the gibraltar climatic record – part 3. temperature
TRANSCRIPT
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The Gibraltar climatic record – Part 3. Temperature
Dennis WheelerUniversity of Sunderland
IntroductionThis article is the third in a series on the climate of Gibraltar, the record of which is best known for the long series of precipita-tion data that runs from 1790 to the present day and was described in a previous article in this series (Wheeler, 2007). The general and remarkable history of weather observ-ing on ‘the Rock’ has been discussed in Hurst (1959) and Wheeler (2006; 2009). Often overlooked, however, is the temperature series, which, whilst not enjoying the same antiquity as its precipitation counterpart, is now known to exist from as long ago as 1821 and offers another valuable insight into the climate and climatic variation of the southwest Iberian region. The precipitation record has been the subject of much justifi-able interest and in addition to the above-cited papers, attention should be drawn to Gordon (1940), Foster (1942) and Wheeler and Martin-Vide (1992). Neither has the barometric pressure record escaped atten-tion – Gibraltar forming one of the southern anchor points of the North Atlantic Oscillation Index (Jones et al., 1997). But the temperature record, regrettably, has not been accorded the status its potential value demands. Particularly neglected have been the first 30 years of the series (1821 to 1851): a neglect this article hopes to remedy by integrating and homogenizing the early nineteenth century data into the official Met Office series that begins in 1852, filling minor gaps elsewhere and, finally, review-ing the temperature regime of this most interesting and well-established of Mediterranean observing stations.
The Gibraltar recordThe official Met Office Gibraltar temperature series began in 1852 and is now continued through the station at the North Front site at RAF Gibraltar (Figure 1). Figure 2 shows the geography of Gibraltar and indicates the location of the weather station that is now situated on the concrete-covered isthmus that links Gibraltar with mainland Spain. The
original observation registers for the Met Office series now reside in the National Meteorological Library and Archive in Exeter. But other data, exclusively from the nineteenth century, can also be called upon from a number of documents. The monthly aggregated data of the Royal Engineers observations covering the period 1852 to 1886 are available in HMSO (1890) although there are gaps and occasional inconsisten-cies in this otherwise valuable publication. Of greater interest and consistency are the daily observations published in the Gibraltar Chronicle from 1 July 1821. It is not known if temperature readings were made before that date and extensive searches of the Gibraltar Government Archives have failed to yield any further information or records. The practice of publishing these daily data continued until the 1970s when weather reports gradually disappeared from the Chronicle’s pages. These daily temperature data appeared only in the form of maximum and minimum temperature readings from 1852, but from their inception in 1821 until 1851 they were in the form of ‘fixed-hour’ observations, the times being originally 9am, midday, and 3pm local time. In the early 1830s observations were also reported
for 8pm and from July 1832 an observation for 5pm was included. An example, indicat-ing the range of phenomena reported, is shown in Figure 3. Only the 9am observa-tion appears regularly through the first 30 years of observations and for reasons of consistency of record we will concentrate on these data in the following analyses. It should be noted also that during these first three decades of the series, the readings, whether presented as vulgar fractions or, as they were from December 1848, in decimal form, were supplied to ¼ (0.25) of a degree Fahrenheit, a fraction which is not vastly dif-ferent from 0.1 of a degree Celsius and therefore offers usefully similar statistical resolution.
The sites at which the temperature obser-vations were made were the same as those for rainfall and these are summarized in Table 1 and Figure 4. The earliest data seem to have been gathered whilst the observa-tory was in the grounds of the Garrison Library on the west-facing side of Gibraltar close to the centre of the town. Consequent-ly it must be assumed that from then until well into the twentieth century, and the move to Windmill Hill on the southern tip of the Rock, the sites have been on the west-facing
Figure 1. General view of the rock of Gibraltar seen from North Front, the site of the current weather station. (© Dennis Wheeler.)
