions cooling at night-time is less inside of the built-up area than in the surrounding districts due to the absorbtion and re-radiation of 20-T5&10-??I energy from the urban surfaces by the elevated layers of polluted air. As a result of the heat island above JFMAMJ J AS0ND Mexico urban area there is a marked reduction City's Fig. 7: Frequency of visibilities less than 2 km at 10:30 in frost occurrence as one nears the center of town; A.M. from Tacubaya Observatory for 1936 and 1966 the average number of night frosts is reduced from 70 (Jauregui, 1969) 302 Erdkunde Band XXVII

The characteristics of inMexico City Table VIII: Average number of bright sunshine are marked by the nature of the dominant effluents (hours per month) inMexico City (Tacubaya) and London (high sulphur petroleum products) and the peculiar (KEW) of the of anti climatology valley (high persistence JFMAMJJASOND Year cyclonic weather with clear calm days and surface Mexico temperature inversions). 259 227 271 238 235 178 186 189 144 192 203 233 2555 London measurements. Measure Turbidity 49 63 110 168 198 219 193 190 137 96 55 40 1460 ments of the instantaneous direct solar radiation car ried out inMexico Galindo for the City by (1962) Surface temperature inversions. It is 1957-58 show a decrease of 10% with respect period well known that atmospheric pollution is associated to measurements Gorzynzki in the early by 1911-28, or with stable lapse rates temperature inversions near as can be seen in table VI; also, when Galindo's the ground. In an urban area likeMexico City located values of the Linke-turbidity factor are compared in a sheltered valley and during nights with high net with those for London in the 1960's, it can be readily outgoing radiation, there may be a simultaneous that Mexico City's air was at the time a appreciated development of both temperature inversion and a more than that inLondon polluted (table VII). light local wind flowing down the hills to theNorth, West and South. In table IX is shown the frequency of A. surface inversions Table VI: Direct solar radiation instantaneous maximum morning (6 M.) temperature for the short of 1971 - 1972 values for Mexico City (urban Station in Tacubaya) period August July 6 A. M. radiosonde observations started for thePeriod 1911-28 andUniversity City (suburbanStation) (regular only for 1958 (Galindo, 1962) since the first date). These early morning surface in versions do not but for a few hours after sun AS OND persist J FMAMJ J rise (when fumigation conditions are frequently ob near Tacubaya served) until dry adiabatic lapse rates are estab 1.631.661.541.56 1.52 1.541.48 1.53 1.57 1.60 1.571.63 lished before noon by abundant insolation, particular the under certain metear Univ. City ly during dry season; only 1.491.491.461.341.241.31 1.321.38 1.41 1.37 1.301.31 ological conditions are they strong enough to ap pear also in the afternoon sonde observation.

Table IX: inversions in Table VII: Linke turbidityFactor for London (KEW) Frequency of surface temperature Mexico the 1972 andMexico City (UniversityCity) Cityfor periodAugust 1971-May AS F winter spring summer fall year ONDJ MAMJJ 7 10 9 15 19 21 25 23 27 18 5 2 London 4.1 4.9 5.1 4.5 4.6 6.5 5.5 5.7 season are Mexico_4A_6.0 During the rainy surface inversions usually not more than 150 m deep; but as soon as dry sets However, since the Clean Air Act of 1956 was en anticyclonic weather in, strong night radiation to as can forced, London has gained about 40% more sunshine losses lead generally greater inversion depths no meas be seen in table X. by reducing drastically coal smoke, whereas Simultaneously, greater tempera ures have yet been taken inMexico City to check the constantly rising level of air pollution. Table X: Frequency of surface inversions inMexico City to Moreover, irradiation of exhaust gases in clear days According depth(%) leads to smog and eye irritation is fre photochemical Top of Mexico residents. Al quently experienced by City Inversion Au Se Oc No Deja Fe Mar Ap May Jun Jul of the exact though the identity compounds produced (meters) in the photochemical smog,which are responsible for 30- 50 16 0 0 7 5 0 0 0 0 0 0 0 eye irritation, has not been quite established, studies 51-150 50 56 38 13 23 23 36 17 15 19 0 50 151-250 17 33 35 47 23 23 32 35 41 45 60 50 of typical irradiation smog, like the one observed in Los indicate that the initial reactants are 251-400 17 11 37 27 23 36 16 14 30 12 40 0 Angeles, 21 8 30 11 18 0 0 nitric oxide and 401-600 0 0 0 6 14 hydrocarbons (oleofins, oxigen). 600 0000548436 0 0 These hydrocarbons originate almost in total from auto exhaust (Hamming and MacPhee, 1967). Since sunny days are very frequent inMexico City during ture differences between surface and top of inversion season the long dry season (see table VIII), it is reasonable to are also more common during the dry (ta mean expect that similar chemical reactions in the capital's ble XI), which would that temperature gradi more or same atmosphere lead to the production of eye irritation ents in the stable layer remain less the the compounds. throughout year. Ernesto Jauregui: The urban climate of Mexico City 303

