Through a glass brightly: Some new light on the Campbell±Stokes sunshine recorder

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1 Through a glass brightly: Some new light on the Campbell±Stokes sunshine recorder G. Stanhill Institute of Soil, Water and Environmental Sciences, Bet Dagan, Israel Although it is now 150 years since the first measurements of sunshine were made using the burning glass method, a number of questions concerning these observations remain open and are considered in this article. The first is historical and concerns the name of this type of sunshine recorder: Who was responsible for developing the instrument into its present form? A second question concerns the working of this instrument: What does it measure and what are the factors responsible for the changes in sunshine that have been recorded? Finally: What have sunshine measurements contributed to our understanding of climate and climate change and is it necessary to continue taking them? Campbell and/or Stokes? More than 200 years elapsed between the publication of Althanasius Kircher s design for a recording sundial, reproduced in Fig. 1 (Middleton 1966), and the first actual measurements of sunshine made in 1853 by J. F. Campbell using a `self-registering sundial of his own design (Campbell 1857). A quarter of a century after Campbell s measurements started, G. G. Stokes, the eminent physicist, Lucasian Professor of Mathematics at Cambridge University and President of the Royal Society, began his paper which described the design for the card support for sunshine recorders adopted by the Meteorological Office with these words: ``The method of recording sunshine by the burning of an object placed in the focus of a glass sphere freely exposed to the rays of the sun, which was devised by Mr. Campbell, commends itself by its simplicity and seems likely to come into pretty general use (Stokes 1880). As indeed it did. Only five years later R. H. Scott, then President of the Royal Meteorological Society, listed five years of sunshine duration measurements made with what he referred to as ``the Prof. Stokes s sunshine recorder from 40 stations in the UK (Scott 1885). A century later sunshine-duration values from more than 250 UK stations were included in the Meteorological Office s Monthly Weather Report. Outside the UK a small number of Campbell± Stokes (CS) sunshine recorders came into use in the 1880s, both in British territories overseas and in Europe; 100 years later the bulletin of the World Radiation Data Center listed CS sunshine duration data from 303 stations outside the UK (World Radiation Data Center 1992). Who was responsible for the many improvements in the sunshine recorder which transformed Campbell s crude original 1853 model into the standard climatological instrument shown in Fig. 2, adopted by the Meteorological Office in 1880 and still in use? Was it, as the instrument s name suggests, G. G. Stokes? The opening sentence of Stokes s (1880) paper quoted above shows that he made no such claim, nor is such a contribution included in the 17-page obituary notice detailing his scientific achievements which appeared in the Proceedings of the Royal Society (Anon. 1905). What is clear is that the development was not the result of co-operation between Stokes and Campbell as not a single letter to or from J. F. Campbell was found in the vast (approximately items) collection of Stokes s correspon- 3

2 Fig. 1 Althanasius Kircher s design for a recording sundial, 1646 (Middleton 1966) dence and papers held in the Cambridge University Library (Wilson 1976). The only reference to sunshine recorders found in this collection was an undated photograph of an instrument similar to that shown in Fig. 2 in an envelope marked ``Campbell± Stokes Sunshine Recorder, although not in Stokes s own distinctive handwriting. Campbell s own long account of his sunshine studies, which he titled Thermography 4 (Campbell 1883), describes nearly all of the changes in sunshine-recorder design which occurred between 1853 and These include the use of improved glass spheres, together with Prussian blue coloured cards ``set in grooves... in a bell metal bowl to obtain hourly and daily records of sunshine in a wide range of insolation climates. It is, however, clear from this little-known book that Campbell s major interest was in the

3 What does the sunshine recorder measure and what factors influence its measurements? Fig. 2 The original Campbell± Stokes sunshine recorder on site at the Real Instituto y Observatorio de la Armado en San Fernando, Cadiz ( N, 6812 W) where it has been in use since 1871 (# R. B. Carlos-Roca) use of the sunshine recorder to study the intensity and spectrum of solar irradiance, rather than its routine use to record sunshine duration as a climatic element; this receives little attention. By contrast, there is considerable discussion of the relevance of sunshine measurements to atmospheric transmissivity, to the exploitation of solar energy as a source of power and, in particular, the relationship between sunshine hours and solar activity as shown by sunspot numbers. Campbell s book was published too late and is insufficiently documented to unequivocally establish his priority for all the changes in design. This is perhaps not surprising as there is no evidence that Campbell, a gifted and wealthy, much travelled gentleman scholar with a wide range of non-scientific as well as scientific interests (Anon. 1908), was concerned with scientific priority although he was careful to name those who collaborated in his research. The likely answer to this minor mystery is that the instrument s promoters thought the addition of Stokes s name to that of Campbell would be helpful through the scientific eminence it conveyed. By the time the doublebarelled name came into general use Campbell was no longer alive and was not able to comment. The design of the CS recorder is such that it registers direct radiation, I (the radiant energy flux of the solar beam measured at normal incidence), whenever its intensity is above the burning threshold of the recording surface. In Campbell s original instrument this was wood and the volume burnt provided the measure of direct solar irradiance. In its current form the measure used is the length of the trace burnt on a standard card marked and measured in hours, n. The intensity of I at the burning threshold has been standardised at 120 Wm ± 2 (World Meteorological Organization 1997). The number of hours of bright sunshine duration is in itself of little significance, although it was at one time widely used to promote UK holiday resorts (Curtis 1898) and is still used, after normalisation by day length, N, as an inverse index of cloud cover. Probably the major use of CS measurements has been to calculate the more significant, but until recently much less commonly measured, parameter, global irradiance, E g ;, the short-wave radiation from the sun and sky reaching a horizontal surface. The considerable literature on the relationship between global irradiance and sunshine duration, reviewed by Martinez et al. (1984) and Linacre (1992), shows that the most widely used way of estimating global irradiance is by the use of the linear regression E g ; =(a+ b n/n) E o to relate normalised sunshine duration n/n, to E g ; global irradiance normalised to that at the top of the atmosphere, i.e. extraterrestrial irradiance, E o, which is determined by the latitude and date. The slope of this relationship, b, represents the sensitivity of normalised global radiation to normalised sunshine duration. The offset term, a, represents global radiation under overcast conditions when direct radiation is below the burning threshold of the CS, and the sum of the two parameters, a + b, global radiation under clear-sky conditions. Mean values of these parameters, together with their standard deviations, are given by Linacre 5

4 (1992) as a= , b = , and a + b = The values of the parameters vary not only with location and cloud type but also with the length of the measurement periods, i.e. daily, weekly or monthly, used to derive the relationship. As is to be expected, CS measurements are also accurate predictors of I, normal incidence of direct solar beam irradiance. This was first demonstrated by Galindo Estrada and Fournier D Albe (1960) who, reverting to Campbell s original method, measured the weight of sun card burnt during 100 days in Mexico City and related these values to daily totals of I measured with a standard radiometer. On the basis of the high correlation found, they suggested using the CS recorder to monitor daily values of direct radiation. This suggestion was followed up in a later study in which 16 years of standard measurements of sunshine duration from Ireland and Israel were compared with measurements of direct irradiance. Highly significant linear relationships were found for hourly, daily, monthly and yearly periods and, despite the very different radiation regimes at the two sites, these did not significantly differ except in the case of annual totals (Stanhill 1998a). The common relationship between n and I derived from 383 individual monthly mean values, shown in Fig. 3, was such that one Fig. 3 Relationship between mean values of direct irradiance and sunshine duration for individual monthly means measured at Bet Dagan, Israel (32800 N, E), (open squares) and Valentia, Ireland (51856 N, W), (solid squares). Square of correlation coefficient = hour of bright sunshine was equivalent to 2.209MJ m ± 2 (614Wm ± 2 ) normal incidence irradiance. The highly linear relationship implies that I is constant for that portion of the day when I exceeds 120 Wm ± 2, the burning threshold of the CS recorder. This could be explained by the well established fact that under clear-sky conditions the diurnal curve of I is flat-topped and only varies markedly during a relatively small proportion of the daylight hours. However, more research is needed before the relationship common to Ireland and Israel can be recommended as generally applicable. During the development of the CS recorder much attention was given to the instrumental features affecting the accuracy and precision of the instrument, in particular the burning threshold of the sun cards, the differences between observers in evaluating sunshine duration (especially important when the traces are intermittent or faint), and the size and quality of the glass sphere (Curtis 1898). The results of these studies led to the adoption of standard sun cards, methods of evaluating the length of the traces burnt, and glass spheres. When the recommended procedures are adhered to, both the uncertainty of measurement and its resolution have been set at 0.1 hour. The burning threshold on any given occasion, however, may well vary considerably from the standard value of 120 Wm ± 2 (World Meteorological Organization 1997). Another obvious but noteworthy requirement for accurate CS measurements is that the site used has an open exposure. This has been defined as one without obstructions obscuring the sun during more than 5% of the period when it is over 38 above the horizon. This is the solar altitude at which normal incidence irradiance can reach the burning threshold of sun cards. Slow-growing trees are probably the most insidious site obstruction and were, for example, the most likely cause of the very steep decline in sunshine found in the first third of the 120-year measurement series from San Fernando Observatory near Cadiz, Spain (Wheeler 2001). At this site the annual total of sunshine measured declined almost linearly from 3089 hours in 1897 to 1772 hours in In the spring of 1932 sunshine duration

5 doubled and in the following years the annual totals returned to values between 3000 and 3500 hours. From 1950 to 1975 the annual totals declined irregularly to 2500 hours, a decrease that was attributed to rapid industrialisation of the region, and this was followed by an irregular increase to an annual total of 3015 hours in 2000, a value very similar to that measured 100 years previously. The original CS recorder at San Fernando, shown in Fig. 2, is still in operation at its original site (Carlos-Roca, personal communication 2001), and the fact that its current readings closely agree (to within 2% for individual monthly means) with those measured at Gibraltar, 85 km to the south-east (Wheeler 2001), strongly suggests that site deficiencies (perhaps the trees seen in Fig. 2) rather than instrument deterioration were the cause of the early decline in sunshine duration measured at the San Fernando Observatory. It also attests to the remarkably long working life of the CS sunshine recorder. Causes of changes in sunshine duration and their relevance to climate change The results of the century-long series of measurement at San Fernando discussed above point to three major factors as the cause of changes in sunshine duration. These are: alteration in the frequency of the synoptic situations associated with clear or overcast sky conditions; changes in atmospheric transmissivity caused, for example, by pollution; and changes in the exposure of the measurement site. Finally, deterioration in the transmissivity of the glass sphere could lead to a spurious decline in sunshine duration, which would be difficult to detect if gradual. Trends in cloud cover, if substantial, could well cause changes in sunshine duration. Over the last 50± 80 years increases in total sky cover of 1% per decade were reported for four continental areas (Houghton et al. 1990); a somewhat larger decrease in cloud cover has been reported from an analysis of the Chinese observation network (Kaiser 2001). However, the uncertainty associated with cloud cover observations is very large and, moreover, there is evidence that trends in cloud cover and sunshine duration are neither simply nor universally related (Angell et al. 1984; Kaiser 2001). Changes in the water content of the atmosphere and/or in the frequency of rainfall, fog, mist, dew, snow and hoar frost could affect sunshine duration indirectly as well as directly by changing the water content of the sunshine card and hence its burning threshold. This could result in a change in the diurnal pattern of sunshine such as that found in an analysis of 40 years of CS measurements at Ankara, Turkey. This showed significant decreases in sunshine, mostly at sunrise and sunset hours when the major increases in relative humidity occurred (Aksoy 1999). Urban pollution effects on sunshine duration are not easy to distinguish and quantify although a number of attempts have been made to do so using measurement series from London. Reductions in sunshine during the first series measured between 1858 and 1874 in central London (Roscoe and Stewart 1875) were significant, amounting to 2.2% per year, and could have been caused by increased urban air pollution. Hatch (1981) drew attention to the reverse process in his study of the 100-year series from Kew, south-west of London. He attributed the increase in sunshine apparent in the early 1960s to the stringent measures against air pollution in the London area enacted at that time. Cowley (1976) quantified this improvement as the increases in mean normalised global irradiance, E g ;/E o, per sunshine hour which were measured at Kew over the 1951± 74 period; these amounted to 25% in the winter and 8% during the summer. Weekly cycles in winter sunshine duration, with weekend maxima and midweek minima, have also been used to demonstrate urban pollution effects in the UK; the fact that the `Sunday Effect weakened after the 1960s was attributed to the reduction of air pollution in the following decade (Wilby and Tomlinson 2000). The relevance of the many and divergent changes in sunshine duration that have been measured, and of the many and divergent reasons given to explain them, to the current debate on climate change and global warming lies in their possible contribution to a relatively 7

6 8 new facet of this debate ± the role and magnitude of changes in short-wave radiation in the earth s energy balance. For over 100 years the major cause of contemporary global warming has been widely agreed to be the increase in long-wave radiation from the atmosphere caused by growing concentrations of carbon dioxide and other man-made trace gases. The modelled heating effect of this positive radiative forcing, 1.2 degc, however, is double the 0.6 degc increase in air temperature actually measured during the last century (Houghton et al. 1996). To resolve this discrepancy, the effects, both direct and indirect, of short-wave radiative cooling, caused by growing concentrations of radiatively active aerosols originating from fossil-fuel emissions, have recently been included in the models used to predict climate change. Direct evidence for a substantial decrease in short-wave radiation at the earth s surface, based on measurements of E g ; made during the last 40 years, has emerged in the last decade. A review of this evidence (Stanhill and Cohen 2001) shows that the measured decrease ± which globally averages Wm ± 2 a year ± is, however, 10 to 100 times greater than the modelled aerosol effect; it even exceeds the long-wave heating effect, calculated as 2.45Wm ± 2 since the Industrial Revolution (Houghton et al. 1996). To resolve this discrepancy and explain the decrease in E g ; will require much study. One difficulty is that time-series of accurate E g ; measurements only extend back some 40 years which is insufficient to study nonlinear, cyclic changes and possible links with changes in solar and volcanic activity. Such investigations could, however, be made using long-term CS measurement series, some of which are three times as long as the longest radiation measurement series, as proxies for global and direct irradiation. As an example, the results of one such study, based on the 100-year sunshine duration series from Cracow, Poland, can be cited. This showed three significant periodicities, two of which are associated with the 11-year cycle of solar activity, in addition to an accelerating decrease since 1955 attributed to urban pollution (Morawska-Horawska 1985). Interestingly, the linkage of sunshine measurements to solar activity as shown by sunspot frequency was noted by Roscoe and Stewart (1875) in their analysis of the interannual variation in the first, short series of sunshine measurements from central London made with the original crude recorder between 1855 and 1874; Campbell himself also devoted a chapter to this subject in his book on sunshine measurements by the burning glass method (Campbell 1883). The many site-specific causes of changes in sunshine duration indicate that different patterns of change are to be expected at different sites, and an example of this can be seen from the measurements illustrated in Fig. 4 which were made at two observatories in the British Isles, both situated on the west coast and remote from population centres and industrial sources of pollution. At Stornoway in the Outer Hebrides, there was a small linear decrease in annual sunshine duration, averaging ± hours per year between 1880 and Although this was not statistically significant, both the Runs and the Wolf and Wolfowitz tests showed that this series was significantly non-homogeneous and demonstrated significantly non-random clustering. At Valentia in the extreme south-west of Ireland, the decrease was ten times larger and very highly significant, averaging ± hours per year over the 1893± 1999 period. The reality of this steep reduction was confirmed by the relatively even greater decreases in global, direct, diffuse and net all-wave irradiance measured at this same site during the second half of the sunshine duration record (Stanhill 1998b). To date no explanation for these reductions nor for the clear outliers in the sunshine record has been put forward, and any suggestions from readers, however speculative, would be welcome. At Stornoway, although not at Valentia, the lowest annual total of sun hours was measured during 1993, two years after the eruption of Mount Pinatubo, the major volcanic event occurring during the measurement series. The connection between sun hour duration and the other major volcanic events, as quantified by the Dust Veil Index shown in Fig. 4, was, how-

7 Fig. 4 Annual sunshine duration at (a) Valentia, Ireland (51856 N, W), (square of correlation coefficient = ) and (b) Stornoway, Scotland (58811 N, 6822 W), (correlation coefficient = ). (c) Major volcanic events quantified by the values of their Dust Veil Indices. ever, not an obvious one; nor was there a simple relation with the peaks of solar activity indicated by number of sun spots. The future of the CS sunshine recorder The question of whether the recording of simply the duration of sunshine be continued was raised over 50 years ago, and answered ``not if it is feasible to record the actual intensity as well as the duration (Brooks and Brooks 1947). At that time the cost of the equipment and the maintenance needed to measure and record the intensity of solar irradiance rendered this unfeasible as a routine climatological measurement. This is no longer the case for global radiation measurements at automatic weather stations although it remains true for measurements of direct solar beam radiation. Where 9

8 records of the duration of bright sunshine are still needed, a number of electronic sunshine recorder models have become commercially available in the last decade. These register the time that global radiation exceeds diffuse radiation by a given threshold; thus they function similarly to the CS recorder although direct radiation is measured on a horizontal surface rather than on one normal to the solar beam. These new models of sunshine recorders have a number of advantages over the standard instrument ± they are automatic and do not require daily attention from an observer, data reduction is faster and more accurate, and routine and absolute calibration is possible. For these reasons they will probably, with few exceptions, eventually replace the CS recorder. The exceptions should be at those long-established, well-maintained and freely exposed meteorological stations where the continuity of long-term records is important for their contribution to our understanding of climate change. Acknowledgements The assistance of Rafael Boloix Carlos-Roca (Director of the Observatory of the Armada, San Fernando, Cadiz), Paraic Carrigan (Met Eireann, Ireland), and Steve Jebson (National Meteorological Library and Archive) in supplying information and data is gratefully acknowledged, as is the help of the librarians at St. John s College, Cambridge, and at Cambridge University Library. My thanks to S. Moreshet and Y. Li for their help with data analysis and presentation. References 10 Aksoy, B. (1999) Analysis of changes in sunshine duration data for Ankara, Turkey. Theor. Appl. Climatol., 64, pp. 229± 237 Angell, J. K., Korshover, J. and Cotton, G. F. (1984) Variations in United States cloudiness and sunshine, 1950± 82. J. Clim. Appl. Meteorol., 23, pp.752± 761 Anon. (1905) Obituary notices of fellows deceased (Part iii.) Proc. R. Soc., 75, pp. 199± 216 Ð Ð (1908) Campbell, John Francis (1822± 1885) In: Dictionary of national biography, Vol. 111, Smith Elder & Co., London, p. 8 Brooks, C. F. and Brooks, E. S. (1947) Sunshine recorders: a comparative study of the burningglass and thermometric systems. J. Meteorol., 4, pp. 105± 115 Campbell, J. F. (1857) On a new self-registering sundial. In: Report of the Council of the British Meteorological Society, read at the seventh Annual General Meeting, May 27, 1857, pp. 18± 26 Ð Ð (1883) Thermography. Wakeham and Son, Kensington Cowley, J. P. (1976) Variations in global solar radiation at Kew. Meteorol. Mag., 105, pp. 329± 343 Curtis, R. H. (1898) Sunshine recorders and their indications. Q. J. R. Meteorol. