INTERNATIONAL JOURNAL OF CLIMATOLOGY, VOL. IS, 801-810 (1995) 551.583.14:551.506.3(468.l)

EARLY INSTRUMENTAL WEATHER DATA FROM CADIZ: A STUDY OF LATE EIGHTEENTH A N D EARLY NINETEENTH

CENTURY RECORDS

DENNIS WHEELER

Geography Department. Sunderland University SRI 3SD, UK

Received I! January 1994 Accepted 18 September 1994

ABSTRACT This paper examines some of the oldest instrumental records in Spain. They cover the period 1789 to 1816 for the Cadiz region and reveal strong evidence of a significant and approximately quantifiable degree of warming since those times. In contrast to the middle latitudes, greater temperature increases have occurred during the summer months. Wind records also indicate that the general circulation may have been more meridional than today.

K E Y WORDS: southern Iberia; historical climatology; instrumental records

INTRODUCTION

Font Tullot’s (1988) study of the climate history of Spain has done much to highlight the long-standing interest that exists in that nation regarding the weather. The present writer has also drawn attention to the particular work of the San Fernando Observatory at Cadiz (Wheeler, 1992a, b, c). This paper examines the earliest records that exist for this Observatory, whose principal function was concerned with astronomical studies but, as was so often the case, required meteorological information to correct for observed star locations and the motion of the great pendula then in wide use. These data cover two periods and, indeed, two locations. The earlier of the two embrace the years between 1789 and 1795, with some scattered data from as early as 1776. At those times the Observatory was located close to the present-day centre of Cadiz. By the time of the second period of record, covering the years between 1805 and 1816, the Observatory and its associated facilities had moved to the small town of San Fernando some 6 km from Cadiz. In terms of exposure, proximity to the sea and altitude, however, the sites differ little from each other (Figure 1).

The data consist principally of temperature readings, with some occasional barometric and wind direction information: the temperature data are, for the most part, continuous within the two periods of time but were inconsistently recorded; occasional days are missing, the data are presented in different forms and may even be scattered through quite different documents and papers. They clearly were not seen as having intrinsic climatological importance at the time and were an adjunct to the astronomical studies. Their value today is that they represent possibly the oldest instrumental temperature data for Iberia and, with greater certainty, for this latitude. They also provide a supplement to the long-term, and continuous, rainfall records from San Fernando (which started in 1805) and Gibraltar (in 1795). It is regrettable that the temperature information ceases in 1816 and was not re-established until 1870 (Wheeler, 1992c) when formal meteorological studies appear to have begun. Nevertheless, the opportunity exists to estimate general temperature levels in the Cadiz area during that distant time.

CCC 0899-8418/95/070801-10 0 1995 by the Royal Meteorological Society

802 D. WHEELER

Figure 1. Relief map of south-west Spain showing the location of Cadiz and San Fernando. These two nearby centres lie on the broad plain of the lower Guadalquivir valley and are exposed to Atlantic influences from the Gulf of Cadiz.

High ground to the east minimizes the role of the Mediterranean. Elevation is shown in 200-111 bands

THE ARCHIVE SOURCES

All the data examined were obtained from the archives of the Real Observatorio de la Marina in San Fernando and were, without exception, data recorded by astronomers during their daily activities. Most abundant are the temperature records, which span both principal periods. For the earlier (1789-1795) period we also have wind direction and barometric data. For a while, observations of all three phenomena were made thrice daily, the times being carefully noted. Nevertheless the principal focus of attention in this paper is the mid-day data. The other two most commonly used observation times were between 0800 and 0900 hours and between 2000 and 2100 hours. For the period 1805 to 1816 only the temperature data are extant. Again the time of observation was fixed at mid-day. The majority of the temperature readings were made in the telescope rooms of the respective observatories but the latter data set enjoys the advantage of being duplicated between 1811 and 1816 by temperatures made outside of the Observatory buildings.

The documents within which the data are recorded vary between carefully tabulated sheets containing nothing other than climatic data and sheets devoted principally to astronomical data and

HISTORICAL DATA FROM CADIZ 803

information within which temperatures, barometric pressures and other information are listed. With one exception, the archive materials exist as hand-written documents-that one exception is the item published in 1777 and written by Vicente Tofifio and Joseph Varela, the well-known astronomers and directors of the Cadiz academy for the training of Spanish naval officers of which the Observatory has always been part. This publication contains a detailed near-daily account of astronomical and solar observations made towards the close of 1776. It includes, importantly, occasional reference to the midday temperature in the telescope room.

