a fog climatology for mexico - eth...

• Fog – Data interpolation – Regional climatology – Mexico

DIE ERDE 139 2008 (1-2) Special Issue: Fog Research pp. 45-60

A Fog Climatology for Mexico

1. Introduction

Fog plays a major role not only in the hydrologi-cal cycle but also for many human activities, such

as agriculture and land, sea and air transport. Likemost hydrometeorological phenomena, fog occur-rence strongly varies with geographical locationat both the local and the regional scale. In addi-

Fernando García-García and Víctor Zarraluqui (Mexiko City)

Fog can be defined as a cloud in the vicinity of the earth’s surface that affects visibility. Itdiffers from a cloud only in that the base of fog is at the surface of the earth while clouds arefurther above. Fog plays an important role in the hydrological cycle, mainly in the transport ofwater from the atmosphere to the earth’s surface through wet deposition and interception bytrees and vegetation. It is considered also a natural hazard that causes low visibility (according tothe international, meteorological definition, fog reduces visibility at the ground below 1 km) andis a particular danger for all varieties of air, land and water transportation. On the other hand, fogcan be also considered a potential non-conventional source of water supply when removed byartificial methods for human consumption. Fogs of all types originate when the temperature andthe dewpoint of the air coincide. This may occur through cooling of the air to a little beyond itsdewpoint, as a result of advection, radiation or upslope movement of the air; or by addingmoisture and thereby elevating the dewpoint, thus producing so-called frontal fogs. These synop-tic and mesoscale mechanisms are modified by local terrain features, such as topography, landand vegetation cover and, in turn, small-scale circulation. Thus, varied climatic regimes result indifferent distribution patterns of fog occurrence and development. In spite of its importance, theimpacts of fog formation, development and distribution have not yet been properly assessedthroughout the world. In particular, in Mexico there are very few specific studies on the topicand there are none of national character known to these authors.

Eine Nebelklimatologie für Mexiko

With 8 Figures

46 Fernando García-García and Víctor Zarraluqui DIE ERDE

tion, fog observation and recording methods arestill very dependent on direct human presence andperception, since its detection by automated in-strumentation is not a widespread practice instandard weather stations. These characteristicsmake it difficult to develop detailed fog climatolo-gies that cover the whole world.

The case of Mexico is no exception. Mexico isa country with a great variety of climatic re-gimes that reflect the uneven distribution ofwater resources throughout its territory. Thisis one reason why there are no comprehensivefog climatological studies on the national scale,

and most fog data are scattered across diversesources. Data availability is highly variable interms of quality and spatial and temporal cov-erage, and most existing studies have a regio-nal character in addressing specific aspects ofthe problem for particular applications. It canbe pointed out, for instance, that the NationalAtlas of Mexico (Coll-Oliva 2007) does not in-clude a single map devoted to fog.

The main purpose of the present study is to de-velop a detailed fog climatology for Mexico. Theprocedure takes into consideration the inhomo-geneous characteristics of the available database.

Fig. 1 Map of Mexico showing the locations of the 2,888 climatological stations used in the study. Eachsymbol on the map represents one station. Note that the non-uniformity in the spatial distributionof the stations is related to the difference in population density. Average data density isapproximately one station per 680 km2. / Karte von Mexiko mit den 2.888 Klimastationen, derenDaten in der Studie berücksichtigt wurden. Jedes Symbol stellt eine Station dar. Die ungleichmäßigeräumliche Verteilung der Stationen ist auf die unterschiedliche Bevölkerungsdichte zurückzuführen.Die durchschnittliche Datendichte beträgt etwa eine Station auf 680 km².

2008/1-2 A Fog Climatology for Mexico 47

These results are then used to classify regionsof fog incidence in terms of their main meteoro-logical and physical formation and developmentmechanisms. Thus, a secondary purpose is toshow the potential of the methodology used whenapplied to different spatial and temporal scales.

2. Data and Methodology

The database used for the present study wasstandard climate records provided by the Mexi-can National Meteorological Service (SMN).Standard normal values are in this context definedas averages of weather observations – precipita-tion, air temperature and pressure at the surfaceetc. – over consecutive thirty-year periods. Thisis the most comprehensive and up-to-date pub-lic database of atmospheric records and productsavailable in the country and is available from theNational Water Commission (CNA 2007). In par-ticular, standard average monthly fog days(number of days with fog occurrence as reportedby an observer) were used. These were calculatedfrom daily observations in the period from 1961to 1990 at the 3,300 climatological stations man-aged by the SMN (CNA 1999).

