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Dennis Wheeler University of Sunderland

Gibraltar can fairly claim one of the longestprecipitation records in the Mediterraneanregion. Observations began in 1790 underthe auspices of the Royal Engineers (RE) but,as noted in Part 1 of this series (Wheeler,2006), the stimulus behind this endeavourcame more from the need to ensureadequate management of water supplyrather than from any scientific enterprise.Gibraltar's relationship with mainland Spainhas sometimes been a difficult one andwater supply had to be organized locallyand was for many years dependent on rain-fall and some wells, both of which proved tobe unreliable, the latter because of theirbrackish quality and the former because ofmarked seasonality in its distribution andproblems of the summer drought of unpre-dictable duration (Figure 1). At the outbreakof World War Two the situation was givingcause for alarm and the systems then avail-able are described in Forster (1942). Morerecently the traditional methods of rain-water storage have been supplemented bydesalinization units (Wright et al., 1994), butrainfall and water supply continue to be amatter of justifiable preoccupation. Carefulobservation and understanding of thepatterns of rainfall assumes, therefore, anunusual significance.

History of observing sites andobserversThis paper concentrates on the principalseries that is now provided by the Met Office(MO) from its site at the Airport at NorthFront (Figure 2). This is the series that beginsin 1790 but, not surprisingly, the observa-tion site has been moved on a number ofoccasions and Table 1 lists those that havebeen used over the past two centuries.There were also changes in instrumentation,and the 5-inch standard MO gauge has beenused only since 1935 following the move tothe Windmill Hill site. Nothing is known ofthe ‘pluviometer’ with which observationswere made before 1852, nor of its exposure.

There is, however, greater certainty regard-ing the instruments used after that date andJames (1861), an RE officer, has left a com-prehensive account of the procedures to beadopted by observers in the regiment. Tworaingauges were recommended for use atthe time. One was circular in design, 8 inch-es in diameter and could be supported on astand or sunk into the ground. The otherwas a ground gauge only, but was square insection and 10 inches in width. The rainwa-ter was measured after transfer to a graduat-ed cylinder. It is known that two gaugeswere used in the early years of the Gibraltarseries; one on the ground and the otherraised 25 feet higher. The latter was regard-ed as less reliable, being thought to beinsensitive to light falls of rain.

Between 1790 and 1812 the official recordis annual only, the data being expressed as12-month totals over the period August toJuly each following year. This period wastaken to represent the ‘rainfall year’ whichbegins with the first rains of late August orearly September and concludes with theonset of the summer drought. The seriesbecomes monthly in January 1813 and con-

The Gibraltar climatic record: Part 2 – precipitation

Figure 1. The view from La Linea towards Gibraltar as it appeared in 1879. The picture of Mediterraneanaridity captured in this charming landscape is today obscured by the airfield, border crossing and MoDbuildings that now cover this narrow isthmus that connects the Rock to the mainland of Spain. Suppliedcourtesy of Luis Photos (Gibraltar).

Figure 2. Map showing the raingauge sites andother locations mentioned in the text. The key forthe names is given in Tables 2 and 4.

tinues so until February 1834 when therecord reverts to ‘rainfall year’ totals. Only inJanuary 1850 is the monthly series re-estab-lished. The daily record, preserved in thestation’s ‘blue books’, begins in the 1880s.Fortunately other sources have come tolight that help to fill some, but by no meansall, of the gaps in the monthly and dailyseries. From 1821 (and until 1936) theGibraltar Chronicle carried daily weatherreports based on the official observations,but these included rainfall observationsonly from the 1830s. In addition, monthlysummaries for all RE sites for the period1852 to 1886 have been published (HMSO,1890). Both sources draw attention to theuse of two raingauges in the 1850s and1860s but preference was given to thoseobservations taken at ground level althoughwhen these were not available recourse was made to data from the gauge at 25-feet.The RE were responsible for the observa-tions in Gibraltar until 1863 when the dutieswere assumed by the Royal Army MedicalCorps (RAMC) and the Gibraltar Governmentwith whom they remained until 1936 whenthe Air Ministry took charge of the work.