Table XI: Frequency of isothermal surface layers and temperature inversions inMexico City according to temperature difference between surface and top of layer (in?/0)

Intensity (?C) Au Sep Oc No Dec Jan Feb Ma Ap May Jun Jul 0 66 20 25 7 17 13 8 26 8 28 80 50 1 0 50 50 33 33 9 4 19 15 22 20 50 2 33 20 13 13 28 27 16 12 26 39 0 0 3 0 10 0 40 22 20 14 26 29 11 0 0 4 0 10 12 7 11 20 24 13 15 0 0 0 5 0000088070 00 6 0000038000 00 6 000000 13 000 00

The strongest heat islands observed inMexico City are associated with these deep dry season inversions which in turn are conductive to higher pollution con centrations. The wind was calm in 82?/o of all cases a was when surface inversion observed during the period under study; under these conditions it is likely that a very light centripetal circulation would gener ate across ^\ km the marginal thermal gradients fomenting 0_5_10 _&_S_I the slow drifting of pollutants towards the central 8: dioxide distribution areas, as reported for other cities (Chandler, 1965, Fig. Sulphur (in mg SC^/dmVday) for October 1962 Bravo and Viniegra, Davidson, 1967). Pollution can, under these circum (after 1966) stances, rise to high level values before the surface inversion is or a fresh out destroyed by insolation, Regular sulphur dioxide and smoke records exist break of continental air arrives polar sweeping for the city since August 1967 for five sites in the the It is in these away offending gases. only post urban area (these have been augmented to 10 since front situations that the traditional of transparency 1970). Figure 9 shows themonthly variation of smoke the thin air of the capital is restaiblishedand for a day concentrations for the first five stations for which or two the residents ofMexico can admire City again data are available (Marquez, 1969). These smoke the of the mountains. beauty surrounding concentrations are obtained by drawing for 24 hours a It should be noted that when however, compared known volume of air through a filterpaper and meas with conditions in other cities at sea simi large level, uring the darkness of the stain. A seasonal variation is lar air levels at time of the would pollution any year evident for all stations, but particularly for the one be more in the of since the aggressive capital Mfexico; located in the center. Higher concentrations of smoke of at 2250 m is 77?/o partial pressure oxigen high only occur during the dry months due to both a high fre of that at sea the inhabitants need a level, greater quency of intense surface radiation inversions and a volume of air for as out lung ventilation, pointed strong heat island. The decrease in smoke levels during at the International on the recently Colloquium Phys the rainy season is the result of both, a reduced fre of theHuman in held in iology Body High Altitudes, quency of surface inversions and also of atmospheric La Bolivia under the of the World Paz, auspices scavenging by precipitation. Health Organization.