Due to the nature of the data, several corrobora-tion checks and adjustments had to be performed.First, in the case that a given observing stationhad not reported a full thirty-year record, provi-sional normal values from data for at least tenyears within the study period were used. Reportsfor stations with less than ten years of fog recordswere rejected from the database. Second, occa-sionally it was found that some stations had ei-ther reported incorrect geographical coordinatesor changed their locations during the recordingperiod. In both these cases, station coordinateswere, whenever possible, adjusted against usinghistorical data from the National Statistics, Geo-graphy and Informatics Institute (INEGI 2008a),or the corresponding data were not consideredfor the study. Third, in cases when normal val-

ues were obviously dubious or questionable, theywere recalculated from the original daily recordsalso provided by the SMN (Quintas 2000). Afterthese data quality tests the resulting database,consisting of the average monthly number of fogdays from 2,888 climatological stations with a non-uniform spatial distribution over the whole coun-try (Fig. 1), was stored in electronic spreadsheets.

Fog climatologies and their geographical patternswere elaborated using the commercial contouringand surface mapping program Surfer V8.01(Golden Software, Inc, Golden, CO, U.S.A.) in theMS Windows environment. This program allowsto choose from different data interpolationschemes and to calculate “best fits” statistics. Var-ious gridding methods were used to interpolateand represent the data in digitised base-maps ofthe Mexican territory – administrative boundaries,general physical features, topography etc. – ob-tained from the National Commission for Bio-diversity Knowledge and Utilisation (Conabio2007). The tested gridding methods includedKriging with different variograms (linear, quad-ratic and wave hole effect), triangulation with lin-ear interpolation, and radial basis function (mul-tiquadratic, inverse multiquadric and thin platespline). Kriging is one of the more flexible meth-ods and is useful for gridding almost any type ofdata set. In general, Kriging with a linear vario-gram is quite effective for most data sets, al-though it can be rather slow for larger data sets.Triangulation with linear interpolation is fast but,for small data sets, generates distinct triangularfaces between data points in their graphic repre-sentations. The radial basis function (RBF) is ac-tually a diverse group of data interpolation meth-ods. All of the RBF methods are exact interpola-tors that employ an equation dependent on thedistance between the interpolated point and theneighbouring sampling points. In terms of theability to fit data and produce a smooth surface,the multiquadric function is considered by manyto be best since it produces a good representa-tion of small-sized samples (see for example Hardy


1971). The performance of the different interpola-tion methods mentioned above was tested andevaluated, to finally produce fog-occurrence mapsfor both spatial (national, regional) and temporal(yearly, seasonal, monthly) scales.

3. Results

The macro-spatial, national-scale fog-occur-rence maps were produced with yearly and sea-sonal (winter, spring, summer and autumn) time

resolution. Given the nature of the fog database(see section 2) it was relatively easy to adapt itto regional, smaller-scale studies provided thatthe proper interpolation methods were used.The choice of these methods depended verymuch on the spatial distribution of the data andtheir spatial density. In all cases it was general-ly found that the method of radial basis func-tion, with an inverse multiquadric kernel usedto define the set of weights to be applied to thedata points when interpolating a grid node, ren-dered the best statistical results. Both the co-

Fig. 2 Relief map depicting the main orographic systems of Mexico: I. Sierra and Peninsula of BajaCalifornia, II. Sierra Madre Occidental, III. Sierra Madre Oriental, IV. Trans-Mexican VolcanicBelt, V. Sierra Madre del Sur, and VI. Sierra of Chiapas and Sierra of Guatemala. Black bullets showthe different locations mentioned in the text: 1. Mexico Basin, 2. Teziutlán, 3. Ensenada, and 4. LasAlazanas (after Conabio 2007, INEGI 2008b and USGS/EROS 2006). / Reliefkarte mit denwesentlichen orographischen Systemen von Mexiko: I. Sierra und Halbinsel Baja California,II. Sierra Madre Occidental, III. Sierra Madre Oriental, IV. Transmexikanischer Vulkangürtel,V. Sierra Madre del Sur und VI. Sierras von Chiapas und Guatemala. Die schwarzen Punkte zeigendie verschiedenen im Text erwähnten Standorte: 1. Becken von Mexico, 2. Teziutlán, 3. Ensenadaund 4. Las Alazanas (nach Conabio 2007, INEGI 2008b and USGS/EROS 2006).


efficient of determination (R2) and the F-teststatistics confirmed this. It was also found,however, that sometimes other methods suchas triangulation and Kriging resulted in moreappealing graphic representations.