The archive at San Fernando offers one final source of information-an inventory of scientific instruments from which it is possible to discover the type of thermometer used in the Observatory. In those times Spain had little manufacturing capacity in accurate scientific instruments and most were imported, mainly from England. Thus item 1 in the inventory, begun in 1789 but subsequently added to, identified a thermometer and barometer manufactured by George Adams of London. Item 9 lists a thermometer made by Nairn and Blunt. Unfortunately the inventory does not indicate whether the thermometers were of alcohol, or mercury-in-glass. Nevertheless the conclusion can be drawn that the instruments were of the best type available and not likely, either by manufacture or by neglect on the part of their owners, to have been seriously inaccurate. The problem that presents itself is principally that of standardizing these data to contemporary requirements of exposure and observational hours.

Any corrections must take into account that the majority of the observations were made within a large room, although possibly one with relatively free exchange of air with the outside. Only in the case of the 1811 to 1816 series are exterior shade temperatures noted. These do, nevertheless, give some scope to determining the corrections needed to convert the telescope room readings. The availability of midday readings only also hinders the calculation of reliable daily means and comparisons with present-day conditions are made through the mean maxima, with which they are more likely to correspond. Even this comparison must be made with caution as the true maximum in most climates is delayed until after midday. In this case, and because of the absence of information on which to base any fixed-hour corrections, no attempt was made to re-express the midday readings. These limitations should be borne in mind in the interpretation of the temperature data that follows. On the other hand we can be more confident that the barometric and wind direction data are comparable with exposure and observational practice to those gathered today and in more recent times generally.

THE CLIMATE OF CADIZ-1776

The scarcity of data for this short span of time renders it of little scientific value and it is included here mainly because of its historic interest. Table I summarizes the temperature data abstracted from Tofiiio and Varela (1777). At this time the Observatory was located in the city of Cadiz. Its principal functions were related to astronomical matters, often for the purposes of navigational instruction to the officer corps of the parent institution ‘La Academia de las Guardias Marinas’. Using great circles constructed by the English instrument maker John Bird, the astronomers made regular observations of passage of the Sun,

Table I. Mean midday temperatures from the Cadiz Observatory record for 1776. All temperatures are in degrees Celsius

August September October November December

Mean midday 25.1 23.7 22.1 17.7 15.9 temperature Number of 2 12 8 12 9 observations

804 D. WHEELER

Moon, planets and the more important zodiacal stars through the local meridian. They were also concerned with the progression of the Observatory's great pendulum and found it to be influenced by the temperature and humidity of the air (Lafuente and Selles, 1988). It was in this connection that occasional temperature readings were made, all at midday, using the Adams thermometer. The readings were made in degrees Fahrenheit but, in common with all the following discussion, have been re-expressed in degrees Celsius. The general temperature levels clearly indicate a shaded location for the thermometer and it is almost certain that they were made within the main instrument room where the pendulum was housed. The degree to which free air could circulate through the room is unknown but it is likely, given the nature of the studies, that large openings in the walls would have been necessary. The room was a relatively small one covering an area of 11.5 varas (a vara is 0.84 m) square at the top of a fortified tower, popularly believed at the time to have had Roman origins. Table I shows the midday means for 1776. Along with the obvious problem of comparing midday with maximum temperatures, it must be emphasized that the former are based on small samples. Nevertheless the impression of generally cooler conditions is one that will be repeated in the other two, more substantial, data sets.

The suggestions of cooler conditions in 1776 does not contradict those of Font Tullot (1988) regarding Iberian climatic conditions at that time. He cites the greater frequency and earlier starts to winter snowfalls and generally lower, although variable, temperature regimes. The closing years of the decade were also much wetter than usual in those parts of Iberia with an Atlantic aspect. Lamb (1982) quotes Pfister as stating that north of the Pyrenees Apline glaciers advanced throughout the cool period 1764 to 1778, and in Britain summers were cool and wet. The overall picture, along the Atlantic seaboard from southern Spain northwards is, it would appear, one of consistently cooler and wetter conditions.