Air polution measurements. Statistics micro-gr/rrrVday for SO2 concentrations are available for the year 1962 250-j - from a studymade by Bravo and Viniegra (1966). 200-S Figure 8 shows the average sulphur dioxide levels from 30 points of observation, inmg/100 cm2/day for 150-/ ...o.o.. the dry month of October 1962. The values decrease 100- center away from the except towards the Tlalnepant ^^J^ la suburban industrial area where a secondary peak is ? observed, but in general, average smoke concentrati A S 0 N DJ FMAMJ J A ons are more than three times those of the outer sub 1967 1968 o.o o-o 0---0 urbs. It should be noticed from this figure also that TLALNEPANTLA TACUBA TLALPAN o? -o CENTRO (downtown) o-.-o AEROPUERTO towards theWest, at the foot of the hilly terrain, (airport) are more as pollution gradients slightly intense would Fig. 9: Monthly variation of smoke concentration inMexico be expected from the topography. City for5 points (Marquez, 1969) 304 Erdkunde Band XXVII

It is evident that both climate and topography Union where these phenomena have a frequency of 50 combine to make Mexico City smog one of themore to 60 days/year (Zakharov, 1966). urgent problems of the capital to be solved. However, despite regulatory legislation since 1971, much re mains to be done technically and administratively to ensure purity of the capital's air for the future.

Natural air pollution. Wind erosion on the dusty semiarid plains to theNorth and East has be come a serious problem for the capital. A crucial time for erosion is the January-April period, at the end of the dry season. The inhabitants of Mexico City have to as long been accustomed these storms known ,tol vaneras*. The areas more liable to wind erosion are mainly the dried soils of the exposed bed of ancient Lake Texcoco. The scarcity of rain and the dry winds from polar continental air masses dry the surface and lower thewater table providing very dry top soil con ditions. Under these circumstances blowing dust occurs when intense downdrafts during the afternoon from the so called ,high-level thunderstorms' sweep over the plains (see Krumm, 1954). The most dense duststorms are associated with these dry thunderstormsat the end of the dry season, when westerly winter circulation is S 0 5 10km giving way to themoist easterly current.These _&_s_storms peculiar to the semiarid plateu areas of North Fig. 10:Mean monthly dustfall (in tons/km2)for 1959 (after America have been described by Harris (1959) for Bravo and Baez, 1962) the locality of El Paso Texas. Convective heating few thunder during sunny days permit high-level Table XII: Average frequency of duststorms inMexico City storms to develop over the hot plains of the valley of as observedfrom Tacubaya (urbanSite) for theperiod 1923-58 Mexico, producing only a trace of precipitation at the (Jauregui, I960) surface because of the rate of extremely high evapora Duration Year tion in JFMAMJJASOND the hot dry air beneath the cloud. The down One hour 7 9 13 10 7 7 3 1 1 2 3 5 68 draft winds associated with the storm stir up huge Three hours clouds of dust lasting several hours. ormore 45 7422100012 28 Once the dust cloud is formed it usually drifts across the urban area fromNE to SW. Visibility may When they are not produced by dusty winds associ be restricted to near zero for the firsthour and airport ated with the arrival of a cold front, duststorms form operations must be paralized during this period. The in the valley of Mexico mainly during the afternoon little rain that reaches the ground from the high cloud and last from one hour to five hours. Ta bases has collected so much dust in its fall that this may up ble XIII shows the frequency (and corresponding is named ,mud rainc Mexico City precipitation by wind of duststorms as observed dwellers. average intensity) from the airport, located on the shores of Lake Tex As the leading edge of the dust wall moves across coco, for different hours of the day. In the middle of the city from theNorth or East it loses strength by the dry season more than 70 percent of these dusty deposition and in some cases the dusty air does not winds occur between 3 to 8 P. M., the average wind reach the southern sector of town. Figure 10 shows the velocity being 15 to 20 mph. mean monthly amount of dustfall for the year 1959 as measured by Bravo and Baez (1960). It is evident 7. The Heat Island. Many authors have investigated that the North and East fringes are the most affected in a great number of cities the so-called urban heat by these phenomena. island effect resulting from the contrasting heat re occur Duststorms mainly between January and sponses of city/rural surfaces. In order to study this a measurements May during the dry season, with the greatest number phenomenon series of temperature - in February April as shown in table XII. The were carried out by the author. Between October 1968 yearly occurrence of blowing dust is similar to that and February 1969, traverseswere made in clear calm observed in other semiarid regions of the world like nights, with a psychrometer attached to the right Kasakhstan in the virgin land region of the Soviet windshield wiper of a back-motor car at about one Ernesto Jauregui: The urban climate of Mexico City 305