The maps representing the annual and season-al distributions of the average number of fogdays in Mexico are shown in Figures 3 and 4.Average values of fog occurrences above 50 fogdays per year are found more frequently in thecountry’s main orographic systems (see Fig. 2),i.e.the Sierra Madre Oriental, the Sierra MadreOccidental, the Sierra Madre del Sur, the Sierraof Chiapas and the Trans-Mexican Volcanic Beltthat runs from east to west across south-central

Mexico. There are also some coastal regions withmoderate incidence of fog, particularly thenorthwest coast of the Baja California Peninsu-la around the Ensenada region. The maps showthe general lack of fog events in the northerndesert and semi-desert regions.

Several representative cases in the main fog re-gions mentioned above were analysed for season-al and monthly variation. For the sake of brevity,only two of these regional cases are presented anddiscussed in further detail in the following section:the case of the Mexico Basin, where Mexico Cityis located; and an area of the southern Sierra Ma-dre Oriental, around the Teziutlán region, where fogoccurrence has its maximum value in the country.

Fig. 3 Fog climatology for Mexico: distribution of the annual average number of fog days in the country.Isolines (red) are drawn every 50 fog days. The maximum annual average value for a single site isabout 280 fog days per year and registered in the Teziutlán region. / Nebelklimatologie für Mexiko:Verteilung der durchschnittlichen Anzahl der Nebeltage pro Jahr. Die roten Isolinien bezeichnenjeweils einen Abstand von 50 Nebeltagen. Der maximale Mittelwert tritt im Gebiet von Teziutlán aufund beträgt ungefähr 280 Nebeltage pro Jahr.


Fig. 4 Fog climatology for Mexico: distribution of quarterly (seasonal) average number of fog days in the country:spring (March, April, May), summer (June, July, August), autumn (September, October, November), andwinter (December, January, February). Isolines (red) drawn every 10 fog days. The maximum quarterlyaverage value for a single site is 82 fog days per season – in the summer season in the Teziutlán region.


Verteilung der durchschnittlichen Anzahl der Nebeltage pro Jahreszeit in Mexiko: Frühling (März, April, Mai), Sommer(Juni, Juli, August), Herbst (September, Oktober, November) und Winter (Dezember, Januar, Februar). Die rotenIsolinien bezeichnen jeweils einen Abstand von 10 Nebeltagen. Der maximale Durchschnittswert für ein einzelnes Gebietbeträgt 82 Nebeltage je Saison – im Sommer in der Region Teziutlán.


4. Discussion

The general synoptic features involved in theformation and development of fog in Mexico canbe inferred from analysing the seasonal fog cli-matologies presented in Figure 4. In most of theMexican territory the rainy season occurs dur-ing summer and fall, between May and October(see Fig. 5). Thus, it is not surprising that theleast number of fog days in the year occurs inspring, with a minimum in April. The occurrenceof fog increases towards the end of spring, co-inciding with the beginning of the rainy andhurricane seasons in both coastal areas of thecountry (in May on the Pacific coast, and in Juneon the Atlantic-Caribbean coast). The availablesources of humidity during summer (maximumincidence of hurricanes and peak of the rainyseason) also coincide with the maximum season-al frequency of fog days all over the country,except for the Baja California Peninsula thatpresents a Mediterranean climate and precipita-

tion regime. Autumn marks the transition be-tween the end of the hurricane season and thebeginning of the season of cold frontal systemsaffecting Mexico. The frequency of fog in thenorthern regions diminishes whilst the seasonalmaxima concentrate in the mountainous, inter-tropical zone towards the south and east, in par-ticular in the southern part of the Sierra MadreOriental and in the Sierra of Chiapas. Finally,during winter fog incidence is almost exclusive-ly found in the higher altitudes under the influ-ence of the lower temperatures brought about bynortes, which also advect humidity from the Gulfof Mexico towards the continent (see below).