THE CLIMATE OF CADIZ-1789 to 1795

For the period 1789 to 1795 the data are more exhaustive, systematically recorded, and embrace not only temperatures but also wind direction and barometric pressure. These data are contained within two large bound volumes of principally astronomical information. The observations were frequently, but not always, collated into separate sheets containing only climatic data. In 1789 observations of temperature, air pressure, and wind direection were made three times a day but thereafter only the midday readings were made. The temperature readings were made within the Observatory and there is no suggestion that they were taken in outdoor shaded locations. Indeed the lack of variation between early morning, midday and evening readings would seem to preclude any external siting of the instruments and the main telescope room is again the most likely position. There are, helpfully, very few gaps in the record.

Table I1 lists the monthly means of the midday figures. Assuming again that these approximate to the mean maxima, the present-day equivalents are also included. The annual figures reveal a 1.5"C increase since the 1790s and the period 1931-1960. All 12 monthly means also show increases over the same time span, although the differences vary, being greatest during the summer months and least in winter. This suggests that the summers of 1789 to 1795 were generally cooler than today, perhaps by as much as 3"C, whereas the winters were less markedly cooler, possibly in the order of only half a degree. Manley's (1974) Central England Temperatures (CET) shows a more modest increase of only 0.4"C in the annual means over the same period, with warming concentrated into the winter months. This latter contrast confirms, however, some earlier work. Writing on the warming since the early decades of the nineteenth century, Lamb and Johnson (1959) stated '. . . For winter this warming has been greatest in the higher latitudes whereas in summer the lower latitudes have shown most warming'. It must be emphasized again, however, that these are the greatest probable estimates of warming since the 1790s because they are based on midday readings and the true mean maxima at that time may well have been slightly higher, hence reducing the magnitude of the warming deduced from Table 11.

Fewer such problems exist concerning the summary of daily wind observations. These cover the same period and again are based on the midday readings. The written information gives directions by the eight

HISTORICAL DATA FROM CADI2 805

Table 11. Mean midday temperatures from the Cadiz Observatory records 1788 to 1795. All temperatures have been converted to degrees Celsius. Comparisons are made with the mean maximum temperatures for the period 1931 to 1960. Gaps between October 1788 and June 1795 are a result of a total absence or insufficiency of data

Year Jan Feb Mar April May June July Aug Sept Oct Nov Dec Mean

1788 1789 1790 1791 1792 1793 1794 1795

Mean Mean maximum 193 1- 1960 Difference

22.3 17.8 14.9 16.1 16.3 16.0 17.7 21.4 23.6 25.7 266 25.2 208 15.9 143 20.0 14.6 15.8 17.5 18.7 19.7 25.5 27.3 28.3 26.0 22.5 17.9 15.3 20.8 14.9 14.0 17.7 20.3 20.4 23.1 27.3 29.0 27.8 22.3 17.7 14.9 20.8 15.0 166 17.8 26.8 26.5 24.2 22.2 205 15.6 14.5 14.4 15.8 17.7 21.6 27.2 27.8 26.1 23.9 18.4 16.1 13.3 16.8 17.5 19.4 20.6 21.6 25.0 26.2 23.9 22.6 18.4 14.2 19.9 15.5 15.9 17.0 18.7 22.0 228

14.6 15.7 17.0 18.8 21.0 23.3 26.6 27.4 25.5 22.4 18.1 15.0 20.5

cardinal points. A further category of calms was included. Some observers also referred to the wind and weather as honancible, which translates as fine and clear with the implications of only light winds, such entries, which did not include also a wind direction, were entered as calm. The month-by-month frequencies varied a great deal, but Table 111 lists the individual monthly, seasonal, and annual mean percentages of winds. The dominance of westerlies and south-westerlies is notable, although it declines during the winter months, when northerlies become a major component. In summer the latter were almost entirely absent. It must not be forgotten, however, that the local sea-breezes, well-developed in summer, are westerly and this doubtless contributes to the midday count of winds from that direction. Comparing the seasonal aggregate statistics assembled by Watts (1975) with those from the study period the differences are few. Spring northerlies were then more frequent, and this might help to account for the longer and harsher winters of that time. It is unfortunate that Watts did not include a winter season count because the higher frequency of eighteenth century northerlies is a striking feature, suggesting a large number of occasions