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the stand, to allow the air to circulate freely… (p.19). As regards instrument exposure for the years before 1852 one can only specu-late. This period comes before the invention of the Stevenson Screen in the mid-nine-teenth century, and the Glaisher Screen invented a little earlier would not have been available for the start of activities in 1821, whilst the general nature of exposure and observation al procedures may have varied. Although there is no conclusive evi-dence for exposure of thermometers at the Garrison Library, evidence from elsewhere in the Gibraltar Chronicle might be cited. Several weather reports were published in the 1840s signed merely by ‘J S’ about whom we have no other information. This observer lived in the Rosia district (south of the town centre as it then was) and the following lines appeared on 8 June 1847 to
The other sites used subsequently fall within the period of the Met Office official record (henceforth the MO series) although responsibility for the observations passed in 1863 to the Army Medical Corps and only latterly to the Met Office. As for the condi-tions under which the observations were made, it is known from the registers in the National Meteorological Archives that the thermometers in use in 1852 were of Negretti and Zambra manufacture. Moreover, their exposure at this time is known from James’s (1861) Instructions for taking Meteorological Observations issued to all Royal Engineers observers, in which the instrument stand is described as fol-lows: The stand is a double at the back, and revolves on a post about four feet from the ground; the Case of thermometers is kept out by blocks about two inches from the face of
side of the Rock, sheltered by buildings and trees and have enjoyed a partially urban setting. The only exception, and a notable one, to this helpful level of consistency in the exposure occurred between April and July 1851 when all observations were made at the Signal Station at the top of the Rock in a notably exposed situation at an altitude of 385 metres! The observation site returned to the town on 19 July when the Gibraltar Chronicle reported The meteorological obser-vations inserted in the Chronicle from 5th April to Saturday 8am on 19th inst. inclusive were taken from the Signal Station, but in future these observations will, as formerly, be taken from the Garrison Library [author’s empha-sis]. Until 1852, the Gibraltar Chronicle is mute on the place where the observations were made. It seems highly probable, but cannot yet be proved conclusively, that the Royal Engineers Observatory referred to after 1852 was one and the same site as that in the grounds of the Garrison Library at the foot of the west-facing slopes of the Rock and that observations continued in that place until the first move, to South Bastion, in 1852.
Figure 2. Aerial photograph of Gibraltar showing the situation of the airport. The weather station lies to the immediate north of the airport buildings on the right hand (eastward) side of the main road. (© Google.)
Figure 3. An example of the presentation of weather data in the Gibraltar Chronicle. This layout was characteristic of the 1840s before the official series began in 1852. (Courtesy of Gibraltar Government Archives.)
Table 1List and details of meteorological observatories in Gibraltar.
Location Altitude (metres amsl)
Dates Notes (summarized from Hurst (1956))
Garrison Library 7.6 1780 to 1852 This location cannot be confirmed with absolute certainty, 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 an increase in mean annual rainfall of 75 millimetres 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-metre fall to avoid ‘carry over’ in westerly winds
Windmill Hill (3) 107 1944 to 1947 This site was further from the break of slope noted above and is closer to condi-tions on the Windmill Hill (1)
North Front (1) 2.4 1947 to 1955 On the RAF airfield between the Rock and mainland Spain
North Front (2) 2.4 1955 to present Slightly more exposed than North Front (1)
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that time of year introducing (for Moberg’s Stockholm and Uppsala series) a positive bias of 0.7 to 0.8 degC. Böhm et al. (2009) suggest a similar positive bias for summer temperatures in Central Europe: the same might be expected in the early Gibraltar record.
The Gibraltar Chronicle data set: abstraction and treatmentThe official series, although having a few gaps, is available as noted from 1852, but in order to extend this series backwards it was necessary to extract from the pages of the Gibraltar Chronicle (henceforth GC series) all the daily data from their first pub-lication date (1 July 1821 – although the Gibraltar Chronicle had started publication in 1801). In all, over 17 000 fixed-hour obser-vations were extracted for the GC set and the derived daily series is continuous over the 51-year study period to 1872. The only immediate correction applied to the obser-vations was for the three-month period in 1851 when observations were made at the Signal Station (385 metres above mean sea level (amsl)), these corrections being esti-mated using the standard environmental lapse rate of 0.65 degC per 100 metres.