The synoptical and mesoscale characteristicswhich prevail in Mexico and have an influence onthe development of fog during the dry and rainyseasons are described in the following. In the sum-mer, most of the Mexican territory gets under theinfluence of the trade easterlies along the south-western end of the semi-permanent Bermuda high-

Fig. 5 Winter (December, January, February) and summer (June, July, August, September) climatologiesof rain for Mexico, for the period 1958-2004 (after Vázquez 2007) / Niederschlagsverteilung inMexiko im Winter (Dezember, Januar, Februar) und im Sommer (Juni, Juli, August, September),für den Zeitraum 1958-2004 (nach Vázquez 2007)


pressure system (Fig. 6a). These east-northwestprevailing winds gather humidity over the Gulf ofMexico and, after moving over the Gulf CoastalPlains, are orographically forced to ascend tohigher altitudes, thus producing typical cases ofupslope fog along the Sierra Madre Oriental. Twoexamples of this are Teziutlán and Las Alazanasregions (see Fig. 2). This circulation pattern iscommonly observed until autumn and also reach-es the central part of the territory, including theTrans-Mexican Volcanic Belt to the south and theHigh Plateau to the north. On the western coastthe circulation pattern is very much dependent onthe dynamics of the intertropical convergencezone, since the development of a warm water poolover the northeast Pacific Ocean induces a regionof deep convection and propitiates the formationof hurricanes that affect the Mexican coast.

On the other hand, in the winter the region liesin the Subtropical High and the synoptic situa-tion is dominated by a deep trough in low lati-tudes that defines an elongated area of relative-ly low atmospheric pressure along its axis ortrough line (see Fig. 6b). This large-scaletrough may include one or more closed circula-tions of low pressure, or cyclones, that producenortheasterly cold frontal systems which blowtowards the shores of the Gulf of Mexico. Theseso-called nortes or northerns that result froman outbreak of cold air from the north are a com-mon disruption of the mean synoptic conditionin the winter. At this time of the year it is com-mon to observe banks of stratus clouds nearthe coast of the Gulf of Mexico, both over thesea and over the coastal plains, that are advect-ed inland by the dominant winds towards the

Fig. 6 Mean circulation in the Mexico region: (a) Mean patterns of omega-vertical wind at 700 hPa (shadedareas in Pa s-1) and horizontal wind at 925 hPa (arrows in m s-1) during summer. (b) Mean patternsof surface pressure (isobars in hPa) and horizontal wind at 925 hPa (arrows in m s-1) characteristicof the passage of a norte during winter (after Magaña et al. 1999) / Vorherrschende Zirkulationim Raum Mexiko: (a) Mittlere Stärke und Richtung des Vertikalwindes bei 700 hPa (schraffierteFläche in Pa s-1) und des Horizontalwindes bei 925 hPa (Pfeile in m s-1) während des Sommers.(b) Mittlerer Luftdruck (Isobaren in hPa) und Horizontalwind bei 925 hPa (Pfeile in m s-1),charakteristisch für den Durchzug eines „Nortes“ während des Winters (nach Magaña et al. 1999)


mountainous, high altitude (1,500 to 2,000 m a.s.l.)areas, thus producing orographically modifiedcoastal fogs along the southern Sierra MadreOriental, often accompanied by drizzle and rain.These frontal systems also advect cold air to-wards the central Mexican Plateau, where theflow is modified by the local orography.

To illustrate the influence of mesoscale and lo-cal terrain features, two regional studies are dis-cussed in the following. The first case corre-sponds to the Mexico Basin (Fig. 7), located inthe Trans-Mexican Volcanic Belt, covering anarea of about 30 km in radius centered roughlyat downtown Mexico City and at an average al-titude of 2240 m a.s.l. The basin is mostly sur-rounded by mountains, including some of thehighest peaks in the country, like the Popocaté-petl (5,462 m a.s.l.), the Iztaccíchuatl (5,286 ma.s.l.) and the Ajusco (3,930 m a.s.l.) volcanoes,except to the northeast. This latter area wasoriginally occupied by Lake Texcoco, the larg-est of a system of interconnected lakes in thebasin that have been systematically drainedsince colonial times during the last four hun-dred years (see Fig. 7b). Fog events in theMexico Basin are not very frequent, reachingaverage maximum local values of up to 7 fogdays per month throughout the year, except inthe summer and early fall (June to September)when monthly maxima amount to up to 12 fogdays per month. However, when present, foghas important economic impacts because it in-terferes with the operation of the major airportin the country, especially during winter whenthe fog events are more persistent and takelonger to dissipate. In the winter the frontalsystems described above advect cold air towardsthe central Mexican Plateau, the flow being mod-ified by the local orography, reaching the Mexi-co Basin and producing typical radiative-advec-tive and post-frontal fog episodes with visibili-ties of less than 400 m in the early morning hours.As the day passes by, solar radiation warms upthe lower atmospheric layers and the fog dissi-

