Climate of Mexico-1929

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w. B.No.999Issued January, 1930UNITED STATES DEPARTMENT O F AGRICULTURE

WEATHER BUREAU

MONTHLYWEATHER REVIEW[Supplement No. 331

CLIMATE OF MEXICOBY

JOHN L. PAGE

Submitted fo r publication November 14, 929

UNITED STATESGOVERNMENT PRINTING OFFICE

WASHINGTON: 1980

'O'Baleby


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NaticsnsaB Oceanic and Atmospheric Administration

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CLIMATE OF MEXICOCONTENTS

PREFACEThe climatic conditions of Mexico are little knownbecause of the meager amount of published material onthis subject. The purpose of this SUPPLEMENTs toPortray and interpret various phases of Mexican climate.

No attempt, however, is made to cover in detail everyPhase of the climate of Mexico.The data upon which this study is based are in the formof (1) instrumental observations made by the observerst h o u hout the country, compiled at the Central Meteoro-logicaf Observatory at Tacubaya, Mexico, and (2 )Published descriptions of some phases of the climate.All available instrumental data of most elements forstations of more than three years, and some cases, wherestations were widely separated, records of three years orless, were tabulated b months, reduced to averages, andused in this study. &e number of stations and length ofPears of records differs with the climatic elements. Forrainfall, days with rainfall, and maximum rainfall in 24hours 220 stations, varying in length from 3 to 50 years,Were used. For temperature and days with frost thenumber was 130 and the length from3 to 50 years, though,1n general, the records were shorter and more broken thanfpr rainfall. The records for the other elements as rela-b e humidity, hours of sunshine, days with thunder-storms, cloudiness, cloudy, partly cloudy, and clear days,Pressure and wind directionwero for only about 45 stationsand length of years somewhat less than for temperature.Only in case of the available records for pressure andwhd direction was a uniform period of years used, thatof 1921-1927, and in these elements no adjustment wasQade of the shorter to the lsng?r stations.Th e Bigelow tables for reducmg pressure readin s togreat probability of inaccuracy 111 reducing these pressurereadings to sea level a few isobars were dashed where thedid not seem in accord with general conditions.Only average conditions were mapped, because therecords a t many stations are short and recorded extremeswould very likely be far from what the extremes would befo r a long period of years. Some maps had to be general-'zed and lines drawn on the basis of information otherthan instrumental records.The metric system has been used throughout thisbecause it is the system of the country and conse-quently the system in which the records were found.In the preparation of this SUPPLEMENTcknowledg-'?ents are made to Charles F. Brooks, under whose direc-"09 it was written, for many helpful suggestions andCrltlcisrns both as to content and method of treatment88 well making it possible to secure the data from

Sea level were used in this study. And because of the

Mexico; to Sefior Jose C. Gomez, chief of the CentralMeteorological Observatory at Tacuba a, for makingrecords readlly accessible during a perioiof two monthsand for invaluable assistance in havmg a large part of thedata complle ; o the observers in many parts of theweather in their locality; and to Clark University for afellowship through the academic year 1928-29.country for fetters describing various phases of the

I N T R O D U C T I O NAn adequate understanding of the climate of anycountry necessitates,. first of d, consideration of themajor controls, for it is these that explain why the climateof one localit differs from that of another region, near byor remote. 6h us the major factors that control the cli-

mate of Mexico as well as other regions are: (1)Latitude;(2 ) land and water, including ocean currents; (3 ) elevationand exposure; (4) prevailing winds, and (5) storms.As the sun is obviously the control of all climate, andas the. amount of solar heat received at a given localitydepends upon the length of the day and the angle atwhich the sun's rays strike the earth, a consideration ofthe lat itude of the region must be given first place in thestudy of the climatic controls, even though it may notbe the dominant one.Mexico, lying as it does between 14' and 32' 40'N. latit,ude, should, as far as i ts latitude is concerned,be a count with a tropical and subtropical climate.than might be supposed since most of the country ISat an elevation sufficiently high to counteract the effectof latitude. Extending over but 18' of latitude, there isbut little difference in the length of the day between themost northern and most southern parts-the longest dayin northern Mexico being only 1 hour and 15 minutesmore than the longest day in sodthem Mexico-and butslight difference between the annual temperature of thetwo sections. Only in winter is the north-south tempera-tiire gradient pronounced. In general, south of theTropic of Cancer the month of maximum temperatureis before June, whereas to the north of it the maximum ISnot recorded until June, July, -4ugust, or even September.The latitude of Mexico is also an important factor inthe distribution of pressure and consequently. the occur-rence of storms and t,he amount and distrlbutlon of rain-fall. The northern part of the country 1s.far enoughnorth to come under the influence of cyclonic storms inwinter, and the southern part far enough south to bewithin the doldrum belt in summer. Nearly all parts ofMexico may be lashed in the fall of the year by tropicalhurricanes, And only few sections are exempt from thefury of the norther in winter. The more northerly sec-tions, Lower California, Sonora, and parts of Chihuahuaand Coahuila, lie within t$e belt of subtropical calmsand are normally dry, while the southern sections lyingwithin the low-pressure belt in summer are excessivelywet and humid.The second major control of the climate of Mexico isthe distribution and temperature of the water bodieswhich bathe its shores. These water bodies arc Gulfof Mexico, Caribbean Sea, Gulf of Tehuantepec, Pacific

Yet its low7 titude has less effect upon its temperature

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2 SUPPLEMENT NO . 33Ocean, Gulf of California, and, locally, lakes and marshes.To the difference in temperature of these bodies may beattributed some curious anomalies of climate. Thusthe west coast of Mexico, excepting Lower California,has a higher temperature than the Gulf coast a t the samelatitude. Since the prevailing winds are on-shore atall places the difference must be explained chiefly onthe basis of the surface temperatures of the water bodieswhich bathe these shores. The Satavento coast of VeraCruz is much more refreshing than the scorching Valley

retardation of the months of maximum and minimumtemperature, so typical of a marine climate, is beexemplified along the west coast.Though the surrounding water bodies have but littlinfluence upon the temperature of Mexico, they havhowever, by supplying an abundant quantity of watevapor a very profound influence upon the rainfall. ThGulf of Mexico, especially, serves Mexico well in thregard. For not only are the mountains drenched witrain as the moisture-laden winds are forced upward ov

FIGURE.-Relief map of Mexicoof Tehuantepec on the south, for the former enjoys seabreezes from the relatively cool Gulf of Mexico, whereasthe latter is scorched by descending winds from themountains. And the Peninsula of Lower California,though bordered on three sides by water, remains arid,hecause the wind blowing on-shore from the cool Cali-fornia current rarely is forced to give up its moisture.Only along the lowlands of the coast does the marineinfluence upon the climate make itself particularly felt,for the mountains rise so rapidly and precipitously.Thus at only a few coastal stations can the effect of thesea in reducing the annual temperature be studied. The

their flanks, bu t sufficient moisture is left to make thcentral valleys within well watered and fertile.Indeed, it is to its mountains and elevation that Mexicowes Its salvation. For mountains and high plateautransform much of Mexico from a tropical to a trultemperate country, and changes regions tha t might havbeen almost waterless to regions of copious rainfelObserve Figure 1, a relief map of Mexico. Innumerablvariations in relief cause numerous local variations iclimate. Thus Mexico City has a mean tem erature inmonths about 10' C. lower than Vera &z, but 2Tmiles to the east; a north wind a t Puerto, Mexico,


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CLIMATE O F MEXICO 3winter is cool and humid, but in the Valley of Tehuan-tepec it is dry and scorching; the west coast may bebasking in the warm sun, while the Gulf coast is expen-encing the biting fury of a norther; and four times asmuch rain falls a t Coatepec, Vera Cruz, as at Perote inthe same State only 20 miles away. From tropical heatto arctic cold is merely a matter of a few hours travel.

During some of the year all of Mexico is in the so-called belt of northeast trades, yet many parts of the

hurricane moving westward and northward from theCarribbean Sea may lash the southern part of the countrywith violent wind and rain.PRESSURE WINDS AND STORMS

Pressure differences being the cause of all wind move-ment, whether It be gentle zephyr, cyclonic storm, tropicalhurricane, or tornado, it is not unnatural that a study ofthis pressure element should precede that of winds and

cOlmtry have prevailing northeast winds a t no time duringthe year. Here again mountain barriers cut off or deflect;the wind into directions as varied as the topographyItself. Along the Gulf coast, however, the winds arehortheast and east for much of the year, and exert a mn-Qderable influence on the climate of the coast as well as

storms. The latitude of Mexico places it, in part, in thesubtropical high-pressure belt but largely in the so-callednortheast trade wind belt. As has already been pointedout the more northerly part of the country comes underthe influence of the westerly winds and their associatedcyclonic storms during the winter season, while the more

FI QU R E.-AVerage sea -lev el pressure, Aprilthe east,ern and northern slopes of the Sierra MadreOriental. Along the west coast westerly winds exert aQRrinc influence along the lowlands, and bring rain ashe rise along the slopes of the western mountains.$nally, under the influence of storms, extratropical ortropical, the normal effect of the other climatic controlsOf little avail to the Gulf coast is itsUbtropical latitude when ravaged by a norther. Awinter cyclone moving far south of Its normal course maydrQch the arid regions of northern Mexico. Or a

be obscured,

FIGWEE.-AV0rsge sea -lev el pressure, Octobersoutherly part is dominated by the tropical low-pressurebelt in summer.Sea-level pressure maps for Mexico (figs. 2-5) show ageneral decrease in pressure from northeast to southwest.The pressure gradient 1s strongest in January, when 5 to7 millimeters difference 1s foynd between the Gulf andPacific coasts. (Fig. 2 .) Ths general January pressuredistribution is to be expected when one considers thelower temperatures along the Gulf than along the Pacific,a fact attributable to the lower trade wind latitude of the