Table XIII: Frequency of duststorms and associated wind velocity(in mph)for differenthours, as observedfrom the Airport(in percent) (Jauregui, 1971) (lowernumber is average wind speed)

Period ofDay JFMAMJJ AS OND 12 to 14 7 4 20 3 18 2 - - - - 16 35 15 23 21 15 21 5 - - - - 19 21 15 to 17 29 39 31 52 46 37 - 100 100 - 54 48 13 18 17 19 19 17 - 18 16 - 18 20 18 to 20 15 53 40 33 29 60 100 - - - 19 3 14 15 17 15 18 15 - - - - 11 10 21 to 06 10 4 6 12 7 - - - - - 9 10 11 98 13 5- - - - - 7 10 06 to 11 __3______23 25 30 20

meter above the street during approximately two were secure hours. Speeds always sufficient to ade quate ventilation of the instrument.An example of the two dimensional form of Mexico City's heat island is shown in fig. 11 illustrating the conditions at - 04: 15 06: 15 A. M. on 23 with clear km February 1969, ^\ 0_5 10 area skies and wind calm. The urban of the capital_&_S_I under these conditions a well marked tem produces Fig. 12: Minimum temperature distribution for 8 February perature gradient. A slight displacement of the heat 1972 (Deg. C) island towards the SW is observed. Towards theWest At edge of the city cold air drains downslope from the open country hills into the urban fringes and a slight 6?-,-,-1-1 sharpening of the edge of the heat island is observed there. Due to the great extension of Mexico City's urban area, itwas only possible, with one vehicle, to survey the heat island as far as the suburbs. Fortunately how ever, the climatological network extends further away

ft 2?-^-^S? -o-o

0J-1-U0 2 U 6m/s 13: Fig. Urban/country temperature contrast and intensity of synoptic wind inMexico City

to the neighbouring rural areas. Using data from these stations, isotherms of minimum temperature were also drawn under different meteorological conditions to show the heat island effect. Fig. 12 illustrates condi tions for the clear, calm morning of February 8 1972. City/rural temperature differences appear more marked in this example and the temperature gradient ismore strong on the urban fringes at the foot of the hills the West and North where cold air flows

downslope. An intense surface temperature inversion (3?) was present in this particular night; in other cases when no inversion was observed, the heat island was 0_5_10S km found to be most likely weak. On the other hand, _?_S_ geostrophic wind speeds greater than 3-4 m/s near the Fig. 11: Temperature distribution in Mexico City on 23 Fe ground prevent the formation of the heat island bruary 1969 between 04:15 and 06:15 A.M. (fig. 13). 306 Erdkunde Band XXVII

8. The Urban Rain Island. Apparent urban-produced precipitation increases have been studied by many authors (Kratzer, 1954, Emonds, 1954, Eriksen, 1964, Changnon, 1970, Dettviller, 1970). The major potential effectsmentioned in the literature are increased convection from added heat, added updraft motions from friction barrier effects and added freez ing and condensation nuclei. Although only the oro graphic effect on precipitation is evident for the annual distribution of rainfall in Mexico City (see fig. 4), a marked high area of precipitation is often found for individual storms over the capital during the rainy season. Figure 14 shows the distribution of rainfall for July 26 1971. This map was constructed from the daily rainfall figures (during the 24-hour period ending at 8 A. M.) at 33 urban and suburban stations. The observed distribution suggests that heavy precipi tation over the central urban area could be initiated by either (or both) the higher temperature or the added updraft motions generated by the city. In some cases the area of maximum rainfall is diplaced to the South or West by the prevailing Easterly or North Fig. 15: Average annual number of days with thunderstorm easterly current. At other times the rain island may

town. As well developed cumulus embedded in the Trade current approach the limits of the urban area from the East during the rainy season, they encounter additional updraft currents originated by the city. As a result, thunderstorms develop quite frequently along the eastern fringes as can be seen in figure 15 where the distribution of average annual number of thunder storm days has been plotted.