pates within a few hours after sunrise. It is alsoevident, however, that most relative fog-occur-rence maxima are associated to local terrain fea-tures that modify and reinforce the synoptic andmesoscale circulation. In particular, in fog areasclose to foothills there is an obvious influenceof a mountain-valley circulation that drains cool,humid air to the lower lands overnight. For theairport area, the additional presence of smallwater bodies like the remnants of Lake Texcocoprovides an additional local source of low-levelatmospheric humidity. The general charac-teristics of fog formation and the consequencesof the location of the airport near small shallowlakes (less than 10 km2 in surface area) have beendiscussed by Magaña et al. (2002). The above-described conditions, which are responsible forthe occurrence of fog episodes that force theshutdown of all operations on the airport, agreewell with the general features of the fog map forJanuary presented in Figure 7a.

The second regional case study corresponds tothe Teziutlán area located in the southern SierraMadre Oriental, where the maximum annual fogoccurrence in the country is observed. Fog in thisregion has been studied in some detail from dif-ferent viewpoints that include: climate-vegeta-tion relationship (Maderey et al. 1989; Ern 1972;Lauer 1978; Vogelmann 1973), hydrological bal-ance (Barradas 1983), chemical characteristicsand effects of fog water deposition (Báez et al.1998), and some meteorological (Fitzjarrald 1986)and microphysical aspects (García and Mon-tañez 1991). The incidence of fog is particularlyhigh during autumn and winter, with a maximumlocal average of up to 80 fog days per season.Monthly fog maps for the Teziutlán region forOctober and January, considered representativeof these two seasons, are shown in Figure 8.These maps adequately represent the above-described synoptic-mesoscale situation for theformation of upslope fog throughout the year,reinforced by the advection of stratus clouds fromthe Gulf of Mexico and the coastal plains towards


Fig. 7 a) Fog climatology for the Mexico Basin (left panel): average number of fog days in January. Eachsymbol on the map represents one station, with data density being approximately one stationper 25 km2. The underlying relief map indicates the approximate altitude of the basin floor (2,240m a.s.l.) as well as the maximum elevation shown in the map of about 3,000 m a.s.l. (the Sierrade Guadalupe) to the north.b) Vegetation cover and land use in the Mexico Basin (right panel). Note that most of the regionis occupied by the metropolitan area of Mexico City (after INE 2007).

a) Nebelklimatologie des Mexiko-Beckens: durchschnittliche Anzahl an Nebeltagen im Januar.Jedes Symbol auf der Karte steht für eine Station mit einer Datendichte von etwa einer Stationpro 25 m². Die darunterliegende Reliefkarte zeigt die ungefähre Höhe des Beckenniveaus(2.240 m ü. NN) sowie die maximalen Höhen im Norden, die ungefähr 3.000 m ü. NN erreichen(die Sierra de Guadelupe).b) Vegetationsbedeckung und Landnutzung im Mexiko-Becken. Ein Großteil der Region gehörtzum Agglomerationsraum von Mexiko City (nach INE 2007).

the region in the winter. In addition, they can beeasily related to modifications due to local fea-tures associated to the complex, high mountainterrain and vegetation of the region.

5. Conclusions

A fog climatology developed for Mexico at vari-ous spatial and temporal scales was developed with


historical weather records. The corresponding da-tabase was constructed using monthly averagevalues of daily observations acquired over a thirty-year period at about 2,900 climatological stationsof the Mexican National Meteorological Servicenetwork. Different data interpolation schemes andtheir performance were tested and evaluated in

order to produce fog-occurrence maps on variousspatial (national, regional) and temporal (yearly,seasonal, monthly) scales. It can be concluded thatthe program Surfer is a powerful tool for this typeof studies, provided the interpolation methods areadequately chosen given the general characteris-tics of the data. In particular, radial basis function