It is known, however, that other sites werealso in operation. Prompted no doubt by the demands for water supply planning, theGibraltar Government established rain-

gauge sites at various locations around theRock. These are shown in Figure 2 and theirdetails are summarized in Table 2. BothForster (1942) and Hurst (1956) made use ofthese data in their exhaustive analyses oflocal rainfall. These records seem, regret-tably, to have been lost. Other, more frag-mentary, records from the 1840s and 1850shave survived. The oldest are those pre-pared by Sir John Hall (Principal MedicalOfficer) that begin in November 1838 andcontinue to January 1841. His weather diaryis now in the care of the MO and includesalso daily temperatures, supplemented bynotes on the wind, the weather and ‘pre-vailing diseases’. The 1840s and 1850s alsosaw a number of private monthly reportsand summaries published in the GibraltarChronicle. Those for the period 1844 and1845 were compiled by another member ofthe RAMC, E.F. Kelaart. His observations alsoappear in summary form in his publicationFlora Calpensis: contributions to the Botanyand Topography of Gibraltar (Kelaart, 1846).He was followed as the Gibraltar Chronicle’sweather correspondent by ‘JS’ (his identity isunknown) but whose monthly reportsappeared from 1846 to 1848. It is not knownexactly where the Kelaart and JS observa-tions were taken. Both refer to ‘the South’,which may suggest the area around Rosia

Bay on the western side of the peninsula.Between 1844 and 1848 these twoobservers provided 50 detailed monthlyreports. Similar monthly summaries basedon the RE and RAMC records appeared from1852 and Figure 3 shows a typical exampleof the period. It is also known from occasion-al items in the Gibraltar Chronicle monthlyreports in the 1860s that the RE operated araingauge (25 feet above the surface model)at Europa Point, but none of these data havesurvived.

The MO monthly series was collated by J. Rothwell and D. Hyde in 1985 with minorchanges by K. Grant in 1995. This recorddoes not include the various suggestions forhomogenization suggested by Hurst (1956);neither have the early gaps noted abovebeen filled. It seems unlikely that docu-ments have survived to permit the monthlyrecord before 1813 to be reconstructed, butthe Gibraltar Chronicle published occasionaldaily figures from 1829 and did so on a con-tinuous basis from 1840. Fortunately, suffi-cient data are available to enable themonthly figures to be calculated or deducedfrom the various reports and it is possible toreconstruct a set of data, based on sourcesconsistent with the extant series, to com-plete the monthly series from 1834 to 1850.The daily record can be extended back

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Table 1

Summary of locations of the ‘official’ Gibraltar raingauge sites.

LocationAltitude

(m above sea-level (ASL)) Dates Notes (summarized from Hurst (1956))

Garrison Library 7.6 1780 to 1852 This location cannot be confirmed with absolutecertainty, but the evidence offered in Wheeler(2006) strongly supports this suggestion.

South Bastion (1) 15 1852 to 1913 A garden site and possibly too protected.

South Bastion (2) 12.5 1913 to 1929 This record suggests some interference by war-time installations after 1914.

Alameda Gardens 31 1929 to 1935 A well-exposed site but the record reveals anincrease in mean annual rainfall of 75 mm after the move from South Bastion.

Windmill Hill (1) 120 1935 to 1940 A representative and well-exposed site.

Windmill Hill (2) 119 1940 to 1944 This site was too close to a sharp 30 m fall to avoid ‘carry over’ in westerly winds.

Windmill Hill (3) 107 1944 to 1947 This site was further from the break of slopenoted above and is closer to conditions onWindmill Hill (1).

North Front (1) 2.4 1947 to 1955 On the RAF airfield between the Rock and main-land Spain.