The difficulty of drawing together the pre- and post-1852 data results from the fact that, whilst the official MO series consists of maxima and minima and the means derived therefrom, the GC set comprises fixed-hour observations only until 1852 when the daily maximum and minimum series begins. Fortunately the fixed-hour observations continued to be published alongside the maximum and minimum readings until the 1870s. Thus for the key overlap and calibra-tion period 1852 to 1872 there are three sets of data: (i) the mean monthly estimates based on fixed-hour (0900) observations; (ii) the monthly means of the MO-archived series based on maximum and minimum temperatures and (iii) the monthly means of the GC series also using the observations of maximum and minimum temperatures. This overlap period provides the possibility of calibrating the fixed-hour observations against the max/min-based means and re-expressing the monthly means from the 0900 hours readings as equivalent values. James Glaisher’s correction tables for this purpose were considered inappropriate as they were derived for the quite different climate setting of the British Isles. The first step was therefore to take a fresh approach and consider the mean differences between the GC 0900 hours monthly figures and those derived from the mean of the GC maximum and minimum readings for the period 1852 to 1872. Table 2 summarizes these differences and shows an annual rhythm in which the differences are nega-tive and greatest in summer and tend to
The evidence, scant though it is, suggests that the record and methods used after 1852 were employed before that date and, given that the Royal Engineers were respon-sible for the rainfall observations from the earliest days (Wheeler, 2007), the probability exists that they assumed the same role for temperature observations. However, it is generally agreed (Parker, 1994; Hughes, 1995) that such non-Stevenson-Screen exposures as might have been in existence for this distant point in time would result in thermometer records with exaggerated diurnal ranges. More recently, Moberg et al. (2003) have argued that such effects would be greater in summer than in winter as a result of back-scattered solar radiation at
accompany the monthly reports for April and May . . . the thermometer is a self-regis-tering one, placed in an open situation in the shade, and facing due north. It is more than probable that the same practice held sway at the Garrison Library.
After 1852 and the start of the Royal Engineers published series, the monthly means were based on the average of the maximum and minimum thermometer readings but prior to that a standard ther-mometer was used, the readings being taken at fixed hours as noted earlier. The most likely location for the thermometer would have been some sheltered spot, per-haps the north wall of the Library itself: a venerable stone structure that stands today.
Figure 4. Location of the various observation sites used in the compilation of the Gibraltar temperature series (see Table 1 for details).
N
5000 metres
GibraltarHarbour
Gibraltar
Gibraltar(U.K.)
Airport
EuropaPoint
Bay of Gibraltar
Strait of Gibraltar
AlboránSea
MarbellaCadiz
Ceuta (Sp.)
GL
SB
AG
30m
300m
Catalan Bay
Sandy Bay
Little Bay
Rosia Bay
NeutralZone
De facto boundaryNF
GibraltarHarbour
Gibraltar(U.K.)(U.K.)
S P A I N
S P A I N
Gibraltar(U.K.)
MOROCCO
NeutralZone
Airport
EuropaPoint
La Linéa de la Concepción
De facto boundary
Bay of Gibraltar
Strait of Gibraltar
AlboránSea
Gibraltar WeatherObservatory Locations(1821 - 2009)
MarbellaCadiz
Ceuta (Sp.)
NF
GL
SB
AG
WH
30m
300m
Catalan Bay
Sandy Bay
Little Bay
Rosia Bay
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maximum of 0.8 degC, is also in agreement with the magnitude of the warm bias identi-fied by Moberg et al. (2003).
A second and final correction was there-fore applied to the 1821 to 1851 GC 0900 hours series in order to bring it into con-formity with the official MO series as it now stands, this correction being the sum of the two contributing elements described above. Table 4 lists these final monthly cor-rection factors. At the same time corrected data from the GC series were used to fill those gaps that existed in the MO series to 1872, these being in January 1852; April, May and June 1853; August, September and October 1855; December 1856; April to December 1862; January to December 1863, and December 1872. Quite why they should be missing from the official series when the observations had evidently been made and published in the Gibraltar Chronicle is unclear. It might also be noted that the digital MO series has December 1852 and 1853 means that are rounded to the nearest whole number: these have been changed here to include the first decimal place. The final corrected series is therefore based on GC 0900 hours data from 1821 to 1851, with GC max/min readings used to complete the gaps in the MO series as out-lined above. The only notable gap remain-ing is that for 1940, when the consequences of the outbreak of war and the extraordi-nary situation in which Gibraltar found itself (the whole of the local population was
appears that the original data (as presented in the Gibraltar Chronicle) have been adjusted at some stage although enquiries at the Met Office and consultation of the Archives have failed to yield any official policy on this matter. The original registers now housed in the Met Office Archives make it clear however that the GC data and the original MO observations were one and the same and the newspaper published the readings as they were received: the correc-tions to the MO data were applied at a later and unknown date to the series now avail-able in digital form from the MO. Most of the individual monthly differences in Table 3 are of the order of fractions of a degree but there are some that are larger, particularly in February and March 1853. Examination of the two sources has failed to account for these disagreements and they were excluded from the monthly means used in Table 3 in order not to obscure an otherwise clear annual rhythm.