Fig. 8 Fog climatology for the southern Sierra Madre Oriental region of Teziutlán: average number offog days in: (a) October (monthly maximum local average of 24 fog days) and (b) January (monthlymaximum local average of 26 fog days). The maximum local annual average is about 280 fog days.Each symbol on the map represents one station, with data density being approximately onestation per 160 km2. The underlying relief map indicates that fog tends to concentrate in thevalley, the approximate altitude of Teziutlán is 1,920 m a.s.l.; it is surrounded by elevations ofup to about 3,000 m a.s.l. to the west. / Nebelklimatologie für die südliche Sierra-Madre-Oriental-Region von Teziutlán: durchschnittliche Anzahl der Nebeltage in: (a) Oktober (das monatlicheMaximum des örtlichen Mittels beträgt 24 Nebeltage) und (b) Januar (das monatliche Maximumdes örtlichen Mittels beträgt 26 Nebeltage). Der maximale lokale jährliche Durchschnitt beträgtetwa 280 Nebeltage. Jedes Symbol auf der Karte steht für eine Station, die Datendichte beträgt etwaeine Station pro 160 km². Die darunterliegende Reliefkarte zeigt, dass Nebel zu einer Konzen-tration in den Tälern tendiert; die Höhenlage von Teziutlán beträgt ungefähr 1.920 m ü. NN, dieTallagen sind im Westen umgeben von Erhebungen mit bis zu 3.000 m ü. NN.


interpolation using the multiquadric method gavegood statistical results for the national scale butlacked resolution for isolated data points. For theregional, better-resolved scale, this method provid-ed excellent results. It should be mentioned that insome cases triangulation gridding with linear inter-polation resulted in more appealing graphic repre-sentations. Needless to say, quality-assurancetechniques applied to the data were vital for ob-taining these satisfactory results.

Major regions of fog incidence in terms of the mainmeteorological and physical formation anddevelopment mechanisms were identified. Theseresults show the high variability of fog incidenceover the Mexican territory, both at the spatial andtemporal scales, and the difficulties that this im-plies for its proper and accurate handling, graph-ic representation and analysis. Finally, the de-scribed graphical representations were used tobetter understand synoptic, mesoscale and localfeatures related to the formation and developmentof fog. In particular, from regional studies localcharacteristics related to topography and landcover, including the presence of nearby waterbodies, can be inferred as modifying factors of thelarger-scales atmospheric conditions.

The results presented here represent the first at-tempt to develop a detailed national and regionalfog climatology for Mexico. In the future, the meth-odology will be tested and may be extended to otherhighly localised hydrometeorological phenomena,such as frost and hail incidence, for which there isalso a lack of detailed observational data.

Acknowledgements

The authors are indebted to Jorge Luis Vázquez-Aguirre and Dr. Víctor O. Magaña-Rueda for provid-ing the original maps presented in Figures 5 and 6.The underlying relief maps appearing in Figures 7aand 8 were drawn with digitised data provided byMaría de Lourdes Godínez-Calderón.

5. References

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and Science Data Center, U.S. Geological Center. –Sioux Falls, SD. – http://edc.usgs.gov/products/elevation/gtopo30/gtopo30.htmlVázquez-Aguirre, J.L. 2007: Variabilidad de la Pre-cipitación en la República Mexicana. – M. Sc. The-sis, Universidad Nacional Autónoma de México. –Mexico City. – Available from TESIUNAM Data-base, Clasif. 001-03060-V2-2007 at http://www.dgbiblio.unam.mx/Vogelmann, H.W. 1973: Fog Precipitation in theCloud Forests of Eastern Mexico. – In: BioScience23: 96-100

Summary: A Fog Climatology for Mexico

The results of a fog climatology developed forMexico are presented. The study is based onstandard average monthly fog days calculatedfrom daily observations acquired over thethirty-year period from 1961 to 1990, at the3,300 climatological stations of the MexicanNational Meteorological Service network. Af-ter applying corroboration checks and adjust-ments to the data, different interpolat ionschemes and their performance were tested andevaluated in order to produce fog-occurrencemaps on various spatial (national, regional) andtemporal (yearly, seasonal, monthly) scales,using the commercial contouring and surfacemapping program Surfer. For the data interpo-lation, it was found that the method of radialbasis function with an inverse multiquadric ker-nel rendered the best statistical results. Theseindicate that average values of fog occurrences,of more than 50 and up to 280 fog days peryear, are found more frequently in the coun-try’s main orographic systems. It is also foundthat the maximum seasonal frequency of fogdays in the country occurs during summer,thus coinciding with the peak of the rainy sea-son. These results are also analysed in view ofthe synoptical and mesoscale characteristicsthat prevail in Mexico during the dry and rainyseasons and then used to classify major regionsof fog incidence in terms of their main meteo-rological and physical formation and develop-