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4 SUPPLEMENT NO. 3latte r and the free exposure of thqformer t o cold invasionsfrom the United States and Canada. The higher pressurealong the Gulf is further enhanced by the precipitousmountain wall of the Sierra Madre Oriental which actsas a barrier to the free movement of the northeast windsJus t to the west of the Sierra Madre Oriental in January

pressure gradient over Mexico is from northeast to southwest with a slight alteration of the pattern from seasonto season.Much of Mexico being a plateau of considerable elevation, pressure reduction to sea level chan es the generastations. This fact plus the effect of very diverse loca

and causes a piling up ?f these winds along the coast. plan of pressure from that a t the actual e!vation of the

Frona~ ).--Prevailing win ds for Januaryan area of relatively low pressure is found, while fartherwest is again a ridge of h ghe r pressure.By April (fig. 3) pressure throughout the country islower. The pressure gradient is still, in general, fromnortheast to southwest, but much weakened. Moderatelyhigh pressure covers the middle and southern Gulf coast,Yucatan, the southern plateau, and northern Pacific coast,". .. I 1

I I I" .. !.----I-.&---Jlwhile the northern plateau has become an area of rela-tively low pressure.With the coming of summer and the rains (fig.4, uly)Yucatan and the southeastern portion of the Mexicanlateau have become the areas of high pressure, while thegacific coast and northwestern Mexico remain as areasof low pressure.In October the southeastern part of the plateau con-tinues as an area of high pressure, while the general pres-sure gradient is still from northeast to southwest. (Fig.5 . ) Thus throughout the year the general sea-level

. r -FIOURE.-Prevailing wind s for April

FIOURE.-Prevailing win ds for Julytopography often causes the prevailing wind direction tbe different from that indicated by the sea-level pressmmaps. Compare Figures 2-9. The direction of the surface winds is not determined by the sea-level pressure distribution but by general pressure distribution a t the leveof the vanous stations and by the local topographYNothing in particular can be said about the general mn d

FIOURE.-Prevailing winds for Octoberdirection except that along the Pacific coast the winds fothe most part are on-shore all year. The sea breeze, ODof the saving graces of the Tropics, is perhaps largelrespon$ble for these prevailing on-shore wind conditloasExceptions-to this may be found at stations around tbGulf of California where the direction varies from montto month, and the prevailing offshore wind around thGulf of Tohuantepec. This offshore wind is a result Othe general pressure gradient from northeast to southwest, the gradiest being stronger in this region than elsewhere ;S Mexico, and to the geserally low elevation O


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CLIMATE OF MEXICO 5the Isthmus of Tehuantepec which permits the northeasth d s o cross without much diaculty.On the Gulf coast the prevailing winds are more inaccord with general pressure conditions than in anyother par t of the country. The subtropical Atlantichgh-pressure area, an arm of which extends over theQulf, produces northeast winds all along the Gulf coast.In the plateau section of Mexico the wind direction isdetermined by local pressure distribution and relief.Consequently marked differences in direction prevailat different places not far distant. Compare Toluca andTacubaya fo r January. (Fig. 6.) Tacubaya has aPrevailing north-northwest and Toluca a south-southeastMind.The eneral distribution of pressure over North Americaand a$jacent oceans has a profound influence on theweather and climate of Mexico. Lopez, who has probablybeen the greatest contributor to Mexican climatology,?rites th at the weather of the Republic of Mexico18 controlled by (1) the center of action of the Pacific ofthe north, (2 ) the equatorial depression, and (3 ) the centerof action of the Atlantic of the north. His treatment ofFeather controls is so valuable that a part of his discus-Qon, as translated by the writer, will be presented here.

The intensity and position of the center of the Atlan tic highressure exercises a marked influence on the weather of the country .B ring th e winter season if t he pressure over the Atlantic is abovenormal the weather is hot and rainy ip the.Republic * * * Inthe summer these same conditions give rise to good weather andincrease of temperature. On th e other hand if the pressure over the%ter [Atlantic] is below normal in winter, aold, dry weather isthe result; and a summer with these same pressure conditions isscold one. * * *The relation between the center of low pressure in Alaska andthe high over Hawaii demonstrates th at a n accentuated depressionin Alaska translates itself into a high in Hawaii and shortly after-bards r ainy weather in th e Republic [Mexico]. On the contrary ahlgh in Alaska coincides with a depression in Hawaii which presentsitself after wards on the west coast of the United States followed byheat and good weather in our country [Mexico]. * * *The long dry an d hot periods of weather which are so damagingto agriculture are governed by a ype of weather which frequentlyPresents itself during the months of April and May. This is char-a&rized by a center of low pressure of semistable character, ofvery moderate intensity, and whose center is s ituated little t o t hehorth of the frontier with th e United States, embracing in it s areal1 or almost all of the Republic. In this case the country is situ-ated to the south or southwest of th e center and therefore th e windsare dry an d hot, a nd blow from directions between south and west.I n Mexico the frosts and cold waves are generally the result ofQticyclones which descend from the territory of the United Statesat the back of a cyclone which has crossed th e country. ThePremature fr osts of the autumn, however, are caused by a moderate%yclone which develops by radiation in the Mesata, persistingthere many days with lit tle movement (48, p . 136-1 37).. Perhaps one of the most interesting as well as mostlaportant phenomenon in Mexican weather is the strong,old, and often humid wind which blows in winter,on the Gulf coastal region. This wind iseferred to in Mexican literature as. el Norte. I thas been the object of study and investigation by variousPeople. Lopez likens it to the Mistral of France and thebora of the Dalmatian coast (60, . 91). It often makesitself fel t in its low temperatures and rainfall in Yucatan,Iabasco, and Vera Cruz. At times its effect is also feltthe Valley of Tehuantepec, not a s a cold moist wind,bW as a foehn with all the features that characterizefoehn of the Swiss Valleys of the Alps (60,p . sa).w. . Redfield earl maintained that northers aroundTa5pico and Vera &uz were always accompanied byQ?lcrtnes or rather were the results of hurricanes (2).It 18 now generally known, however, that a norther IS% necessarilv the result of a hurricane but mav Drevail

when there is a widespread barometric depression overthe western part of the Gulf of Mexico or Caribbean Searegion, or when a well-developed cyclone moves acrossthe northern part of Mexico with a pronounced anti-cyclone over the north central part of the United States.Jose Guznikn recognizes the following conditions asnecessary for northers on the Gulf coast: (1) Barometricdepression on the coast of the Gulf of Mexico, (2) hightemperature in this region, (3 ) southerly winds in thesame, and (4) anticyclone in or very near the RockyMountains of considerable intensity with relation to thedepression on the coasts (60, p . 94).According to Batturoni the northers often reachFrontera before they reach Vera Cruz or even Tampico.He attributes this fact to the less resistence between thewind and water than between the wind and land. As anexample he cites the occasion of the heavy norther ofFebruary 7-9, 1892, in whlch he says:He saw the cloud bank in the north and east on the 8th; therecords showed that the norther had begun at Frontera on th emorning of the 7th (200 miles east-southeast of Vera Cruz) andthat it began at Tampico about 4 a. m. of February 9 (200 milesnorth-northwest of Vera Crus) but did not begin at the la tter placeuntil 8.30 a. m. of th e night 9th ( 1 ) .Batturoni distinguishes two classes of northers, eachhaving its own characteristics: ( 1 ) Northers that comefrom the TJnited States south to Vera Crux, and (2)northers that begin and die away on the Gulf coast ofMexico ( I ) .As examples of the first class he cites the ones of Feb-ruary 7-9, 1892, while the following northers illustratethe second class: February 9, 1892, and September 23-25,1892 (1).Finally, Mexico is frequently beset in summer and fallby the tropical hurricane. The warm waters of the Car-ibbean Sea and the Pacific adjacent to the coast ofMexico are favorable to the formation and growth ofthese tropical storms. The chief of the forecast divisionof the Mexican Meteorological Service, Senor Pablo V ~ S -quez Schiaffino, recognizes the following five classes oftropical hurricanes that effect the weather of Memco.(1) Antillian hurricanes of normal course which either curvetoward the northes t in the lati tude of Florida or in the easternportion of th e Gulf of Mexico.(2) Cyclones whioh originate in the Caribbean Sea or the theeast * * * take a course generally toward the west crosslngthe Peninsula of Yucatan and she Gulf of Mexico only to recurvein t h e western Dortion of the same or enter the Mexican territory_ __._.on the coast of Ithe State of]Tamaulipas.(3) Cyclones which originating in the Caribbean Sea or to theeast, follow a normal course and afte r crossing the southern pa rt ofthe Peninsula of Yucatan continue moving toward the west, pass-ing the Gulf of Campeohe and penetrating the Republlc between18 an d 20 of nor th l ati tude, conserving sufficient energy t o crossth e Mexican Mesata a nd aDDear on th e Paclfio where they dlsappear(4)- Cyclones which have their origin in the Caribbean Sea or tothe east, and take a similar cour8e as (3) except that, after theycross Centra l America and pass to t he Pacificlocean th ey pursue anorthwest course, recurving between 18 and 22 north latitude tonenetwte the Gulf of California or strike again@ th e west slope of------- -- -the country where they dissolve themselves. In this group mayalso be included the c clones originating in the. waters of th e Pacific,to the south of the 6ulf of Tehuantepec or in more westerly lon-gitudes.(5 ) Cyclones of similar origin as he above which cross the Mexi-can Mesata from southwest t o northzast appear in the Gulf ofMexico and move toward Florida (61).A map showing the c o m e of these five classes appearsas Figure 10.