Table 3 confirms that the GC max/min monthly means may have been too high, especially in summer, and that they were reduced accordingly. It is possible that this step was taken in order to accommodate the over-exposure of the thermometer before adoption of the Stevenson Screen. This pattern of differences conforms to Moburg et al.’s (2003) view of ‘back radiation’ in summer on thermometers that were at that time over-exposed, creating a warm bias. The level of correction, reaching a
zero in the winter. November and December 1860 were excluded as the 0900 hours read-ings (but not the maxima or minima) were consistently low through each of the two months to an extent that exceeded the mean difference by five standard deviations and probably represents an instrumental error in the standard (dry-bulb) thermom-eter that seems to have been corrected for January 1861 when the maxima, minima and 0900 hours readings were in accord. The negative tendency in summer suggests that the mean monthly 0900 hours observations are higher than those derived from maxi-mum and minimum observations. These corrections, as they might now be termed, were applied to the GC 0900 hours monthly means from 1821 to 1851 to bring them into line with the later GC max/min-based series.
The next task was to compare the GC max/min averages with those of the MO for the period 1852–1872. It had been assumed that the GC monthly means and MO monthly mean of the official series were one and the same, but investigations revealed this not to be the case: a distinction that raises some pertinent questions. Table 3 shows the differences between the two sets of means. The MO series has some gaps within it but the differences with the GC series are interesting and although they vary between individual months, there is a progressive and regular increase in the mean differences from close to zero in mid-winter to close to 1 degC in the summer. It
Table 2Summary of differences (degC) between mean monthly GC 0900 hours and GC means derived from observations of maximum and minimum temperatures. These differences are based on GC mean monthly–GC 0900 hours.
year J F M A M J J A S O N D
1852 –0.7 –0.9 –0.9 –1.0 –0.8 –1.1 –0.4 –0.4 –0.4
1853 0.2 0.4 1.1 0.3 –0.1 –1.3 –0.8 –0.9 –0.6 –0.2 0.0 0.8
1854 1.0 0.4 0.3 –0.4 –0.9 –0.7 –1.1 –0.2 0.1 –0.3 0.0 0.3
1855 0.4 0.4 0.2 –0.3 –0.5 –0.6 –0.6 –0.2 –1.0 –1.0 –0.7 –0.6
1856 –0.4 –0.6 –1.5 –1.2 –1.4 –0.8 –0.7 –0.1 0.1 –0.3 0.1 0.6
1857 –3.0 0.5 –0.3 –0.5 –0.9 –0.7 0.0 –0.1 –0.6 –0.2 0.2 0.3
1858 –0.2 0.9 1.1 0.1 0.1 –0.7 –0.7 0.0 0.5 –0.2 0.4 0.0
1859 0.4 0.4 –0.3 –0.1 0.5 0.2 –0.2 –1.3 –1.2 –0.3 0.1 0.3
1860 0.4 0.4 –0.3 –1.3 –0.6 –0.8 0.1 0.8 0.1 0.6 (–4.2) (–5.1)
1861 0.3 0.5 –0.1 –0.2 0.0 0.0 –0.1 0.3 0.2 0.4 0.3 0.0
1862 0.5 0.2 0.0 0.3 0.6 0.5 1.4 0.1 0.7 0.3 0.8 1.2
1863 1.4 0.5 0.9 0.5 –0.7 –0.1 0.2 0.1 0.3 6.0 –0.4 0.6
1864 0.2 0.0 –0.3 –0.6 –0.7 0.5 0.3 0.0 –0.2 –0.3 0.1 0.4
1865 0.9 0.2 0.1 –0.2 0.3 0.1 0.0 –0.6 –0.1 0.0 –0.2 0.5
1866 0.1 0.5 –0.6 –0.8 –0.4 –0.3 –0.1 –1.2 –2.5 –1.8 –0.4 0.3
1867 –0.6 –0.8 –0.5 –0.9 –0.7 –0.4 –0.4 –0.3 –0.7 –0.8 –0.8 –0.5
1868 –1.1 –1.0 –0.9 –0.7 –0.4 –0.4 –0.7 –0.1 –0.4 –1.2 –0.7 –0.2
1869 –0.7 –0.7 –0.5 –0.5 –0.4 –0.2 –0.1 0.6 –0.7 –0.7 –0.9 –0.5