ment mechanisms. Finally, two regional casestudies are presented with the aim to illustratethe influence that mesoscale and local terrainfeatures, such as topography and vegetationand land cover, have on the formation and de-velopment of fog. This study represents thefirst attempt towards a comprehensive and de-tailed fog climatology for Mexico. It is con-cluded that fog incidence over the Mexican ter-ritory shows high variability at both spatialand temporal scales, showing the difficultiesthat this implies for its proper and accuratehandling, graphic representation and analysis.

Zusammenfassung: Eine Nebelklimatologie fürMexiko

Dieser Artikel beschreibt die Ergebnisse einerStudie zur Nebel-Klimatologie Mexikos. Die Studiebasiert auf der Anzahl der durchschnittlichenmonatlichen Nebeltage, die berechnet wurde ausDaten von täglichen Erhebungen an 3.300 Klima-stationen des mexikanischen Wetterdienstes, wel-che über eine 30-jährige Periode, von 1961 bis 1990,durchgeführt wurden. Nach Plausibilitätsprüfungenund Anpassungen der Daten wurden verschiedeneInterpolationsverfahren und deren Ergebnisse ge-testet und ausgewertet, um Karten zu Nebel-Häufig-keiten auf verschiedenen räumlichen (national, regi-onal) und zeitlichen (jährlich, saisonal, monatlich)Ebenen zu erstellen. Hierzu wurde das kommerzielleKonturierungs- und KartenoberflächenprogrammSurfer verwendet. Für die Interpolation wurdefestgestellt, dass das Verfahren der radialen Basis-funktion mit einem inversen multiquadratischenKern die besten statistischen Ergebnisse lieferte.Diese zeigen, dass die Durchschnittswerte hin-sichtlich des Auftretens von Nebel mit mehr als 50bis zu 280 Nebeltagen im Jahr häufiger in denwesentlichen Gebirgszügen des Landes liegen.Ebenfalls fand man heraus, dass die maximale sai-sonale Häufigkeit von Nebeltagen im Land währenddes Sommers auftritt und somit identisch mit demHöchststand der Regenzeit ist. Diese Ergebnissewurden außderdem mit Blick auf die synoptischenund mesoskalen Merkmale analysiert, die in Mexi-ko während der Trocken- und Regenzeit vorherr-

schen, um die Hauptgebiete von Nebelvorkommenbezüglich ihrer vorherrschenden meteorologischenund physikalischen Konstellation sowie ihrer Ent-stehungsmechanismen zu typisieren. Schließlichwerden zwei regionale Fallstudien präsentiert mitdem Ziel, den Einfluss der mesoskalaren und loka-len Besonderheiten des Raumes – wie Relief, Vege-tation und Bodenbedeckung – auf die Bildung undWeiterentwicklung von Nebel deutlich zu machen.Die Studie stellt den ersten Versuch einer umfas-senden und detaillierten Nebel-Klimatologie fürMexiko dar. Sie macht deutlich, dass das Auftretenvon Nebel über Mexiko sowohl auf räumlicher wieauch auf zeitlicher Ebene eine hohe Variabilitätaufweist. Ebenso zeigt sie die Schwierigkeit, dieseDaten sachgerecht und genau aufzubereiten, gra-phisch darzustellen und zu analysieren.

Résumé: Une climatologie du brouillard pour leMexique

Les résultats d’une climatologie du brouillard déve-loppée pour le Mexique sont présentés. L’étude estbasée sur le nombre mensuel moyen des jours debrouillard calculé à partir des observations quoti-diennes acquises au cours des trente années de 1961à 1990 sur 3300 stations climatologiques du réseaudu Service Météorologique National du Mexique.Après l’application des contrôles de corroborationet des ajustements des données, les différents régi-mes d’interpolation et leurs performances ont ététestés et évalués afin de produire des cartes del’apparition du brouillard sur diverses échellesgéographiques (nationale, régionale) et temporel-les (annuelle, saisonnière, mensuelle), en utilisant« Surfer », le logiciel commercial de contouring et decartographie de surface. Pour l’interpolation desdonnées, il a été constaté que la méthode de lafonction d’une base radiale avec un noyau inverseà élévation multiple au carré rend les meilleursrésultats statistiques. Ceux-ci indiquent que cesvaleurs moyennes des événements de brouillard,allant de plus de 50 jusqu’à 280 jours de brouillardpar an, se retrouvent plus fréquemment dans lesprincipaux systèmes orographiques du pays. Il estégalement constaté que la fréquence saisonnièremaximale de jours de brouillard dans le pays se