North Front (2) 2.4 1955 to present Slightly more exposed than North Front (1).

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continuously from 1888 to 1840 and in morefragmentary form to 1831.* For this latterperiod the daily record is partial; days oflight rain were not always reported, butthose of heavier falls were regarded as ofsufficient interest for publication. Finally, theGibraltar Chronicle records have beenabstracted to reconstruct the ‘rain day’seriesback to the start of meteorological report-ing in July 1821 (Table 3). Between that dateand 1831, although rainfall depths were notreported, the daily summaries indicatedthose days when rain was observed to havefallen. Such is the variability of day-to-dayrainfall, however, that it is impossible toconvert these ‘rain day’ frequencies into anyreliable form of rainfall depth.

It might be expected that the MOmonthly series and that abstracted from theGibraltar Chronicle and from the RE pub-lished report should be identical, and thissupposition is correct for the periods from1833 to 1849 and from 1879 onwards. Butbetween 1850 and 1878 there are occasion-al inconsistencies between the three. Themost common disagreements are betweenthe Gibraltar Chronicle and the other two.The differences are consistent neither inmagnitude nor direction; systematic errorscan be eliminated, and they might be attri-buted to typesetting or similar failings thatcould arise as a consequence of the rush togo to print each day. Of more concern arethe occasional differences between the MOrecord and that of the RE. The largest suchdifference is just over 50 mm in the annualtotals for 1859, almost all of which resultsfrom the May totals of 70.9 mm (converted

The Gibraltar clim

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Table 2

Sites of Gibraltar Government raingauges. Initial letters refer to the key sites in Figure 2.

Location Altitude (m ASL)

Dates Notes (summarized from Hurst (1956))

Moorish Castle (MC) 79 1900–1956 The ground is irregular and the site exposed.

Green’s Lodge (GL) 247 1925–1951 On steep ground, a recording gauge was used in anexposed site made more so by the gauge being located on the roof of a small hut.

Willis Gate (WG) 140 1902–1956 Only 150 yards (137 m) from Moorish Castle and onsimilar ground.

Catalan Bay (CB) 128 1927–1951 Overlooked by the steep, east face of the Rock. Variousgauges were used here including a recording type, and an 8-inch and, later, 5-inch gauge.

City Hall (CH) 25 1949–1956 Over-exposed recording gauge on a flat roof.

North Front (NF) 3 1902–1940 Located not far from the present site (Table 1).

Rock Gun (RG) 375 1932–1941 Highly exposed, yielding data always considered to be of questionable quality.

Convent Garden (CG) 15 1950–1956 A 5-inch gauge within the Governor’s walled gardenand possibly over-protected.

* These data are available from the author.Figure 3. Monthly weather summary for December 1861 showing the degree of meteorological detailprovided by the Observatory. Supplied courtesy of the Gibraltar Government Archives.

from inches) in the MO series but 121.6 mmin the RE series. The 1864 annual totals differby 28.1 mm. On this occasion the MO serieshas the higher figure. Such differences can-not be accounted for by mixed use of datafrom the ground and 25-foot gauges notedabove. During much of 1857 and 1858 theGibraltar Chronicle published both sets ofobservations and over this period the accu-mulated totals differed only by 0.31 mm,while the mean absolute daily differencewas 2.11 mm. Other annual differencesbetween the MO and RE series are, however,in the order of a few millimetres and onlytwice exceed 10 mm (in 1872 and 1878).

Homogeneity of the MetOffice seriesRainfall is one of the most localized ofmeteorological phenomena and can varyover short distances in response to localfeatures and degree of exposure. This prob-lem is exacerbated because of Gibraltar’stopography and its position in relation tothe dominantly eastward or westwardnature of the winds that prevail through theStraits at all times of year. Indeed, so acute isthe problem created by the Rock itself thatthe Air Ministry commissioned a report atthe outbreak of World War Two to assist inbetter forecasting (Gordon, 1940).