1870 –0.2 0.4 –0.8 –1.1 –1.1 0.1 –0.2 –0.7 –1.3 –1.3 –1.2 –0.9
1871 –0.7 –0.2 –1.0 –1.6 –0.6 –0.9 –0.3 0.6 0.8 –0.2 0.3 0.4
1872 0.7 0.7 1.1 0.7 0.7 0.4 0.6 0.8 –0.2 –0.3 –0.2 0.3
mean 0.0 0.2 –0.1 –0.4 –0.4 –0.3 –0.2 –0.2 –0.4 –0.1 –0.2 0.1
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Andalucia, and the Atlantic influence through the Straits of Gibraltar does much, as Drinkwater noted, to subdue the heat encountered over southern Iberia. Only rarely do temperatures exceed 35°C and the record maximum of 39.5°C in July of 1994 was exceptional. On the other hand winters are, as expected, mild and only one air frost has been recorded at low level, on 7 December 1870. At higher levels closer to the peak, however, there is a record of …ice 2 inches thick at the Signal House on January 9 1842. This occurred on the same day as the lowest fixed-hour observation in the series to 1872 when 1.7°C was recorded at 0900 local time. Such departures from the overall picture notwithstanding, Gibraltar’s climate is one of an essentially Mediterra-nean character but with a notable Atlantic/maritime influence that tempers the more extreme temperature ranges found else-where in southern Iberia.
Figure 6 plots the four seasonal and the annual mean temperature series, with Gaussian filters being applied to remove the so-called high-frequency variations and emphasize the longer-term (low frequency) trends. The general correspondence between the seasons and other features, such as the general increase in tempera-tures over the past two decades and the fall in temperatures during the 1960s and 1970s, are scarcely unexpected. Other fea-tures, however, are worthy of closer attention. Not least of these is the notable
Table 3Differences (degC) between the monthly mean temperatures of the GC and MO series (both derived from maximum and minimum tempera-tures). These differences are based on MO–GC.
Year/Month J F M A M J J A S O N D
1852 –0.4 –0.4 –0.7 –0.8 –0.4 –0.6 0.4 0.0 –0.8
1853 0.0 (–2.3) (–1.2) –0.9 –0.7 0.4 –0.4 –0.1 0.0
1854 –0.2 –0.1 –0.1 –0.3 –0.7 –0.7 –0.8 –0.7 –0.4 –0.2 –0.1 0.3
1855 0.0 0.1 –0.1 –0.2 –0.4 –0.7 –0.8 0.0 0.2
1856 0.7 0.5 0.3 0.4 0.1 –0.6 –0.8 –0.6 –0.3 –0.3 0.1
1857 –0.1 0.0 –0.2 –0.7 –0.6 –0.9 –0.6 –0.4 –0.2 –0.2 0.2
1858 0.1 0.0 –0.3 –0.2 –0.4 –0.7 –0.8 –0.8 –0.5 –0.2 –0.1 0.1
1859 0.1 0.0 –0.1 –0.4 –0.6 –0.7 –0.9 –0.6 –0.5 –0.1 0.0 0.2
1860 0.1 0.1 0.0 –0.2 –0.7 –0.6 –0.7 –0.7 –0.3 –0.1 0.2 0.1
1861 0.2 0.2 0.1 –0.3 –0.4 –0.5 –0.8 –0.6 –0.6 0.0 0.0 0.1
1862 0.2 0.1 –0.1
1863
1864 0.3 0.0 –0.1 –0.1 –0.3 –0.4 –0.7 –0.5 –0.4 –0.2 0.1 0.2
1865 –0.1 0.0 0.1 –0.3 –0.5 –0.5 –0.5 –0.5 –0.2 –0.4 0.0 0.1
1866 0.0 0.1 0.0 –0.3 –0.4 –0.5 –0.8 –0.2 –0.4 0.0 0.0 0.0
1867 0.4 0.1 0.0 –0.1 –0.5 –0.6 –0.7 –0.5 –0.4 –0.2 0.0 0.2
1868 0.1 0.1 0.1 –0.3 –0.6 –0.5 –0.8 –0.6 –0.6 –0.2 –0.2 –0.1
1869 0.2 0.0 –0.1 –0.2 –0.5 –0.6 –0.9 –0.5 –0.5 –0.3 0.0 0.0
1870 0.1 0.0 –0.1 –0.3 –0.7 –0.7 –0.9 –0.6 –0.5 –0.4 0.0 0.0
1871 0.1 0.0 –0.1 –0.2 –0.5 –0.4 –0.7 –0.6 –0.4 –0.2 0.1 0.2
1872 0.1 0.1 0.0 –0.1 –0.4 –0.4 –0.7 –0.6 –0.5 –0.2 0.0
mean 0.1 0.1 0.0 –0.2 –0.5 –0.6 –0.8 –0.6 –0.4 –0.2 0.0 0.1
Table 4Corrections (degC) to be applied to (a) GC 0900 hours monthly means and (b) GC max/min monthly means to bring them into agreement with the official MO series.