,Translatedbv he writer.


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6 SUPPLEMENT NO. 33Tropical cyclones in the Pacific waters of Mexico havebeen the subject of recent investigation by Willis EdwinHurd. To quote him:[An average of five cyclones annualy have been recorded duringthe last 19 years, with a n extreme number of 13 for a single season.One has occurred in May.] There is an average occurrence for Juneof a trifle more than one in two years. I n July and August there is

a slight increase in numbers and activity , but Sep tember is pre-eminently the month of most frequent storminess of this character,abo ut two-fifths of th e entire number of cyclonic disturbancesoccurring then. In October they wane appreciably in frequency,though gaining slightly in intensity, and November adds only anoccasional outburst-a violent one more often th an otherwise-toclose the activities of the season (41, p . 43).Hurd classifies the cyclones of this region as follows:(1) The coastwise storms, (2) cyclones that run perpen-dicular to the coast, (3 ) cyclones of the RevillagigedoIslands, and (4)cyclones west of the one hundred andtwenty-fifth meridian (41, p . 46).

TEMPERATUREAlthough lying largely within the Tropical Zone,

Mexico presents great diversity in its temperature. It isfrequently thought of as a hot country because of its lati-

FIQUREO.-Tracks of tropical cyclonestude. Mexico in reality is transformed from a tropicalto a truly temperate country by its altitude.Mexico offers practically all types of climate from thepermanently snow-capped volcanic peaks to the regionsof tropical palms on the lowlands of the Pacific and Gulfcoasts. It is only a matter of hours in going from oneextreme to the other, for on clear days one can stand on apermanently snow-capped peak and view the tropicalpalms and wild bananas on the lowlands not far away.Most of Mexico falls within this Temperate Zone.One need only glance at a temperature map to note themarked influence of alt itude on temperature in Mexico.The average vertical temperature gradient betweenVera Cruz and Mexico City is 1' C. per 200 meters andmuch of Mexico is above 2,000 meters, an altitudesufficient to cause a markedly lower temperature on theplateau than on the coastal lowlands. The effect ofaltitude is well seen in Figure 11,which shows the annualcourse of temperature for a Pacific coast station (Man-zanillo, Colima, altitude 4 meters), a Gulf coast station(Vera Cruz, altitude 16 meters), and a plateau station(Mexico City, altitude 2,250 meters), all in approximatelythe same latitude,

In climatological literature Mexico is often referred tas having three thermal regions: Tierra caliente or holand, tierra templada or temperate land, and tierra fria ocold land. Figure 12 shows the areas represented beach of these thermal regions. In delimiting these regionthe same degree of temperature was used as that employeby Hernandez (36,p . 1). The tierra caliente is that paof Mexico which has an average annual temperature abov22.5' C. It is restricted to the lowland section of thcountry. This region has as its boundaries altitude linrather than latitude lines, though latitude, exposure, ancool water of the Pacific along the west coast of LowCalifornia assert themselves in the northern part of th

FIGURE1.-Mean monthl y temperatures for Vera Cruz, altitude 16 meters; MexCity, altitude 2,ZM)meters, and Manzanillo, altitu de 4 meterscountry. The tierra templada is tha t par t of Mexiowhere the temperature is between 15' and 22.5', anddetermined more by altitude than by latitude. Thregion embraces the major portion of the countgpractically all of the plateau and most of Lower CaliforplThe tierra fria is restricted to the higher mountain sectloaoof Mexico, where the annual temperature is below 15some areas of which do not appear on the map becausof their very small size or the absence of data and thlack of a good relief map.For equal latitude the Pacific coast of Mexico, neglecing Lower Cslliforniq, is warmer than the Gylf coas


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CLIMATE OF MEXICO 7The curves for the coastal stations of Manzanillo andVera Cruz show that during all months except March,April, and May the Pacific coast station is warmer thanthe Gulf coast station. (Fig. 11.) This is true especiallyin winter when the northers bring low temperatures toall of the Gulf coastal region, a phenomenon which rarelyoccurs on the west coast and never so severe in the latteras in the former. To this may be added the further in-fluence of the warmer Pacific than the Gulf waters, withPrevailing on-shore winds in both cases. The water ofthe Pacific along the steamship route from 15' to 22' N.latitude has a mean temperature at a depth of 7 meters of29' C. for January, March, June, August, and November,

country the decrease northward along the plateau islessened by the decrease in elevation toward the north.The greater aridity in the northern part also permits moreinsolation, less evaporation, and consequently highertemperatures in summer, while a considerable amount ofcloudiness in winter attending extratropical cyclonesprevents the temperature from getting so low as it wouldotherwlse do. The absolute maxima as well as the abso-lute minima temperatures for Mexico as a whole occur inthe northern art of the country.from either the Gulf or the Pacific coast to the plateau.A trip by train from Vera Cruz to Mexico City in summer

Pronounce p temperature gradients are found in going-

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T I E R R A F R I AT l E R R A T E M P L Ar l E R R A C A L I E N

FIGURE2.-ThermaI regions1927 3 and the average temperature of the surface water?f the Gulf of Mexico for the same months for 1920-19271s 26O.4A few generalizations may be .made regarding annualtemperature of Mexico. There 1s a slight temperaturedecrease from south to north. The weak temperatureFadient along the coast is accounted for by the lowatitude of the country and the almost uniform tempera-ture of the waters which bathe these coasts. An excep-tion to this is the transition from warm to cool water along-he west coast of lower California. In the interior of the'Temperatwos recorded by Clark University's sea-water thermograph on the steam'Average o8u rface teniperaturesreported by hips at Greenwich mean noon, courtesyhnland lying between New York and Snn Francisco, 1027.a. Tingley, of U.8.Weather Bureau.f

84137-30-2

surely brings about a realization of this fact. Vera Cruzhas a mean July temperature of 27' C., while n t MexicoCity the mean temperature for the same month is only16.7'. A cornpaxison of temperature and relief mapsrevefLisa close conformity of the two.In January one isotherm cuts th e Pacific coast morethan 5 O farther to the north than I t does the Gulf. (Fig.1 3 . ) In this month the States of .Chihuahua and border-ing sections of Sonora and Coahui!a are the coldest partsof the country, the highest volcanic pealis excepted, witha mean temperature of less than 10' C., while the hottestpart of the country i along the Pacific and Gulf ofTehuantepec coasts w t h *a mean temperature of morethan 25'. The cold area in the north is due to its more


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8 SUPPLEMENTNO. 33northerly location and consequently freer exposure to thecold invasions from the United States, as well as to itselevation, a considerable portion of which is more than5,000 feet. The warm areas in the south are due to theirmore southern location and low elevation, much of onearea being located in the valley of the Rio Balsas whorelittle or no rain falls and bright sunshine prevails d w n gthis month.A comparison of the relief and January temperaturemaps reveals a more dominant altitude than latitude

F IG URE3.-Mean temperature (centigrade) Januarycontrol. I n -'July the al titudeTcontrol5is even more(Fig. 14.) Disregarding Lower Califor-nia, only one isotherm cuts the coast in Mexico and thatis the 30' C. isotherm1in the desert region bordering theGulf of California, the temperature df which is onlyslightly above 30'. Practically all of coastal and low-land Mexico has a July temperature about equal to that ofI ' I

I IF IG UREl.-Mean temperature (centigrade) July

Omaha, Nebr. (24.8') and a considerable portion of theplateau lower than Portland, Me. (20.3'). The recordedextremes of average July temperature are 12.5', at LasVegas, Vera Cruz, and 31.1', at Navajoa, Sonora.An average July temperature chart does not give a truepicture of summer temperature conditions in Mexicosince July is not the hottest month in all parts of thecountry. A map has been constructed showing the aver-

age temperature for the hottest month, which varies fromApril to September. (Fig. 15 . ) The general trend omost of the isotherms is similar to th at of July. Thereare, however, a few significant differences. The area hav-ing a temperature of less than 20' C. is much smallethan in July, and the areas with a temperature below 15'are fewer in number.Because of the higher latitude and arid conditions theaverage annual ranges of temperature are more marked inthe northern than in any other part of Mexico, some othe stations having ranges of almost 20 ' C., while the extreme ranges are more than 4O0-that is the rangebetween the average extreme maxima and the averageextreme minima. I n the south the temperature rangesare small. The most southern station of the countryTapachula, in Chiapas, has only 0.5' difference betweenthe January and July temperatures and at Tonalain the same State, the July and January temperaturesare the same. July, however, is not the month omaximum temperature a t either place. The annuarange is only 2' a t Tapachula and 2.5' at TonalaThrou hout southern lowland Mexico the annual rangeis sm al . The difference between the average extreme

I

maximum and the average extreme minimum at Tapachula is only 18.8", while a t Villa Ahumada in the Stateof Chihuahua is 49.2'. The temperature curves foall parts of Mexico except the desert region show theeffect of the rainy season. They all have a ratherpronounced increase in temperature from Januaryto the starting of the rainy season, then a flatteningout of the curve during the rainy season. Some stationseven .show a decrease in temperature while the seasonof rain persists, with a slight increase in temperatureafterwards, and then a decrease to the winter monthsIt has been shown that there is a considerable differencin the. degree as well as the range of temperature 11the different parts of Mexico. There is likewise adifference in the month in which the maximum temper-ature occurs, April, May, June, July, August, andSeptember all being represented as the month of ma%imum temperature at one or more stations in MexicoThe regron which is hottest in April is small andembraces the most southerly part of the country,is to be expected. The vertical ray of the sun reachesthe southern .part of this region about April 29, andin May cloudiness and rainfall increase, lastmg through-o u t the months which would ordinarily be hottest