Table 111. Mean percentage wind frequencies from the Cadiz Observatory record 1788 to 1795. Means are expressed by months, seasons and the year and are based on midday observations

Month/season North North-east East South-east South South-west West North-west Calm

January February March April May June July August September October November December Spring Summer Autumn Winter Annual

16.1 17.3 10.6 2.8 1.9 0.0 0.0 0.0 0.0 1.3

13.6 38.5 10.2 0.6 5.0

23.7 8.5

8.3 4.5 2.2 3.4 3.8 0.0 0.0 1.3 0.1 0.0 5.4

10.8 3.1 1.3 1.8 7.9 3.3

7.7 8.3

18.3 13.8 15.1 19.5 15.7 16.4 12.0 10.7 3.4 5.4

15.7 17.2 8.7 7.1

12.2

4.8 2.6 4.4 7.6 1.9 2.3 2.6 2.6

11.2 8.2 2,7 3.3 4.6 1.8 7.4 3.6 4.5

4.2 5.8 6.1 9.0 4.4 8.4 7.0 9.9 4.8 8.2 14.3 6.8 6.5 8.4 9.1 5.6 7.4

20.8 14.1 21.1 15.9 17.6 25.6 10.4 18.4 19.2 15.1 18.4 8.1

18.2 18.1 17.6 14.3 17.1

7.1 8.3

12.2 37.2 46.5 39.1 61.7 47.3 42.4 30.2 7.5 5.4

32.0 49.4 26.7 6.9

28.7

13.7 26.3 18.3

5.5 3.8 4.5 2.6 0.0 3.2

19.5 21.0 17.6 9.2 1.8

14.6 19.2 11.3

17.3 12.8 6.7 4.8 5.0 0.1 0.0 3.9 6.4 6.9

13.6 4.1 5.5 1.3 9.0

11.4 6.8

806 D. WHEELER

when polar air reached these latitutes perhaps along the western flank of trough circulations or as cut-off lows. Supporting evidence for this suggestion is found elsewhere. Lamb (1966) has established that the period 1790 to 1830 was one '. . . of rather notable minimum strength of the zonal circulation in January . . .' The Cadiz record would certainly substantiate that possibility of greater meridionality in the general circulation if only through the inferred predominance of northerly circulations seen in the winter record. Lamb's (1966) study has also shown that the principal east Atlantic trough was located close to the Greenwich meridian (Cadiz is 6"W) and drifted eastwards through the nineteenth century, again raising the possibility of greater meridional activity at that longitude. Westerlies and south-westerlies appear to have changed little, although variations may be masked by the sea breeze effect. The contribution that sea-breezes may make to the aggregate midday frequencies can be determined by reference to the data for 1789 which includes also daily wind directions at 0800 hours local time. At that hour sea-breezes are less likely and the frequency of westerlies the same year falls from 28.4 per cent at midday to just 19.2 per cent. South-westerlies are also less frequent, falling from 18.4 per cent in the 1790s to 13.1 per cent. The most notable of the increases is in the north-east category: these winds average only 8.4 per cent frequency at midday, but rise to 18.9 per cent for the 0800 hours observations. Unfortunately the latter were not extended beyond 1789 and more conclusive comparisons cannot be made, but there is a strong indication of a significant sea-breeze component in the midday readings, perhaps adding a further 9 per cent to the frequency of westerlies. It is noteworthy that Watts' (1975) figures show a far greater sea-breeze effect in the present-day record, the frequency of spring and summer westerlies rising by nearly threefold between 0800 and 1300 hours.

Table IV summarizes the daily midday air pressure readings. As far as possible these have been corrected to sea-level at 0°C but uncertainties surrounding the precise height of the cistern and the type of barometer used leave a margin of error of around 1 hPa. The derived mean monthly pressures vary between 1031 hPa for February 1790 and 1015 in June 1790. Current air pressure averages for Chdiz are not available but the Gibraltar record can be used. The remarkable feature of Table IV is the degree to which the eighteenth century air pressures are higher than those found today in this area. The difference between the two annual means is nearly 6 hPa-too great to be explained wholly by pressure contrasts between two such closely interposed sites as Cadiz and Gibraltar. The greatest contrasts are in July and August, with 8.6 and 9.0 hPa differences between Cadiz then and Gibraltar now. This is partly accommodated by Lamb and Johnson's (1959) suggestion that in July during the decade 1790 to 1799 there was a tendency for a north-eastwards extension of the Azores anticyclone as a ridge towards central Europe. Nevertheless the absolute pressures deduced from the Cadiz record appear to be too high to fit easily within the annual pressure map reconstructions in Lamb and Johnson (1966). Given the importance of the barometric readings for astronomical corrections, and the reliability of their instruments, it is unlikely that the observers in Cadiz were given to inaccurate observations and the disagreement remains unexplained.