Literature

Bravo, H., Baez, A.: Variations of pollutants in Mexico City's atmosphere, Journ. Air Pollution Control Ass., Vol. 10 (6). 1960. - : Estudio del polvo depositado por gravedad en la ciudad de Mexico, Ingenieria Quimica, Mexico 1962. Bravo, H., Viniegra, G.: The sulphur dioxide horizontal distribution in Mexico City, Int. Air. Pollution Control Ass. Symposium, London 1966. ' Chandler, T. J.: The climate of London, Hutchinson 1965. 10km ^\ 0_5 Changnon, S. A.: The La Porte weather anomally, Bull. _&_s_ Amer. Meteor. Soc, Vol. 49, 1968, p. 4-11. B.: A of the New Fig. 14: Distribution of 24-hour rainfall for 26 July 1971 Davidson, summary York urban air (mm) pollution dynamics research program, J. Air Poll. C. A., 17, 1967.

Dettviller, J.: Incidence possible de l'activite industrielle drift so far to the West that the urban effect can sur les a Paris, Urban climates, W. M. O., hardly be distinguished from the orographic uplifting T.N. 108, 1970. themountains to theWest and South. en by Dominguez, E.: Elementos de previsi6n a corto plazo la The effect of the is also evident when city average Republica Mexicana, Veracruz 1940. of with more than 20 mm rain in frequency days Eriksen, W.: Beitrage zum Stadtklima von Kiel, Schriften 24 hr over a of several is for the period years plotted des Geographischen Instituts der Universitat Kiel, Vol. urban area; a maximum appears near the old center of 22 (1), 1964. Klaus Rother: Stand, Auswirkungen und Aufgaben der chilenischen Agrarreform 307

H.: Das Bonner zur Emonds, Stadtklima, Arbeiten Rheini la ciudad de Mexico, Ing. Hidr. en Mexico, Vol. 23 (1), schen Landeskunde, Vol. 7, Bonn 1954, Selbstverlag, 1969. Institut Universitat Bonn. ? : Geographisches La erosion eolica en los suelos vecinos al lago de Tex Hidraulica en 25 1971. Gab, G.: del clima de la ciudad de coco, Ing. Mexico, Vol. (2), Investigaciones Puebla, ? :Meso-microclima de la ciudad de Instituto de Comunicaciones, Nr. 2; 25-40, Puebla 1970. Mexico, Geografia, UN AM, Imprenta Universitaria 1971a. I.: La en ano Galindo, radiacion solar Mexico durante el ? : Variaciones de largo periodo de los tipos de tiempo de Geofisico Internacional, Instituto de Geofisica, UNAM superficie en Mexico, Boletin No. 4, Instituto de 1962. Geogra fia UNAM, Imprenta Universitaria 1971b. L.: Radiacion solar en Gorczynski, Tacubaya, segun las D.: Klaus, Zusammenhange zwischen Wetterlagenhaufig mediciones pirheliometricas desde 1911, Folletos Serv. keit und Niederschlagsverteilung im Zentralmexikani Meteor. Mexicano, S. A. G. 1932, numeros 1-4. schen Hochland, Erdkunde, B. 25, Lfg. 2; 81-90, Bonn Harris, E.: A desert thunderstorm stikes El Paso, Texas, 1971. Weatherwise, 12 1959, 115-116. (3), Kratzer, A.: Das Stadtklima, Braunschweig 1956, F. Vie Hamming, W., Macphee, R.: Relationship of nitrogen weg. auto to oxides in exhaust eye irritation, Atmospheric Krumm, W. R.: On the causes of downdrafts from thunder Vol. 577-584. Environment, 1, 1967, storms over the plateau area of the United States, Bull. American Meteor. Soc, Vol. 35 1954. Hill, H.: Temperature variability and synoptic cold fronts (3), in the winter climate of Mexico, McGill Univ., Climato Lauer, W.: Naturwissenschaftliche Arbeiten im Rahmen Series des logical 4, Montreal 1969. Mexiko-Projekts der Deutschen Forschungsgemein schaft, Deutsche in der Welt Jauregui, E.: El aumento de la turbiedad del aire en la geographische Forschung von Heute, Verlag F. Hirt, S. 29-38, Kiel 1970. ciudad de Mexico, Ing. Hidraulica en Mexico, Vol. 12 (3), 1958. Marquez, E.: Estado actual de la contaminacion del aire - : en la ciudad de Las tolvaneras del valle de Mexico, Ing. Hidraulica en Mexico, Salud Publica de Mexico, Vol. Mexico, Vol. 14 (2), 1960. 11 (2), 1969. - : L'erosion eolienne dans la vallee de Mexico, Cahiers des Mosino, P., Garcia, E.: Evaluacion de la seuia intraesti Paris 1962. val en ingenieurs agronomes, Sept. Mexico, Escuela Nacional de Agricultura, Cha - : Aspectos meteorologicos de la contaminacion del aire en pingo, Serie 6, 1968.