produit au cours de l’été, ce qui coïncide avec lemaximum de la saison des pluies. Ces résultatssont également analysés en vue des caractéristi-ques synoptiques et à échelle moyenne qui règnentau Mexique pendant la saison sèche et la saison despluies, et qui sont ensuite utilisés pour classer desgrandes régions d’apparition de brouillard en ter-mes de leurs mécanismes principaux de formationet de développement météorologiques et physi-ques. Enfin, deux études de cas régionales sontprésentées dans le but d’illustrer l’influence que lestraits du relief, à échelle moyenne et au niveau local,telles que la topographie, la végétation et la couver-ture de terre, exercent sur la formation et le dévelop-pement de brouillard. Cette étude représente lapremière tentative d’établir un inventaire exhaustifet détaillé de la climatologie du brouillard pour leMexique. Il est conclu que l’apparition de brouillardsur le territoire mexicain montre une variabilitéélevée aux échelles spatiale ainsi que temporelle,démontrant les difficultés que cela implique pourson traitement approprié et soigneux au niveaud´une représentation graphique et de l’analyse.

Resumen: Una climatología de niebla paraMéxico

Se presentan los resultados de una climatologíade niebla elaborada para México. El estudio sebasó en normales climatológicas mensuales dedías con niebla, calculadas de observaciones dia-rias realizadas durante el período de treinta años1961-1990 en las 3,300 estaciones climatológicaspertenecientes a la red del Servicio MeteorológicoNacional. Luego de aplicar varias pruebas de co-rroboración y ajustes a los datos, se probó y eva-luó el desempeño de diferentes esquemas de in-terpolación de datos para así producir mapas cli-matológicos de ocurrencia de niebla, tanto en es-cala espacial (nacional, regional) como temporal(anual, estacional, mensual), mediante la utiliza-ción del programa comercial Surfer. Se encontróque el método de función radial con kernel multi-cuádrico da los mejores resultados para la inter-

polación de los datos. Estos resultados se utiliza-ron para clasificar las principales regiones de inci-dencia de niebla en términos de los principalesmecanismos meteorológicos y físicos para su for-mación y desarrollo. Estos resultados muestranque los valores promedio más grandes de ocurren-cia de niebla, de entre 50 y hasta 280 días conniebla al año, se dan en los principales sistemasmontañosos del país. También se observa que lamáxima frecuencia de días con niebla en el ámbitonacional ocurre en el verano, coincidiendo con latemporada de lluvias. Los resultados también seanalizaron en términos de las características de lacirculación, tanto sinóptica como de mesoescala,prevaleciente en México durante las temporadasde secas y lluvias, para así clasificar las regionesde niebla identificadas con respecto a los princi-pales mecanismos físicos y meteorológicos de for-mación de niebla. Finalmente, se presentan doscasos de estudio a nivel regional con el fin de ilus-trar la influencia que la circulación de mesoescalay las características locales del terreno, tales comola topografía y la cubierta de vegetación y el sue-lo, tienen en la formación y el desarrollo de laniebla. El estudio representa un primer esfuerzode obtener una climatología detallada y completade niebla para México. Se concluye que existe unagran variabilidad en la incidencia del fenómeno so-bre el territorio mexicano, tanto en escala espacialcomo temporal, y se muestran las dificultades queesta variabilidad acarrea para su adecuado manejo,representación gráfica y análisis.

Dr. Fernando García-García, Víctor Zarraluqui,Centro de Ciencias de la Atmósfera, UniversidadNacional Autónoma de México, Circuito de la Inves-tigación Científica, Ciudad Universitaria, 04510México, D.F., México, [email protected],[email protected]

Manuskripteingang: 07.01.2008Annahme zum Druck: 21.04.2008

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