Table IV. Mean monthly air pressures in the Cadiz record. Approximate corrections have been made to reduce the readings to sea-level at 0°C. The final individual monthly means have been rounded to the nearest whole hPa. The means are based on available data for the period 1788 to 1795. Gaps are because of an insufficiency of data

Jan Feb Mar May June July

1788 1789 1021 1790 1027 1791 1026 1792 1018 1793 1028 1794 1024 1795 1020 Mean 1023.1

1028 1031 1025 1024 1022 1028 1021

1025.4

1021 1023 1026 102 1 1022 1026 1023

1021.8

1022 1021 1020

1022 1025 1022

1022.0

1024 1025 1024 1022 1015 1025 1023 1024 1025

1025 1024 1024 1025 1023 1024 1023 1022

1022.3 1021.8 1024.6

1024 1024 1025 1023 1024 1025

1024.0

Sept

1024 1026 1023 1024 1026 1025

1024.6

Oct

1028 1023 1021 1021 1026 1025 1026

1025.1

Nov

1022 1022 1019 1020 1027 1020 1022

1022.0

Dec Year

1016 1028 1023.7 1027 1023.5 1024 1023.5 1029 1020 1021 1024.3

1023.6 1023.4

HISTORICAL DATA FROM CADlZ 807

THE CLIMATE OF CADIZ-1805 TO 1816

In 1798 the old Observatory was moved to a new site in San Fernando at the other end of the sandy peninsula that links Cadiz with the Spanish mainland. Some data exist for the months of January and February 1798-the oldest for this site-but on 26 February the observer made the following note '. . . the poor weather has prevented observations being made.. .', and no further climatic entries have survived, if indeed any were made, until 1805. What then exists is a curious but valuable collection of monthly summaries of temperatures recorded in the main telescope room of the Observatory-which exists today as a splendid marble hall within the main Observatory building, and now houses the archives and museum. The purpose for which the data were collected was again in connection with the pendulum used as part of the day-to-day activities of the astronomers. The bound volume in which they are held informs us that they were made by Don Jose Maria de la Cuesta, a lieutenant in the Spanish Navy. The same document indicates that the pendulum was made by John Ellicot and the thermometer by George Adams, both of London.

From 1805 until 181 1 inclusive, each annual summary contains, month-by-month, the mean midday temperature together with the highest and lowest such readings. No indication is given of the sample size on which the means are based but a quick arithmetic check confirms that the means are not the averages of the two absolute monthly extremes. Given the intensive character of the astronomical studies being carried out then (made clear from the contemporary documents) it might be concluded that the means are indeed based on regular daily observations. Between 1811 until the close of the series these three data sets were supplemented by a further two, which noted the highest and lowest midday temperature recorded outside the building each month. The 'external' temperatures, to judge by their general levels, were made in shaded conditions and because they were taken at midday can be compared with their 'internal' counterparts. Unfortunately there were no wind records made at this time. On the other hand more serious climatological studies had begun and the rainfall record for San Fernando also started in 1805 (Wheeler, 1992b).

The degree to which internal, telescope room, temperatures represent exterior conditions can be verified using the 1811 to 1816 series. The conclusions can be applied confidently to the 1805 to 1810 series and, although with perhaps less confidence, to that for 1789 to 1795. On an annual basis and paying attention firstly to the absolute midday maxima, the external temperatures are on average 4.4"C higher than the internal readings. Looking secondly at the lowest midday maxima, the external mean is 4.1"C lower than the internal mean. This conclusion is not surprising because the thermal properties of the large stone building would attenuate the effects of the more rapidly changing external temperatures. What is more important, the two differences nearly balance and the conclusion can be drawn that the monthly means derived from daily midday internal readings need only the most marginal adjustment-perhaps a reduction of as little as 0.3"C-to represent the external midday means.

The monthly mean midday temperatures for the 1805 to 1816 series are given in Table V and present a picture similar to that in Table 11. The annual mean is 1°C less than that for the 1789 and 1795 series, but, again regarding the 1931-1960 series, the monthly pattern is one of more modest changes in the winter months and more marked warming of the summer.

Although available only for the period 181 1 to 1816, the absolute external shade maxima can be compared with the Cadiz site's 1931-1960 figures (Table VI). The short span of the former and the limited opportunity for unusually high maxima to be recorded makes direct comparisons difficult, yet only the December maxima show the period 181 1 to 1816 to have yielded a higher figure. The remaining 11 months of the year have absolute maxima that fall well short of those recorded more recently. Although this is scarcely conclusive evidence, it again points to the possibility of cooler conditions of the order of a degree or more in the early nineteenth century. Lamb (1970) has already shown that the opening years of the century were marked by major volcanic eruptions such as Etna (1809), Vesuvius (1806, 1811, and 1812), Sabrina (Azores, lSll) , and Mayon (Luzon, 1814). The great Tambora eruption of 1815 also falls within the study period but the so-called 'year without a summer' that followed it scarcely registers as such in the Cadiz record. Jones and Bradley (1992) also identify the first two decades of the century as having been

808 D. WHEELER

Table V. Mean monthly midday temperatures from the San Fernando Observatory (Cadiz). All readings have been converted to degrees Celsius and compared with the mean maxima for the period 1931 to 1960. The 1812 and 1816 annual means use the calculated 1805-1816 means for December and October in their respective estimations. The gaps are because of an insufficiency of data

Year Jan Feb Mar April May June July Aug Sept Oct Nov Dec Mean

1805 13.9 15.0 17.5 18.6 21.1 23.8 25.0 27.8 27.8 23.9 19.2 15.0 20.7 1806 13.6 14.2 15.6 16.1 17.8 22.2 26.9 25.6 25.0 25.3 18.9 16.7 19.6 1807 14.4 13.6 14.4 15.8 20.6 23.3 24.7 25.0 24.7 26.1 16.0 12.4 19.3 1808 12.8 13.3 13.6 16.5 19.4 23.3 25.8 26.1 22.8 20.6 17.5 14.2 18.8 1809 16.0 15.6 15.4 15.6 18.9 21.7 23.3 24.1 24.0 22.5 16.9 13.3 18.9 1810 12.2 12.8 17.2 17.8 21.1 21.1 24.4 25.1 23.8 20.6 18.1 14.0 18.8 1811 13.5 14.9 15.6 17.5 20.6 23.5 24.3 25.3 24.4 23.3 17.5 12.2 19.4 1812 9.8 15.0 16.3 18.1 18.9 23.9 26.0 27.2 25.0 20.6 19.2 19.5 1813 11.7 14.4 15.3 16.1 20.6 23.3 24.3 26.1 23.5 22.4 17.1 16.0 19.2 1814 12.6 13.3 12.8 16.7 18.9 23.2 25.0 26.4 25.0 21.1 17.8 16.7 19.1 1815 12.2 16.1 18.8 18.2 20.8 22.2 26.1 28.3 25.4 23.3 17.4 13.5 20.2 1816 13.3 15.0 16.7 17.2 2 0 6 23.2 24.4 25.0 24.8 16.4 14.7 19.5 Mean 13.0 14.4 15.7 17.0 19.7 22.9 25.1 26.0 24.7 22.5 17.7 14.4 19.4 Mean Maximum 14.9 16.3 18.3 20.7 22.9 26.5 29.0 29.5 27.4 23.4 19.3 15.8 22.0 193 1-1 960 Difference +1.9 +1.9 +2.6 +3.7 +3.2 +3.6 +3.9 + 3 3 +2.7 +0.9 +1.6 +1.4 +2.6

Table VI. Comparison of highest monthly midday temperatures in the 1811-1816 external series with the absolute maxima in the 1931-1960 series

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

Absolute midday 18.3 19.4 22.2 25.0 31.1 31.7 33.9 34.7 31.0 27.6 22.6 22.8 maximum 181 1-1816 Absolute maximum 21.2 24.8 28.1 31.7 34.1 3 7 4 40.4 41.3 37.4 32.1 29.0 22.1 193 1-1 960

cool across much of Europe and, as far as records can reveal, the Northern Hemisphere. In most respects the data set supports these earlier findings.

The coolest years in the 1805-1816 series are 1808 and 1810, both with 1843°C. The warmest year is 1805 with 20.7"C, and a range of nearly 2°C in such a short span of time suggests a level of variability not met with today. Yet such variability was a characteristic of the Iberian climate and very much a feature of the eighteenth and nineteenth century recovery from the Little Ice Age (Font Tullot, 1988). The effects of the latter were no less marked south of the Pyrenees than they were to north, with snowfalls in particular being much heavier, geographically more widespread and persisting for longer than is the case today. Indeed such were the volumes of snow that there was a flourishing industry, even in the height of summer, in the sale of ice and compacted snow in most Spanish cities. Winter snow was stored in mountainous areas in a series of 'wells' and covered in straw to prevent it from melting. The preserved snow was then transported in summer to the nearest town, its sale supporting a widespread and lucrative trade. As far south as Murcia the collective volume of these wells was estimated at 20 to 25 thousand tons in a region where snowfall is today an event of notable rarity. Significantly from the climatological point of view, this trade continued until well into the nineteenth century when the shortage of snow caused the extinction of the trade of the neueros. Moving more firmly up-to-date, it is only within the last few decades that the small ice cap on the highest parts of the Sierra Nevada range has disappeared as twentieth century warming has persisted.

HISTORICAL DATA FROM CADIZ 809

Since the late eighteenth century temperatures in this region have undergone a slow, irregular, but none the less significant increase. The continuous temperature record at San Fernando begins only in 1870 but shows (Wheeler, 1992~) a progressive recovery from the coolest decade of 1880 to 1889, when the decadal mean stood at 16.9"C. The warmest decade in the record is 194C1949 at 18.1"C. The overall rate of increase, ignoring the shorter term fluctuations, is 0.1"C per decade from 1870 to the present-day. The comparable figure for the mean maxima over the period 1805-1816 to 1931-1960 shows a more rapid rate of in increase, averaging 0.2"C per decade, suggesting either instrumental or interpretational errors or a more sensitive response by maximum temperatures to the factors that govern regional and local temperatures.

Finally, in common with so many Observatory sites, the possibility of an urban heat island effect cannot be overlooked. San Fernando now has a population of more than 60000 and has grown substantially over the past century. The Observatory site, however, remains open to the north and west. It stands on a small hill overlooking the lower end of the town whose sprawl now comes up to the southern and eastern sides of the perimeter fence some hundred or so metres from the Observatory. On admittedly subjective grounds it might be suggested that the urban heating effect is minimal and would be expected to be significant only for sites now wholly surrounded by much larger urban units.

CONCLUSION

The Cadiz and the San Fernando records used here are amongst Spain's oldest, and they are probably the oldest extant for their latitude. The character of the observations forbids any interpretation of mean daily temperatures, but the midday readings can be taken to approximate to the mean maxima. On this basis, a picture emerges of cooler conditions in both principal study periods by the order of 2°C. The period 1805 to 1816 appears, on this basis, to have been cooler than that for 1789 to 1795, perhaps suggesting the influences of the volcanic dust veils that were so common at that time. The warming that has taken place since then is more marked in the summer than in the winter months. This runs counter to what has taken place in more northerly latitudes where the CET, for example, shows a much less regular pattern of monthly temperature changes over the same periods. The former may be a statistical oddity of the data and its method of collection, but such checks as can be made of their reliability give no indication that this is the case and other writers have confirmed the possibility that in lower latitudes the degree of summer warming has been greater. We must await the emergence of contemporary sources from elsewhere in Iberia to confirm, or otherwise, the tentative conclusions drawn from this study.

ACKNOWLEDGEMENTS

The author gratefully acknowledges the financial assistance provided by the Royal Meteorological Society in the preparation of this paper and the help and guidance of the staff of the Real Instituto y Observatorio de la Armada in San Fernando. Particular thanks in this respect must go to Francisco Jose Gonzalez Gonzalez.

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