STAND, AUSWIRKUNGEN UND AUFGABEN DER CHILENISCHEN AGRARREFORM

Beobachtungen in der nordlichen Langssenke Mittel chiles

Mit 5 Abbildungen, 4 Photos und 3 Tabellen

Klaus Rother

The current tasks and effects of Summary: position, Seit kurzem ist das interdisziplinare Gemeinschafts reform in Chile. Field observations in the northern agrarian projekt der Arbeitsgemeinschaft Deutsche Lateiname Valle Longitudinal of central Chile. rikaforschung (ADLAF) ?Entwicklungsprobleme im This paper, based on fieldwork in 1972, describes the aufiertropischen Lateinamerika in historischer, geo characteristics and of the reform in position programme graphischer und regionalpolitischer Sieht" im Gange, Chile and stresses the differences between the terms typical das von der Stiftung finanziell of office of the Frei Volkswagenwerk ge and Allende governments. The author tragenwird. An ihm arbeiten die Lateinamerikanische then the effects of measures on the investigates legislative und IberischeAbteilung des Historischen Seminars der rural landscape of the northern Valle of cen Universitat Longitudinal Koln, das Geographische Institut der Uni tral Chile and concludes that: versitat Bonn (Lehrstuhl: Prof. Dr. W. Lauer) und das the and 1) ownership field patterns have largely survived der Forschungsinstitut Friedrich-Ebert-Stiftung in the introduction of communal ownership, Bonn-Bad Godesberg zusammen. In einer ?Modellstu the is dominant in farm 2) co-operative principle operation. die Chile" sollen spezielle unter In are Entwicklungsprobleme many co-operatives there clear tendencies to de dem des Aspekt Stadt-Land-Gefalles dargestellt wer marcation of used in order to work individually plots den. Die Forschungsschwerpunkte der Geo and beteiligten independently profitably, graphen, die in diesem Land z. T. grofiereVorarbeiten 3) land use has become more intensified than was regionally geleistet haben (s. Bahr 1972, S. 283), sind 1. die Mi the case before but stock has reform, farming stagnated, grationsprobleme als Folge von division in the social structure of areas Standortverlagerungen 4) rural have be des Bergbaus im Grofien 2. die come Norden, Wandlungen deeper, der Sozial- und Siedlungsstruktur im siidchilenischen scattered settlement and small 5) hamlets have become Seengebiet seit dem Abschlufi der denser. Rodungskolonisa tion und 3. die Probleme des Kleineigentums in der In conclusion, the future tasks of Chile in relation to the Zentralzone im Zusammenhang mit Landflucht und overall situation in the are discussed. country Verstadterung und die Agrarreform. Zu diesem letz