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CLIMATE O F MEXICO 9preventing a further rise in temperature. The Apriltemperature is, however, only slightly higher thanthat of May.As the vertical ray of the sun moves northward withthe advance of the season the month of maximum tem-perature becomes later. Most of the country south of theTropic of Cancer is warmer in May than in any othermonth. This May maxlmum is attributable to thealmost vertical ray of the sun and the fact th at the rainyseason has not yet start!d. The warmest month onthe coast in some places rn these latitudes is delayed.June is the hottest month in the north-central partof Mexico, The days are longer during this monththan in any other and it is, in general, the month pre-ceding that of maximum rainfall.A July maximum of temperature is recorded in Sonora,northwestern Chihuahua and Sinaloa. In this area thecloudinegs and rainfall are not sufficient to advance norretard the month of maximum temperature from th at ofa normal continental typc of climate. There is also asmall area in the north and east of the Peninsula of Yuca-tan which records its maximum temperature in July.This delayed maximum is in accord with most coastalregions. The temperature of the Caribbean Sea is 0.2' C.higher in July than in June and 0.2' higher in Augustthan in July, and the temperature of the Gulf of Mexicoas a whole is 0.8O higher in July than in June and 0 . 3 Ohigher in August than in July.'In the remaining part of Mexico, largely coastal sec-tions, the maximum temperature is delayed until Augustor even September. In much of the region of the lowerRio Grande Valley the maximum temperature occurs inAugust, because July has the greater percentage of cloudi-ness, slightly more rainfall, and a greater number of daysWith rainfall.The months of minimum temperature in Mexico areDecember, January, or February, though for most partJanuary is the coldest, A general exception to this is theWest coast where the month of minimum is February.Because of this difference in degree and course of tem-perature through the year the d!vision of Mexico intoeght temperature types or provinces seems called for,each differingessentially from the others in degree, annualmarch of temperature, or both.The delimitation of these provinces .with a compositegraph of average temperature for each IS shown in Figure16. The provinces were selected after careful examina-tlon of graphs for about 100 stations in Mexico and border-ing United States.0 Figures 17-24 are composite raphstremes,' average hours of sunshine per day per month,average number of days frost, and me?n and minimumrelative humidity for each of these provinces, except thatthe niinimum relative humidity is not given for provincesA, 3, and D. These composite graphs are.not intendedto show the exact conditions of any one sta.tipn within theProvince. They present the general conditions through-?ut the region they represent, which after all is the more1QPortant. Hours of sunshine are treated along withtemperatures because temperature is very much depend-ept upon the amount of sunshne the region receives,llkewise relative humidity is added, fo r it fluctuateschiefly with the rise and fall of temperature. Where

of the average monthly means, average monthy ex-

-verage of surface emperatures reported by ships at Greenwich mean noon, courtesyOf p- Q. Tingley of U. . Weather Bureau.th( The avera e temperature the average of the extreme maximurn, and the average ofe extreme mtnimum by mobths, were drawn on aph Paper for the individ ualstations.Were then grouped according o their simfla riies 8 s to degree and courm of tempera-ue through the year' ach value is the average of the indivfdual extreme maximum or minimum tempera-They ell into ei ht distinct regions.'JCcurrlngeach mon th year after year through th e period of record.

possible, the stations used in constructing the tempera-ture graphs were also used in producing the other graphs.The humidity and sunshine diagrams, however, had to bebased on fewer stations than temperature. Some of thelocal differences and of the peculiarities of individualplaces will come out in the discussion of each provincewhich follows.The Lower California province (fig. 16) is the mostunsatisfactorily marked-off province in Mexico becauseof insufficient data, there being no stations in the interiorand none on the west coast sout,h of Ensenada. Thegraphs fo r this.province, therefore, had to be based on afew stations with short records in the northwestern partof the peninsula. (Fig. 17.)The annual range of the monthly maximum is 5.5' C.,while that of the monthly minimum is 1 1 . 2 O , and themean annual range is 8.4'. This region is cool along thewest coast, thanks to the cool water.of the Pacific.The curves of the mean and minimum temperatureshave a very similar course through the year, the meanc, 7-he temperature curves are most peculiar.. 'Lqd- - - - - ' - - . - - ' --p!,--.--L!.-

FIGUREO.-Temperature provin ces, centigradetemperature being about l o o C. above the minimum.August is the warmest month, although only 0.1' warmerthan July. There is a marked increase in the temperaturefrom June to July and a marked decrease from August toSeptember.The highest monthly maximum temperatures come inOctober, though others nearly as high occur in November.Temperatures are prevented from reaching theu highestdegree during the summer months by theFrosts are not frequent n Lower Californla, though theyoccur from November t o March. December and Janu-ary, are, however, the only months which average asmany as one per month.Humidity is high throughout the year according tothe one available station-near by, tha t of sa? Diego,Calif. The humidity evidently decreascs considerablytoward the south and ?way from the c,oast, where thetemperature should be hlgher, rainfall less, and percentageof cloudiness very low.The Gulf of California province (fig. 16 , B) is thedesert of Mexico, a region of. abundant sunshine through-out the year. Some portions receive more than 90per cent of the possible amount for some months. Veryhot days in summer are the rule. There is a sign on oneof the restaurants in Yuma, b i z . , a part of this saine

breeze.


12/32

10 SUPPL EMENT NO. 33region extending into the United States, which reads,Free board every day the sun does not shine. (64,p . 601.)Because of the latitude, aridity, and relatively freeexposure to invasions of cold from the Great Basin of the

three temperature curves show their marked continentacharacter.The highest average as well as the highest extrememaximum temperature occur in July, while the extrememinimum reach their highest degree in August. The

T E M P E R A T U R E

R El

J F M A M J J A S O N DFIQUEE7.-Weathor data for Lower Cnlifornin province

T EM P E RATU R E

FIOURE8.--Weather dnta for Gulf of California provinceUnited States, this province has a considerable annualrange of temperature. (Fig. 18.) The summer days arehot and dry, temperatures of almost 43O C. beinrecorded in May, June, July, August, and September. A/!

lowest temperatures in each case occur in January.Frosfs are common in December and January, &Ddoccasionally in November, February, and March. Theyhave occurred in April.


13/32

CLIMATE OF MEXICO 11No Mexican station in this province records relatiyehumidity, but the Yuma, A r k , station may give a fairidea of the low humidities of the desert. The two monthsin which the maximum occur are those of most rainfall,though there is very little rainfall a t any time.The west coast province (fig. 16 , C) is a st rip along the

Pacific coast, embracing the lowlands from Manzanillo,in Colima, to Guaymas, in.Sonora. It is the one typi-cally marine section of Mgxico, with an August maximumand a February minimum of temperature, and an annualrange of temperature small for its latitude and exposure,although greater than th at of the lowlands farther to thesouth and east. (Fig. 19.) There is very little differ-ence in the July, August, and September temperaturesin either the average or the average of the extremes.The average absolute maximum temperatures are alittle higher in July than in August, a fact due probablyto the more direct rays of the sun during the formermonth, as well as to the fact that August is a cloudierand rainier month in most of the region. The maximumtemperatures are high in all months. Thelowest extrememinimum temperature (the absolute minimum) occurs inFebruary bu t is only 11.3' C., or equal to the mean Janu-ary temperature of LosAngeles. The three temperaturecurves are closest together jn September, the month inwhich cloudiness and relative humidity are at theirmaximum and sunshine a t its minimum. The minimnmtemperatures have a greater and the maximum tempera-tures a smaller annual range that the mean temperatureu1 this province.As in the lowlands to the south and east, the monthwith the maximum hoursof sunshine per day, June, pre-cedes th at of heavy rainfall. But June has only slightlymore than April or May. The sunshine increases afterthe rainy season.Because of the prevailing on-shore winds in all monthsof the year, the *mean reiative humidity is high, withlittle variation from month to month. A minmum isrecorded in April and a maximum in September.Frosts may occur in the-province, but no stationsreported them during the period 1921-1927.The western mountains and foothills province (fig, 6,D) embraces a portion of the Sierra Madre Occidentaland their western foothills. It differs from the westcoast province in that it has a greater average annualrange of temperature, and has a June instead of an Augustmaximum and a January instead of a February minimum.(Pig. 20 . ) It is less continental than the Gulf of Cali-fornia province to the northwest.There is a rather pronounced decreasein mean tempera-ture from June to July with the onset of the rainy season,thou h the temperatures remain quite high throughOcto%er.

October isalso sunny, while December is the least. Frosts rarelyOccur in this region except in the higher parts, wherethey have been reported in the months from Novemberto March, but less than one per month, on the average.There are only few stations in the higher parts of theregion and most of the discussion necessarily had to bebased on records from station in the foothills. Lowhumidity occurs in spring and early summer, with theQcrease in temperature and hlgh humidity in late sum-?ler and fall, when the temperature reaches and passesIts highest degree.The northern plateau province (fig. 16, E) is like theGulf of California province in some respects, but differsenough to warrant a separate classification. It has

The sunniest season is March to June.

There is considerable range in relative humidity.

continental temperature curves like those of the Gulf of

J F

i OF S U N S H I N E P E R DAY

E M PER ATU RE

RELATIVE t-

i M J J A S O N OFIOURE lQ.-We8thW data for W e s t coast province

California, but its temperature is several degrees lowerin both summer and winter, and, because of the greater


14/32

12 SUPPLEMENT NO. 33amount of rainfall and cloudiness in the northern plateauthe temperature curves are flattened out more duringthe rainy season. (Fig. 21.) Another difference is that

H O U R S OF S U N S H I N E PEIO5

1 I 1 I DAYS WITH FROSTI l l 1

J F M A M J J A :

DA Y

O N 0FIQUEE0.-Weather data fo r western mountains and foothill province

the average as well as the maximum temperaturesreachtheir highest degree in June instead of July.This is the coldest movince in Mexico during the

being freely exposed to the invasion of cold winds from

FIGWBE1.-Weather data for northern platoau province

winter months. It has&aconsiderable elevation bzsides the United States when a well-developed winter cyclone


15/32

CLIMATE OF ME XICO 13swings far south in its course across North America.in December, January, and February is below 0' C.,while in November and March it is only slightly above.A temDerature of -20' has been reDorted at Ciudad

provinces the highest minimum are delayed one or more

Guerrko, in Chihuahua, and frosts o&ur in 8 of the 12months.Although much of the region has an altitude of 5,000feet, temperatures of 40' C. are not uncommon in summer.However, relative humidity is low, the average minimumat Chihuahua City from November to June not beingabove 18 per cent and the mean for May is only 33 percent. Thus a temperature of 40' is not so unbearable asthe same degree would be at Vera Cruz where the humid-it is high much of the time. The average annual rangetember being the most humid and May the least.The middle plateau province (fig. 16, G) embraces aportion of the middle and southern plateau sections ofMexico. This province, the most temperate section ofthe country, includes most of the large cities and sup-ports most of the population. It might be characterizedas a region which is tropical in its range of temperaturebut temperate in its degree. (Fig. 22.) The wintermonths are cool, clear, and bracing, with a con-siderable amount of sunshine, whereas the summers arecloudy, although not hot, except in the sunshine, for likeother regions of high altitude it is a region of hot sun andcoo1 shade. The higher the altitude the truer this desig-nation.The highest temperatures occur in May with a second-ary maximum in August almost as pronounced. There1s no marked annual range of temperature, but the tem-perature curves show the profound influence of the rainyseason. With the onset of th at season in June there is adecrease in the average temperature of about 2 C. from?fay to June with an mcrease to a secondary maximumln August. Neither the average of the extreme monthlymaximum nor the average of the extreme monthlyminimum temperature curves are like th at of the averagemean temperature. The absolute highest temperature1s reached in May after which a decreaae of more than3' takes place in July, with n o increasem August. Theground is wet after the rainy season-and humidity ishigher as well as the percentage of cloudmess being great,a combination of which prevents the absolute maximumtemperature from being as high after as before the rains.The month of highest minimum temperature is June,With July and August only slightly lower. The absoluteminimum temperatures in December, January, and Feb-ruary often gets below Oo C., but on the average the abso-lute minimum temperature fo r January, the coldestmonth, is 1.2'.Frosts occur in some parts of the region in 9 of the 12months, but only 6 average as many as.o?e per month.The relative humidity comes to a mplmum in Marchand a maximum in September. In spring the ground isd r y , temperature is increasing and humidit conse-quently decreases. In September the gr o u n i is wet,teniperature is decreasing and humidity is high.The northern Gulf coast province (fig. IS, F) has anannual range in its extreme maximum ,temperature of 7.4'c., n its average mean of 11.3', and Its absolute nini-a r m of 1 5 O . (Fig. 2 3 . ) Augnst, being less cloudy and'?my than June o r July, is the warmest month. Thehlghest maximum temperatures are recorded in June.?his and the Lower California province are the only twoln hlexico which record the highest minimum and highestUean temperature in the same month. In the other

o9elative humidity in this province is 20 per cent, Sep-

H O U R S O F S U N S H I N E PE R DAY1050

40

30

20

10

0

FIGURE2.-Weather data;for&iddle plateau province


16/32

14 S U P P L E M E N T N O. 33

O F SUNSHINE PER DP

E M P E R A T U R E

-I-KH-DAYS WITH F R O S T/ / / IRELATIVE HUMIDITY

- J F M k M J J iIo N D-P r o u n ~ 3.-Weather data for Northern Qulf provfnce

J F M A M J J A S O N Dc

FIGWEE 24.-Weather data for southeastern lowlands provlnce


17/32

CLIMATE O F MEXICO 15In the province bhe graph showing the average hours ofsunshine per day per month is for the one station ofMonterrey, which represents the drier and consequentlysunnier section of the province, data for the other stationsnot being available. A minimum of 3 .8 hours is recordedin December with a maximum of 7 .8 hours in July. This

shows thc effect of the generally cloudy conditions inwinter accompanying the cyclonic storms which take amore southerly course in winter than at other periods ofthe year.Frosts occur in the province during the months ofDecember, January, February, and March, although

Because of i ts low latitude the difference between thelength of its longest and shortest day is not great, beingonly' two hours in the most southerly part. January isthe coldest month with a mean temperature of 22' C .,and average extremes of 13.5O and 30.9'. The mean andmaximum temperatures reach their highest degree inMay, with 27.5O and 35.7', respectively. The May max-imum is a result of the vertical ray of the sun and a smallamount of cloudiness preceding the rainy season.The average absolute minimum temperatures trendupward until June, when the temperature of the provincedoes not, on the average, fall below 20.5' C. June, July,

they are not numerous. The frost data were for Mon-terrey only.The mean relative humidity i high throughout theYear, the average annual range being only 7 per cent. It1s kept high in winter by the lower temperature of theSemicontinental climate, and in summer by the indraftOf moisture from the Gulf of Mexico. The minimumhumidity curve is for the stations of Tampicoand Monterrey, no others being available.The southern lowlands province, the largesl; (fig. 16 , H)Qbraccs the lowlands and foothills of southern and east-@Q. Mexico, including the Yucatan Peninsula. It is a*@@on f relativcly high tcmpernture with low annual(Fig. 24.) With the exception of a fe w stationsh a y is warmest and at these stations May is nearly ask4rm.

August, and September record minimum temperatureswithin 1 to 0.2O of each other, on the average. The uni-form minimum temperatures during these months is theresult of much cloudiness, high humldlty, and. wet ground.They do, however, occur m the higherelevations, at times, in December, January, and FebruaryThe construction of a rainfall map for a country of suchmarked relief and loc.ally contrasted exposure as Mexicopresents numerous difficulties. The mountainous partsof Mexico are almost withopt stations, such as there arebeing located in v alleys w h h vegetation shows to havoconsiderably less rainfall than the neighboring heights.An added problem encountered is the lack of any reliefmap adequate as a guide in drawing the isohyets in areas

Frosts are rare.RAI NFAL L


18/32

16 SUPPLEMENT NO. 33of few stations. Also many records are of short durationand include gaps extending over periods of months oryears. Thus, local sparcity of stations, inadeqbaterelief maps, and shortness of records, render the construc-tion of an accurate rainfall map of Mexico diflicult.Nevertheless, an annual rainfall map based on da ta from220 Mexican and near-by United States and CentralAmerican stations has been constructed. (Fig. 25.)N o attempt was made to adjust the records to a uniformperiod of years, because of the extremely local characterof the rainfall in much of Mexico and the broken nature ofthe records. Where stations were few or absent the prob-able effects of relief and exposure determined the course ofthe isohyets.Mexico, with an area of only 750,000 square m i l e s andwith no part more than 400 miles from the sea, has greatdiversity in its annual amount of rainfall. It ranges fromalmost extreme desert conditions in the region borderingthe Gulf of California, where the pressure is relativelyhigh and the region hemmed in by mountains, to veryheavy rainfall in the middle and southern Gulf slopes,which are freely exposed to the moisture-bearing tradesfrom the warm Gulf of Mexico. The generally heavyrainfall of the Gulf slope is made locally excessive by thepresence of mountains which rise precipitously from theflat and at places, narrow coastal plain.Great local diversities occur. Windward sides ofmountains may receive three or four times as much rain-fall as the leeward. The greatest recorded differences arethose between Teapa, in Tabasco, and Cintalapa, inChiapas, with 4,638 and 769 millimeters, respectively,and between Santa Monica Xtilitia, in San Luis Potosi,and Ixmilquilpan, in Hidalgo, with 2,325 and 105 millime-ters, respectively. Marked differences are also recordedbetween stations on the same side of a mountain rangebut at different elevations and exposures, the most notablebeing that between Teapa and Macuspana, in Tabasco,with 4,638 and 1,931 millimeters, res ectively.There are many areas of heavier anB of lighter rainfallthan in surrounding regions, as shown on the map.Even where relief is not a factor, the maximum rain-fall occurs not along the immediate coast but some milesinland. Frontera, with half as much rainfall as Comal-calco, and Progreso, with half as much as Merida, wellillustrate this rule.The northern basin plateau is the lar est area of lowrainfall as well as. the driest area of hig% altitude in allMexico. The andity is attributable to its locationwithin the subtropical high-pressure belt and its basincharacter. The Sierra Madre occidental is too high andunbroken to permit much vapor to enter from the west.And the Sierra Madre Oriental is sufficiently elevatedand unbroken to be a considerable barrier to rain-bringingwinds from the east.Throughout Mexico the rainfall variation from year toyear is considerable. In the drier regions some years havethree to seven times as much rain as others. Muzquiz,in the State of Coahuilla, reports one of the greatest fluc-tuations with 150 and 1,070 millimeters, while Pueblareports one of the smallest, that of 570 and 1,270 milli-meters. Figure 26 shows the yearly totals from year toyear for ti number of stations in the different parts of thecountry. All graphs are drawn to the same scale for amore ready comparison and all show a greater number ofyears below than above the normal. When the yearsare above normal they are often considerably above.Whether there has been a general desiccation of the cli-mate in any part of Mexico is still a matter of conjecture.Professor Lopez maintains tha t the drying up of Lake

Texcoco has caused a desiccation of the climate of theValley of Mexico. He states:The lakes which formerly existed in th e Valley of Mexico wereth e cause of the exuberant vegetation and primeval climate whichformerly dominated here as we were told in th e chronicles ofHumboldt, b ut with th e drying up of these lakes local climates havebeen altered, clouds and rains have decreased, and the coldness ofwinter has been accentuated (48, . 116).The records taken at Mexico City show that therehave been periods of wetter as well as periods of drieryears in the Valley of Mexico, but they do not show ageneral decrease in the amount of rainfall during thepast 50 years. The change in vegetation, which to-dayis said to be less luxuriant than that described by VonHumboldt, may be due to causes other than decreasedrainfall.The records of Mexico are not of sufficient length forrainfall cycles, if present, to be recognized.Mexico is a country of pronounced seasonal rainfall.Summer and fall are the seasons of maximum rainfall inall Mexico except the northwestern part where theMediterranean rainfall regime is found. In most parts

of the country May to October are the rainy months. Amap showing the percentage of the total rainIall occurringin these six months appears as Figure 27 . Between 90and 97 per cent of all rainfall in the southwestern part ofMexico occurs during this period, while in the north-western par t the percentage drops to-less than 15, with aclose crowding of the lines in the region around the Gulfof California.The region bordering the Gulf of Mexico has a smallerpercentaie of its rainvfall during this period than doesthe Pacific in the same latitude. This may be accountedfor by the lower lati tude of the Pacific, the greater numberof tropical hurricanes on the Pacific than on the Gulfcoast a t this season, and the considerable amount ofwinter rainfall brought to the Gulf coast b the northers,coastal region as contrasted with the Pacific coast. 011the Pacific coast the prevailing on-shore winds seem tobe more restricted to the coastal stations.Although May to October is, in general, the rainyseason in Mexico, March, April, May, and Novemberare the only months not appearing as the month ofmaximum in some part of the country, and May isthe month of maximum for a border station in thoUnited States.A feature common tomost of westernMexico is the con-siderably heavier rainfall in December than in Novemberor January. There is almost invariably a decrease fromthe rainy months to November then an increase to D e *cember and a decrease to January with very little ram-fall again until the opening of the rainy season.

On the basis of amount and monthly cous e of rainfallMexico-falls naturally into 13 rainfall types or provinces,each ds er in g essentially from the others. (Fig. 2% )The rainfall types here adopted are based upon theexamination of a large number of plotted monthlyramfall amounts on an enlarged map of Mexico .forstations in all parts of the country and the border@sections of the United States. Each rainfall type. Isrepresented bs a composite graph of from three to n@stations wlthm the area. (Fi s. 29-41.) The rainfallciated wrth these graphs are composite graphs (for thbesame stations where possible) of related phenomena,including averagemaximum rainfall in 24 hours, dayswith thunderstorms, days with any rainfall, cloud

and the more pronounced on-shore winB in the Gulf

amounts. are adjusted to monta o equal length. Asso-partly,cloudy, and clear days, and percentage of clouJ:


19/32

CLIMATE OF ME X I C O 17

-l200-

800 -

- 200-

- 2800-

-2aoo-- 000-

- 600-

- 200-

- 00 -400 -0 -

-2000-

- 600-- 200-

800 -400 -

0 -

- 600-

M O NT ERREYILLA AHUMADA

4 1 1 1 1

AN LUIS P O T O S l

ENSENADA

-3200

-2800

- 400- 2 000

1600

N EC AXAl - - l -

- 200-

V E R A C Ruz- 000- 600- 200

I

800

40 0f 0f tOAXACA SA L'INA 'CRUZ~- 600- 200 111111111111111110FIGWEE6.-Annud courso of rainfall in mfllimotors,1898-1927


20/32

18 SUPPLEMENT NO . 33ness. The graphs for all provinces are drawn on thesame scale for more ready comparison.A composite graph does not necessarily represent theexact conditions of any one station within the province.It does, however, give a better picture of the averageconditions within a given region than would a numberof graphs for individual stations within the same area.It has the additional advantage of smoothing out someof the minor irregularities of individual stations.To . prevent too much repetition in the discussionof ramfall provinces, they will be treated collectively

Much of the rainfall comes in the form of heavy thun-derstorms, but there is a marked difference in the numberof thunderstorms in the different parts. Another fea-ture common to much of the Pacific coast is heavier rain-fall in December than in November or January.The months of maximum rainfall in this general regionare June, July, August, September, and December.Throughout, the rainy months are months of few cleardays, while the dry season is one of much bright sunshine,excepf, in the desert province where sunny conditionsprevail at all times, there being, on the average, only 18

Jin ID 1 0 . mo .I 0 0

FIGUBE7.-Percentage of ainfall occurring from May to OotObeFa-- separately under the following heads: (1) Pacificcoast, (2 ) Plateau, and (3 ) Gulf coast.As has already been stated, Mexico is a country ofpronounced seasonal rainfall. This fact is well shownon the composite graphs for the rainfall proyinces of thePacific coast section. (Figs. 29-34.) The ramy season isnot made up of the same months throughout the Pacificcoast region, nor is it the same length throughout.In all of the Pacific coast provinces, except the Med-iterranean and the desert, summer is the ramy season,and it is a marked rainy season. In the Mediterraneanrainfall province,. just the opposite is the case-winteris the marked ramy season, while in the desert provinceno month has much rainfall.In the most southerly of the Pacific coast provincessix months each has more than 75 millimeters of rainfall.Proceeding northwestward along the coast the numberdecreases to 5, 4 , 3, 0, 0, respectively.

cloudy days per year in this province as shown by thestation of Yuma, Ariz. A brief discussion of each prov-ince along the Pacific coast region follows.The Gulf of Tehuantepec province (fig. 28 L) includesa.rela$vely small portion of the country but there is greetdiversity in its total amount of rainfall. T a p a c h hwhich has free exposure to the moist winds from the Gulfof Tehuantepec, with 2,214 millimeters, and TehuantepeOCity, located in the protected valley of tho same name,w t h 437 millimeters, are the recorded extremes.It is a province with five wet months, June to Octobe'rand five dry months, December to April, with May andNovember as transition months. (Fig. 29 . ) Almost norain falls in tho region from December to April, less than6 per cent of the total in these five months. June, mth27 4 millimeters, is the wettest month. This is t,he timeof the year when the low-pressure belt has shifted n d battended by a great amount of convectional rainfall.


21/32

CLIMATE OF M E X I C O 19July and August have considerably less rainfall than Junealthough they each have 17 0 millimeters. With thepccurrence of tropical. hurricanes and the general decreasein pressure in this region in the fall rainfall again increasesand September records 230 millimeters, with Octobernearly as wet as July and August.During the four raimest months, June to September,there are less than two clear days per month, whileDecember and January each has on the average morethan 18.Thunderstorms are more numerous in this provincethan in any other part of the country. June, July, andAugust average 10 each, while January alone is free of thisphenomenon. During the rainy season the days withthunderstorms are almost as numerous as the days withany rainfall.To the northwest of this province the course of rainiallchanges and gives rise to anqther rainfall type, here namedthe southern west coast ramfall province. (Fig. 28, I.)The rainfall curve is very much like that of the Gulf ofTehuantepec except that the maximum rainfall comes inSeptember instead on June and the secondary maximumin June instead of September. (Fig. 3 0 . ) The totalrainfall in this province is about 1,000 millimeters, andranges from 1,400 millimeters at Acapulco to 700 milli-meters at Isla Maria Madre. I t is most all concentratedin the five months, June to October. Heavy downpoursPccur, September having as its average maximum rainfallin 2 4 hours 96 millimeters.. Thunderstorms nre lessfrequent in this province than m the gulf of Tehuantepecregion, but cloudiness is greater during the rainy season.September, the cloudiest month, has 16 cloudy days, and16 days with some rainfall, on the average. This prov-ince gets a fuller effect of the tropical hurricanes in Sep-tember than does tha t to the southeast. The first fourmonths of the year each has less than one day with rain-fall, making t hat a season of sunnv weather.The middle west coast proyince (fig. 28, F.) extends$land over a considerable portion of the western plateaum these latitudes. This prqvince may be characterizedas one having four wet and eight dry months. (Fig. 31 . )There is a more abrupt change from the dry to the wetand from the wet to the dry season than in any other pro-vince in Mexico. May has 7 millimeters, June 152 milli-meters, September 223 millimeters, and October G3millimeters of rainfall. In the period June to September85 per cent of the total r?ir,fall.is recorded. Seventy-eight per cent of the days with rainfall also occur in thosefour months. During the four rainy months there areonly 12 clear days, on the average, and June has half ofthem. The months January to May are almost rainless,March and April each having only 2 millimaters. As isshown by the graphs, thunderstorms are frequent duringthe rainy season. November is much drier than eithorOctober or December.The average maximum rainfall in. 4 hours is consider-able, though not so large a proportion of the total as inSome of the other provinces.Proceeding northwestward along the coast to the north-ern west coast rainhll province (fig.28, C.) the length andintensity of the rainy season becomes somewhat less. For8 of the 12 months there is only about one-third of thesky covered with clouds. May, the least cloudy month,has only 2 .8 sky cover and only 4 cloudy days. In allQonths except July, August, and September there areQany clear sunny days. In thls province less than 6Per cent of the total rainfall is recorded in the first fivelnpnths of the year. The rainy season starts in June,Wltli increase in percentage of cloudiness, number of

cloudy days, days with rainfall, and days with thunder-storms. (Fig. 32.) The height of the rainy season isreached in August, one month later than in the region tothe south, when 136.2 millimeters, or almost 30 per centof .the total rain falls. There are only 6 clear days duringt h s month. July, August, and September are the threerainy months, with 71 per cent of the total, while to thesouth there were four rainy months. These threemonths stand out prominently on all the graphs. Alarge part of the rainfall of this province comes in oneshower each .month, on the average, as shown by thegraph of maximum rainfall in 24 hours.Thunderstorms are frequent durin the rainy season,July, August, and September each iaving about five,they being aboutJ half as great in number as the dayswith rainfall. In this province December has twice asmuch rainIall as November and January combined.The region bordering the northern part of the Gulf ofCalifornia (fig. 28, B) including much of the.Peninsula ofLower California, is a desert region and is here called thedesert rainfall province. It differs from all other parts ofMexico in having such a small amount of rainfall. (Fig.3 3 . ) No month has more than 12 millimeters for the7i-- - - 2 i---v --i-------i---2

region as a whole, though there appear to be areas which,because of relief and exposure, have a considorably heav-ier rainfall, but there are no records to verify this. Thisregion may be charactcrized as one with 1ittl.eor no rainfallin all months. The average of the region is only 70 mil-limeters, a good portion of which comes in heavy thunder-storms, more or less of the cloudburst type, common indeserts.N o Mexican station in this province records cloudy,partly cloudy, and clear days, but the Yuma, Ariz.,station may give a fair idea of the few number of cloudydays in this desert region. At Yuma there nre only 18cloudy days per year, on the average,The latitude of the region, lacing it in the intorrupted

mountains of Lower Cahfornla from 2,000 to ~ O , O O Ofeetelevation; the cool current along the west coast of LowerCalifornia; and i ts interior !ocation arc the principalfactors combining to make thls a desert province.In the northern part of Lower California (fig. 28, A)the Mediterranean rainfall rbgiine is found. As the nameconnotes to a climatologist, this is a region of winter rainsand summer droughts. (Fig. 34.) The cause of therainfall distribution is generally lrnown to be the nigra-

subtropical high-pressure beat for a part of the yoar;


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20 SUPPLEMENT NO . 33I PERCENT OF CLOUDINESS

{ . - J F M A M J J A S - 0 N IFIGURE .-Weather data for Qulfof Tehusntepec rainfall PrOVhCS

PERCENT OF CLOUDINESS

J F M A M J J A S 0 - N - O*

FIQUBE0.-Weather data for southern west coast rainfall province


23/32

CLIMATE OF MEXICO 21I PERCENT OF CLOUDINESS

PPPPPPf ff3020

10

0

a CL

RSTORMS & DAYS W

LAINFALL N 241-10

rnR A IN FA L; : L +

4YSp

1 R A I NI

S

L

M J J A S O N 0F MFIQUBE.-Weather data for middle West coast rainfall provlnce

PERCENT O F CLOUDINESS

30

20

10

0

DAYS W120

IO

0 11 200

100

0 --300

200

too

0 J F

t THUNDERSTORIEliXIMUM RAlNFAl.L

R A I N 1

DI A M A S

R DAYS

I - tO N 0

FIQURE P.-Weather da ta for northern west coast rlnfnll province


24/32

22 SUPPLEMENT NO. 33tion north and south of the pressure belts, bringing thisregion under the control of cyclonic storms in winter andunder the control of high pressure or northeast winds insummer.The average rainfall for the region, based on recordsfrom three stations, along or very near the coast, is 295millimeters, 6 millimeters of which come in the months ofJune to September. There are no records available forthe highlands of the province, but as in southern Califor-nia the rainfall is probably heavier than along the coast.Nelson (54, p . 95) reports that in the San Pedro Mart irMountains 3 to 4 feet of snow falls in winter. Decern-ber is the month of maximum precipitation, though onlyslightly more than January o r February. During eachmonth of the year, on the average, about one-half of thetotal rainfall occurs in one 24-hour period.December has the greatest number of days with rain-fall, although all months have less than 10. Decemberwith an average of 9.7 and August with 0 .7 are the ex-tremes. August is the month with fewest cloudy days.I n July, August, and September there are more dayswith thunderstorms than days with rainfall. This maybe accounted for by the fact t ha t thunderstorm data wereavailable only for San Diego, while days with rainfallwere for San Diego as well as two Mexican stations. An-other explanation is that thunderstorms occur in themountains back of San Diego and give no rainfall at thatstat ion or else are dry hunderstorms;i.e., no rain reaches. .the ground.Aside from December the months of maximum cloudi-ness and cloudy days do not coincide with the months ofmaximum rainfall. The maximum cloudiness and cloudydays come just after the rainy season, caused in part bythe evaporation from the ground wet by winters rain.July, August, and September are the clearest months,with an average of less than three cloudy days each.This province then is relatively dry, with much sun-shine, little cloudiness, few rainy days, and often hazyduring the summer months.The plateau rainfall types (figs. 35-37), too, are char-acterized by their seasonal distribution of rainfail, al-though the seasonal differentiation is not so marked as onmost of the Pacific coast. The rainfall in some respectsis similar throughout the plateau. Most of the rainfallcomes in the four months, June to September, but thepercentage occurring in these four months is greater inthe south, the northern part being far enough north toget considerable winter rainfall. In the southern andmiddle sections of the plateau there are four months Ofabout equal amounts, whde to the north the number isreduced to three and the total amount is considerably less.Another character of the whole plateau is that themonth just before and the month lust after the rainyseason have about equal amounts and show their transi-tion character.The month of maximum rainfall in the plateau is Juneor July, depending on the province.The interseasonal contrasts of cloudiness are moremarked in the south and become increaslngly less towardthe north. Throughout the plateau thunderstorms occurin every month of the year, on the average, but aremuch more numerous in the rainy season. Days withany rainiall decrease greatly from south to north.The southern plateau rainfall province (fig. 8, J>hasplentiful summer bu t meager winter rainfall. The yearin this province may be divided into two seasons: a rainyseason from June to September, with 75 per cent of thetotal, a dry season from November to April, with 10per cent, while May and October, with 15 per cent are

transition months. .Throughout the province July iswettest, though the other three months of the rainyseason have about equal amounts and only slightly lessthan that of July.The average for the re ion is about 70 0 millimeters.rainfaI1. Toluca receives the average fo r the region,while Valle de Bravo and Justlahuaca with 1,386 and 21 7millimeters, respectively, are the recorded extremes.During the rainy season the average maximum rainfallin 2 4 hours is about one-fourth the total amount, mostof the stations at some time having received 75 mjlli-meters and some 100 millimeters in a 24-hour period.The rainfall here fluctuates less from year to yearthan in any other part of Mexico and comes a t the mostadvantageous season for an agricultural people.The annual course of cloudiness, cloudy days, dayswith thunderstorms, and days with any precipitationfollow closely tha t of the total rainfall.In the rain season some rain falls on about 20 daysper month. b u c h of the rain comes in the local after-noon thunder showers of short duration, a t times accorn-panied by violent thunder and lightning as well as hail.The sudden downpours are helpful in cleaning the streetsand clearing the atmosphere. Rapid evaporation underbright sunshine after the shower dries the surface in ashort time.The dry season in contrast with the rainy is typifiedby many bright, clear, sunny days. During this seasonPopocatepetl and Ixtacihuatl, the two permanentlysnowcapped peaks towering above tho Valley of Mexico,are often visible from Mexico City , while in the rainyseason they are seldom seen. They are obscured in theearly morning hours by fog and haze and later in theday by clouds.Snowfalls are rare in this protince except on the highermountain peaks, some of which remain permanently snow-capped.The northern plateau rainfall province (fig. 28, D) isalmost a desert. Areas within this province are deserts,as the Bolson de Mapimi, where less than 250 millimetersof rain falls. The extreme aridity of parts of this regionwas a problem of great concern in the recent revolution.The province has a seasonal differentiation of rainfall,which, however, is not so pronounced as in the northernwest coast province immediately adjacent. July toSeptember are the rainy months and June and Octoberthe transitions. Some rain, however, falls in all months.July, with 85 millimeters, is the rainiest and January toMay each has about 10 millimeters. Much of the rainwhich this region receives in winter comes with the extra-tropical cyclones which take a southerly course acrossNorth America. Some of the winter precipitationcomes as snow. The summer rainfall is produced byafternoon convoction, often accompanied by thunderand lightning.

On the average a half to a third of all trhe ainfall coinosin one heavy shower each month.In a province so large and so mountainous as this, localdiversity is the rule. It is, however, a basin and there-fore dry. The mountains to the the west ar e too highand continuous to permit the importation of much vaporfrom the Pacific. The mountains to the east, while lesselevated, are a considerable barrier to moisture from theGulf of Mexico.This is a sunny province with a few cloudy or raiflYda s. There are only 70 days in the year with any ra W

(Fig. 35.)There is, however, consi2erable local diversity in the

fa19and half of these are in July, August, and September,


25/32

CLIMATE OF MEXICO 23P E R C E N T O F CLOUDINESS PERCENT O F CLOUDINESS

'P'AYS

I RAlh

RS

L

PIQUREweather data for desert rainfall province FIGURE 3d.-Weather data for Mediterran ean rainfall prov ince


26/32

24 SUPPLEMENT NO. 331 PERCENT OF CLOUDINESS

, o o ~

0 J F M

'YP'DAYS

M J J A S O N 0FIQURZ&-Weathsr data for southern p1atesu ralnfall province

30

20

10

0

20

10

0

200

100

0

300

200

100

aL

P E R C E N T OF CLOUDINESS

- J F IFIGWEE0.-Weather data for northern platea u rainfa ll provino e .A


27/32

CLIMATE OF M EXICO 25The middle plateau rainfall province (fig. 28, G) differsfrom the northern plateau province in amount and month-ly course of rainfall, and from the southern plateau prov-ince in monthly course rather than amount of rainfall. It ,like the southern plateau rainfall province, is a region ofplentiful summer but meager winter rainfall. (Fig. 37.)

June to September are the rainy months, while May andOctober are transition mpnths. The other six monthshave little rainfall. June i~ the month of maximum rain-fall, while a secondary maFmum falls in September. Thenumber of days with rainfall and days with thunder-storms are considerably less than in the southern plateauprovince, while the amount of rainfall is about the same, acombination of which indicates tha t the rainfall per rainyday is heavier than to the south.As far as cloudiness is concerned this province may beconsidered as a transition region between the plateau tothe north and the plateau to the south.Everywhere on the Gulf coast, from Rio Grande toYucatan, summer or fall is the season of maximum rain-fall, The winters, though distinctly less wet than theseseasons, are rainier than any other part of Mexico, savenorthwestern Lower California. The Gulf coast regionreceives its rainIa11 from tropical and extratropical cy-clones, convection, relief, and northers. The middle andsouthern Gulf slopes have the greatest annual rainfall, theheaviest 24-hour rainfall, and the greatest number ofdays with rain in Mexico. To the north as well RB to theeast of these provinces a considerable decrease in amountof rainfall, days with rainfall, and average maximum rain-fd l in 24 hours is found. There are enough differencesul amount and monthly course of rainfall to warrant thedelimitation of four rainfall provinces. (Figs. 38-41 .)The Yucatan rainfall province (fig. 28, &I) embraces theStates of Yucatan and Campeche, and the Territory ofQuintana Roo. It has considerable rainfall all throughthe year but, has much heavier rainfall in the months ofJune to October. (Fig. 38.) March, with 22 millimetersis driest and June, with 160 millimeters, is wettest.These are themonths of lowest relative humidity, because the riseOf temperature from winter to . summer is more rapidthan the rise of relative humidity. By the lat ter par tof May convectional rains begin and continue throughthe summer, apparently reaching their maximum inJune, with a decrease in July, when the center of lowestPressure has shifted t o nqrtliern Mexico. With theOccurrence of tropical hurricanes in the late summerand early fall the rainfall is again brought to a maximum!n September. This fall maximum is most pronouncedU the eastern part of the peninsulrt, whero the tropicalstorms can be most felt.. There are no records in the southern part of the pen-Insula, but because of its more southerly location. andfrom the more luxuriant vegetation it seems likelythat the rainfall is somewhat heavief than in the northernand eastern parts. The average rainfall for the province1s 950 millimeters, falling on the average, on 125 daysof the year. Each of the months from June to Octoberhas about 15 days with some rainfall, and days withheavy rainfall are not lacking. June and October eachhas more than 40 millimeters as the average maximumrainfall in 24 hours.Thunderstorms are not infrequent in Yucatan durhgthe rainy season. July, August, and September each hasabout three, and every month has reported them. ThePartly cloudy days exceed in number those clear or cloudyevery month of the year, and the number varies litt lef r om month to month, There is likewise little monthly

In March and April little rain falls.

range in percentage of cloudiness. March, with 4.6, andJune, with 6.3, are the extremes.To the west of the Yucatan rainfall province a con-siderable increase in rainfall is encountered. Thesouthern Gulf coast rainfall province (fig. 28K) hasthe heaviest recorded rainfall of any province inMexico. It has in it the station of greatest recordedrainfall in the country, that of Teapa, in Tabasco,with 4,638 millimeters. The data for this station werenot used in constructing the composite graph of theprovince, because this very heavy rainfall seems to bemore local than general. The average for the regionis, however, over 2,000 millimeters. The lowest recordedamount 1s 950 millimeters, a t Frontera, in Tabasco.April, with 44 ndlimeters, is the month of minimumrainfall, while October, with 342 millimeters, is themonth of maximum. (Fig, 39.) Winter rains areabundant, being produced in part by the northers.The heavy rainfall of pummel: is a result largely ofconvection, while the stlll heavier rainfall of September,October, and November resulk from tropical storms,convection, and northers.Heavy rains in a 24-hour period are recorded in everymonth of the year, on the average, though A ril, with18 millimeters, and November, with 98 mhmeters,are the extremes. More than half of the days fromJuly to October have some rainfall. Even in Decemberand January the days with some rainfall are 12 and 11,respectively. Days with thunderstorms are aboutone-third as numerous as days with any rainfall.Cloudiness varies little from month to month, morethan half of the sky being covered on the average byclouds during all months. The percentage of cloudinessmay not, however, be very representative of the wholeregion, since it is for the one station of Puerto, Memco.To the northwest, along the Gulf coastal plain and foot-hills of the Sierra hiadre Oriential the amount of rainfalldoes not change appreciably, but the course through theyear does chan e to the middle Gulf coast type. (Fig.28, H.) Like tBe southern gulf coast, this province hascondiderable winter rainfftll, but less than tp the south-east. The month of manmum rainfall in this province isSeptember. There is a secondary in June. The provincehas a more marked seasonal distribution of rainfall thanthe southern Gulf coast rainfall province. Much cloudl-ness, few clear days at any season, although a maximumin winter and spring, characterize this province. (Fig.40.) During the mn te r and spring Mount Orizaba maybe seen quite frequently from Vera Cruz and, under veryfavorable conditions, from 50 miles at sea. But durinthe rainyseason it is not visible for a great distance anat times can not be seen from Orizaba City.. Muchof the rainfall along the mountain slopes comes as a kindof mist which ma change into heavy downpours. Thedense tropical anc9semitropical forest along the slopes ofthe Sierra Madre Oriental bespeaks the abundant rain-fall.The lower Rio Grande rainfal! province (fig. 28 , E)differs from the middle G$f coast 1 n amount rather thanmonthly distribution of rainfall, for i t too has a Septembermaximum and a June secondary maximum. But, t has lessthan one-third as much rain fall as the middle Gulf cowtrainfall province. (Fig. 41.) The lower Rio Grandeprovince is far enough north. to receive considerable winterrainfall from the extratropical cyclones when they taketheir most southerly course across North America as wellas from the c clones which originate in the western par t

iMany rainy days are the rule in this provmce.

of the Gulf of"Mexico.


28/32

26 SUPPLEMENT NO. 33PERCENT OF CLOUD I N E S S

I J F M A M J J A S O N D~

FIGURE7.-Weather data for middle plateau rainfall province

PCRCENT OF CLOUDINESS

I CbOU4Y, PAF30

20

IO

0

20

to

0

200

100

0

300

200

100

0

c,louDv &, CLEAR

WORMS a ~ A Y S 'WI

MdFALl! I N 24 H O I

J F M A M J J A S ( N OI

FIQUBE8.-Woather data for Yucatan rainfall province


29/32

CLIMATE O F ME X I C O 27PERCENT OF CLOUDINESS PERCENT OF ChOUplNESS

'PP 'P'YS, e A R T i Y * 4RTLYIJNDER!fM RA I111

OUDY I!

J

-T

AYS W l

I

30

20

10

0

20

IO

0

200

100

0

300

200

1 0 0

0

hYS, WI T HHUNDERS

3XIMUM RAI8

I 12 4 H O U R SI

-I-$$-'+

S O N 0F , M A M S O N D t J FFIQUILEQ.-Weather data for southern Qulf coast rainfall province FIQlJRE ~o.-Weather data for middle Gulf coast rainfall provinc e


30/32

28 SUPPLEMENT NO. 33A characteristic of this province is that it has its maxi-mum cloudiness from November to May when it is having

30

20

10

0

20

10

0

200

100

'0

IO 0 ,

?oo

100 -

01

PERCENT OF CLOUDINESS

IlTH 1

fAXI1J m JM I

ARTLYP'I 'PP'OUDY & CL- 1 II 1JNDERSTORMS 8 DA'r

M J J A S O N O~ ~~ ~

FIQURE1.-Weather data for lower Rio Qrande rainfall provinceits smallest amount of rainfall. This anomaIy resultsfrom the greater frequency and more southerly course ofcvclonic storms at ths se&son. Davs with thunderstorms

are almost as frequent. as days with rainfall. All in all,this is R province with few cloudy days, fewer rainy days,and a low total rainfall. The September rainfall is con-splcuous by jts relatively great amount, probably causedby tropical storms which at times deluge this valley.S U M M A R Y

Thus Mexico, though only three times the size of Texas,can be thought of as a country of marked local diversitiesm all of its climatic elements. It has prevailing windsfrom every point of the compass. In temperature itran es from hot, humid conditions of the tropical low-]anis to permanently snowcapped heights. In mean an-nual rainfall it varies from 55 to 4,638 millimeters.Some places are said to receive no rainfall in some years.The number of days with rainfall varies fzom 15 to morethan 200.BIBLIOQRAPHY

(1) [ABBE, c.1(2)-893. NORTHER5 O F VERA CRUZ. (General note) u. s.MO.Weather Rev., 21: 226-227.

(Noteby the editor) U. S. Mo. Weather Rev., 21:1893. TH E NORTHERS OF TAMPICO AND V

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Climate of Mexico-1929

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