Hurst (1956) was concerned that thechanges in site of the MO series createdproblems of inhomogeneity as a result ofmarkedly different local conditions. He sug-gested a solution by making adjustmentsbased on comparisons with the long run ofdata available from the Moorish Castle site(then operated by the Gibraltar Govern-ment) that remained fixed between 1900and 1956 whilst the official site moved on anumber of occasions (Table 1). Table 4

summarises Hurst’s derived ratios of meanannual rainfall at the MO sites to that atMoorish Castle. Although some of the seriesare too short for any reliable conclusion tobe drawn from them, the longer subsets ofthe MO series can profitably be compared;these being those for Garrison Library (1790to 1852), South Bastion 1 (1852 to 1912) andNorth Front 2 (1955 to present). The respec-tive means of the August to July ‘rainfallseason’totals are 792.4, 897.6 and 776.9 mm.An ANOVA test was carried on the three sub-sets for which the F (variance ratio) value of3.36 was significant at the 0.05 level, sug-gesting significant differences betweenthem. A Tukey Post-Hoc test revealed, how-ever, that the North Front and GarrisonLibrary subsets were not themselves signifi-cantly different and that the test result waswholly a consequence of the South Bastionsubset’s high mean. There is, however, noincontrovertible evidence that this differ-

ence is attributable to the change of site.Figure 4 shows that the move from GarrisonLibrary to South Bastion was indeed accom-panied by an increase in annual rainfall, butthat the increase was short-lived and annualtotals returned to their previous levels with-in a decade but left an indelible statisticalmark on the subset for that site. On the basisof the above findings, no case can be madefor any useful adjustments to be made tothe MO series other than the additions forthe 1830s and 1840s noted above.

The climatic settingAt 36° 8' N and 5°26' W, Gibraltar enjoys anindisputably Mediterranean location, and itis this that provides the area with its climatictheme upon which there are local variationsthat result from its position within the con-fines of the Straits and its exposure toAtlantic weather systems. The mean annual

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Table 4

Ratios of mean annual rainfall at different MO sites to that at Moorish Castle. Initial lettersrefer to the key sites in Figure 2.

Site Period Ratio to Moorish Castle annual mean

South Bastion 1 (SB) 1900–1913 1.067

South Bastion 2 1913–1929 0.985

Alameda Gardens (AG) 1929– 1942 1.161

Windmill Hill 1 (WH) 1935–1940 1.080

Windmill Hill 2 1940–1944 0.870

Windmill Hill 3 1944–1947 1.052

North Front (NF) 1947–1956 0.990

Table 3

Rain day record abstracted from the Gibraltar Chronicle for the period 1821 to 1832.

Jan Feb March April May June July Aug Sept Oct Nov Dec

1821 n/a n/a n/a n/a n/a n/a 0 0 0 5 3 13

1822 3 10 2 8 11 3 0 1 4 3 10 12

1823 14 7 3 11 5 3 1 3 3 12 13 7

1824 12 12 6 3 0 2 2 3 1 7 0 2

1825 7 2 9 12 1 3 0 6 3 2 7 14

1826 13 3 8 3 9 2 0 2 1 6 9 4

1827 9 19 5 8 3 0 0 0 4 9 4 7

1828 3 9 0 5 8 0 2 2 1 4 15 6

1829 13 8 19 11 7 2 0 0 5 4 14 17

1830 18 7 2 6 7 4 0 0 0 5 9 15

1831 17 4 3 20 4 1 0 0 4 4 1 4

1832 9 6 5 5 0 1 0 0 0 1 7 1

mean 10.7 7.9 5.6 8.4 5.0 1.9 0.4 1.4 2.2 5.2 7.7 8.5

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rainfall (based on the ‘rainfall year’ for theperiod 1790 onwards) is 814.5 mm, but thisstatistic disguises the changes from year-to-year that are witnessed in Figure 4 and aretypical of its Mediterranean setting, whilethe adage ‘hot dry summers and warm, wetwinters’ is emphasized in Table 5. Never-theless, Gibraltar’s particularly exposedlocation makes it noticeably wetter thannearby sites in mainland Spain. Tarifa, to theimmediate west, has an annual mean of685 mm, while at Malaga, to the east, theannual mean is 469 mm. The statisticaldistribution of annual means (based on therainfall year as defined above) is shown inFigure 5 and displays a significant degree ofskewness, due in some measure to the two exceptionally wet years of 1797/1798(1654 mm) and 1855/1856 (1970 mm). Incontrast, the driest year was 1994/1995 with285 mm. The second-driest year was 2000/2001 with 329.1 mm.

The individual monthly series summa-rized in Table 5 suggest similar degrees ofvariability to those revealed by annual data.Completely dry months characterize thesummer but only January has failed to regis-ter at least one such incidence. On the otherhand, November 1829 was the wettestmonth with 654.8 mm, most of which fell inone week. This tendency to persistence ofwet weather is, however, by no meansunusual and rainfall often occurs in spellsthat might be prolonged over several days.To the 557.3 mm that fell between 18 and 25November 1829 should be added the380.2 mm recorded between 20 and 26November 1955. The latter event wasreported by Hurst (1955). Mention needsalso to be made of the occasional very highdaily totals that have been recorded. Theseoutbreaks are almost exclusively related tocyclonic situations. Notable daily falls arewell-distributed through the two-centuryseries, the two highest being those of 14December 1796 (231.9 mm) and 31January1959 (293.9 mm). Such totals are slightlyhigher than those for the nearby Spanishmainland but less than those occasionally

The Gibraltar clim

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Table 5

Statistical summary of monthly precipitation in Gibraltar based on the 1813–2004 series. All values are in millimetres. Records of a 'trace' are not included in the zero rainfall counts.

Jan Feb March April May June July Aug Sept Oct Nov Dec

mean 122.9 98.4 96.8 68.4 37.0 12.1 0.7 4.1 26.1 80.6 139.6 131.5

maximum 549.1 385.3 451.1 213.4 164.6 148.9 15.7 134.8 215.9 314.2 654.8 627.0

date 1856 1924 1915 1817 1853 1930 1881 1987 1906 1864 1829 1996

minimum 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

date 1981 4x 4x 3x 7x 34x 145x 75x 20x 2x 3x 2x

standard deviation 108.4 82.7 73.7 48.4 34.4 22.2 2.1 13.1 34.5 62.9 118.1 110.3

Figure 4. Annual rainfall (August–July) for the period 1790 to 2004. The running mean based on a 12-yearGaussian filter (bold red) is also shown.

Figure 5. Histogram showing the statistical distribution of ‘rainfall year’ totals (1790–2005) for Gibraltar.

experienced along the coastline fromValencia northwards.

This tendency to persistence in rainfallreflects its often frontal and cyclonic origin.Convective activity, more common overmainland Spain, is limited by Gibraltar’s mar-itime setting. Orographic rainfall is similarlycircumscribed and the famous ‘levanter’banner cloud is its most obvious manifesta-tion, but this appears only on easterly windsas moist Mediterranean air is forced to riseover the Rock, forming cloud and overcastconditions over much of the isthmus. Theremay also be orographic enhancement onwinds from the south-south-west leading tolocalised convergence (J. Rothwell, personalcommunication). Spells of abundant rainfallare usually associated with slow-movingcyclonic systems to the west of the Straits offthe Iberian or Moroccan coastlines, perhapsaccompanied by high pressure in theMediterranean leading to persistent andhumid low-level easterly air flows throughthe Straits. Such occasions occur more com-monly from September through till April.Some of the depressions are in the form ofcut-off lows as described by Boyden (1963)and the MO (1978), whose quasi-stationary character allows for the persis-tence of conditions noted above. Thissynoptic state has long been recognized bySpanish meteorologists, who have identifiedit as a specific ‘type’ in their various classifica-tions (Font Tullot, 1983 and Martín Vide,2005).

On the other hand, the summer drought isintimately associated with the annual exten-sion of the Azores sub-tropical anticyclonethat extends its embrace to much of thisregion between, very approximately, Mayand September. Even here, however, thepicture is complicated by the importance ofthe thermal ‘lows’ that develop over theIberian land surface and give rise to distur-bances in an otherwise anticyclonic regime

and help to account for the occasional highsummer rainfalls cited in Table 5. By northEuropean standards, the equinoctialseasons are short periods and the onset ofwinter is sometimes marked by outbreaks ofcyclonic activity that appear with a monsoon-like abruptness, as is testified bythe November totals cited in the precedingparagraph and given further historicalwitness by the Trafalgar storm that set inimmediately following the battle on 21October 1805 (Wheeler, 2001) and intro-duced a week of exceptionally disturbedweather to mark the close of summer.

ConclusionThe Gibraltar precipitation record representsan important contribution to the worldclimate record through its temporal spanthat now exceeds two centuries andthrough the information that exists to allowscientists to confirm its reliability. In theprocess of preparing this paper, new datahave been added that helps to complete therecord, but gaps remain for monthly data inthe early nineteenth and late eighteenthcenturies.

AcknowledgementsThe author acknowledges with gratitudethe assistance provided by Lorna Swift(Garrison Library, Gibraltar) and DennisBeiso (Gibraltar Government Archives) in thepreparation of this paper.

ReferencesBoyden CJ. 1963. Development of the jetstream and cut-off circulations. Meteorol.Mag. 92: 287–299.

Font Tullot I. 1983. Climatología deEspaña y Portugal. Instituto deMeteorología, Madrid.

Forster HR. 1942. Rainfall of Gibraltar.Unpublished report for the MeteorologicalOffice.

Gordon AH. 1940. Topographical factorsaffecting the forecasting of weather atGibraltar. Air Ministry, CD No. 211, London.

HMSO. 1890. Meteorological Observationsat the Foreign and Colonial Stations of theRoyal Engineers and the Army MedicalDepartment 1852–1886. MeteorologicalCouncil, London.

Hurst GW. 1955. Prolonged heavy rain atGibraltar, November 20–26, 1955.Meteorol. Mag. 85: 324–328.

Hurst GW. 1956. A statistical survey ofrainfall at Gibraltar. Unpublished report forthe Meteorological Office (Gibraltar).

James H. 1861. Instructions for takingmeteorological observations. Eyre andSpottiswoode, London.

Martín Vide J. 2005. Los Mapas delTiempo. Editorial Davinci, Barcelona.

Met Office. 1978. Weather in Home Waters,Vol. III: the waters around the Azores and offsouth-west Europe and off north-westAfrica. Meteorological Office, Bracknell.

Kelaart EF. 1846. Flora Calpensis:contributions to the Botany and Topographyof Gibraltar. John van Voorst, London.

Wheeler D. 2001. The weather of theEuropean Atlantic Seaboard duringOctober 1805: an exercise in historicalclimatology: Clim. Change, 48: 361–385.

Wheeler D. 2006. The Gibraltar climaticrecord: Part 1, the history of weatherobservations. Weather, 61: 36–39.

Wright EP, Rose EPF, Perez M. 1994.Hydrogeological studies on the Rock ofGibraltar. Q. J. Eng. Geol. 27: 815–829.

Correspondence to: Dr Dennis Wheeler,University of Sunderland, School of Health,Natural and Social Sciences, Sunderland, SR1 3PZ.

e-mail: denniswheeler@beeb.net

© Royal Meteorological Society, 2007

doi: 10.1002/wea.49

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