J F M A M J J A S O N D
a) 0.0 0.2 –0.1 –0.4 –0.4 –0.3 –0.2 –0.2 –0.4 –0.1 –0.2 0.1
b) 0.1 0.1 0.0 –0.2 –0.5 –0.6 –0.8 –0.6 –0.4 –0.2 0.0 0.0
evacuated to different parts of what was then the British Empire – only to return after 1945) brought a temporary cessation to such activities before the RAF station was firmly established at North Front and the need for weather observations again assumed importance.
Gibraltar’s temperature record: preliminary interpretationDrinkwater’s (1905) famous first-hand account of the great siege of 1779 to 1783 contains the following account:
With regard to the climate of Gibraltar, the inhabitants breathe a temperate and wholesome air for most of the year. The summer months of June, July and August are excessively warm, with perpetual serene and clear sky: this is, however, allayed in great measure, by a constant refreshing breeze from the sea, which usu-ally sets in about ten in the forenoon, continuing till almost sunset; and, from its
invigorating and agreeable coolness, is emphatically called the Doctor. The cold in winter is not so excessive as in the neigh-bouring parts of the country. Snow falls but seldom and ice is a rarity; yet the Granadian mountains of Spain, and the lofty moun-tains in Africa, have a snow lying on them for several months.
Others found the Rock’s climate less agreeable, for example the resident doctor through the 1840s, E. F. Kelaart, who observed:
The heat of summer is more oppressive than even the thermometrical observa-tions would indicate, owing principally to the want of a free circulation of air, which is prevented by the height and peculiar configuration of the Rock…the reflected heat from the rocky surfaces of Gibraltar is of itself a great source of suffering to the inhabitants. (Kelaart, 1846, p.29)
Table 5 and Figure 5 add statistical testi-mony to these accounts. Summer conditions are warm but rarely as hot as in neighbouring
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text) reveal a similar temperature peak for the regional Iberian record in the 1870s. The same, but more qualitative, conclusion on the 1870s is offered by Font Tullot (1988) who observed …the frequency of excessively hot summers was maintained from year to year, being most marked along the Mediterranean coastline where many places noted record maxima….the eleven years from 1868 to 1878 was particularly noteworthy (p.110). The signal is therefore climatological in character and not it seems a consequence of observational practices. Its geographical setting is decidedly Iberian rather than Mediterranean but this conclusion should alert researchers once again to the contrasts between local, regional and hemispheric climate variations.
The 1830s on the other hand was a dec-ade (Figure 6) when the spring, autumn, and winter temperatures were below the long-term mean and represented one of three such cold periods that stand out in the record, the others being in the 1910s and 1950s. The same nineteenth-century period is identified by Font Tullot (1988) as one of exceptionally cold winters with a succession of cold waves persisting into early spring across much of Iberia. The keen commenta-tor and collector of facts, Rico Sinobas (1858), recalls for that time rivers icing to the point of supporting humans and ani-mals, and of holy wine freezing in the chal-ice! This period differs, however, from the other two in the warmth of its summers. This is the only time in the record during which such a distinction exists and whilst it may reflect the influence noted above of inflated summer temperatures being brought about by exposure of the ther-mometer to back radiation in the early years of the series, Font Tullot (1988) was anxious to draw attention to the heat of summers at that time that stood in such marked con-trast to the frigid conditions of the winters. In the absence of reliable metadata or any certainty that this is evidence of warm bias, any attempt to apply arbitrary corrections to the data would be ill-advised.
Most importantly, the reliability of these data over at least part of this period is con-firmed by the fortuitous survival of a wholly independent set of daily readings. These were gathered by Sir John Hall who was then Principal Medical Officer in Gibraltar. His diary (now in the National Meteorologi-cal Archives) provides daily maximum and minimum temperatures between November 1838 and January 1841. These observations were made at the Naval Hospital, which was then located to the south of the main town in a more open setting than that of the Garrison Library. The two data sets are highly correlated (r = +0.96, p < 0.001) and although the overall means differ by 0.8 degrees, with Hall’s series being the higher of the two, the difference is far from
Table 5Extreme temperatures in Gibraltar by year, month, and day.
Date Temperature (°C)
Hottest year 2008 19.2
Coldest year 1936 16.5
Hottest month August 1989 26.1
Coldest month January 1842 9.3
Lowest minimum 7 December 1870 –1.1
Highest maximum 6 July 1994 39.5
Figure 5. Mean monthly temperatures (solid line) and most extreme months in the Gibraltar 1821–2009 temperature record.
Figure 6. Time series smoothed with a Gaussian filter (12-month window) of seasonal and annual z-scores (standard deviations difference from zero) of the Gibraltar temperature series.
peak in temperatures during the 1870s, the warmth of which was not repeated until the last two decades of the twentieth century. This phase does not coincide with any change of site or, as far as can be deter-mined, with any change of instrumentation. There is no corresponding excursion of tem-peratures in the Northern Hemisphere record (Figure 7) and it is important therefore to determine if this is a purely local climatic feature or a consequence of
observational practice. Comparisons were made with the Western Mediterranean tem-perature record prepared by Camuffo et al. (2010) who drew upon several temperature series across the region. Figure 7 plots these two series and reveals no corresponding increase in temperatures in the Western Mediterranean regional set. On the other hand the findings of Brunet et al. (2007) as exemplified in their Figures 6 and 7 (although scarcely commented upon in the
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be done to take us beyond the scope of this preliminary paper. In particular, treatment of the daily maximum and minimum tem-peratures series requires attention and this will be the theme for future publications. The search will continue for metadata with which to resolve some of the uncer-tainties surrounding exposure, observa-tional procedures and instrumentation identified above. Meanwhile this series is offered as the best available for Gibraltar, having been extended, homogenized, and some long-standing gaps filled.
AcknowledgementsThe author acknowledges with gratitude the assistance and guidance offered by Denis Buiso (Gibraltar Government Archives) and Lorna Swift (Garrison Library, Gibraltar). Financial support for this project was kindly provided by the Royal Meteorological Society’s Legacy Fund. The author also acknowledges financial support by the 6th EU Framework program MILLENNIUM.
significant (t = 1.66, p = 0.378). Such close correspondence between independently derived sources gives further and more objective confirmation that this early part of the GC series is reliable and not subject to serious errors.
Finally, it is important to explore as far as the data permit the effects of the important change of site that took place in 1941 when the weather station moved to its present location on North Front and to the site of the newly constructed airfield. The previous site had been in the more rural setting of Windmill Hill on the south of the Rock. There is, however, no unambiguous suggestion of any urban heating from the growing expanses of concrete that now characterize the site. Temperatures did rise, but that increase is partially mirrored (although slightly pre-dating it) in the regional and the Northern Hemisphere series (Figure 7). At this stage it might therefore be concluded that the Gibraltar temperature series has been homogenized as far as current knowl-edge of the conditions permit, and this should do much to improve our knowledge of recent Mediterranean climatic change at regional and local levels.
ConclusionsThis article has set out to extend and com-plete the current Gibraltar temperature series. In doing so it has provided one of the longest temperature series for the Mediterranean region and one to set along-side the precipitation series. In common with all such single-site series, it reflects global, regional and local characteristics. Periods of warming and cooling are wit-nessed that provide an important gauge of climatic variation, whether it be anthropo-genically induced or the consequence of natural variation. Much work still remains to
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Correspondence to: Dennis Wheeler, 11 Ashbrooke Mount, Sunderland, SR2 7SD, UK
© Royal Meteorological Society, 2011
DOI: 10.1002/wea.737
Figure 7. Z-score comparison of the Gibraltar, Northern Hemisphere and western Mediterranean temperature series smoothed using a 12-year Gaussian filter.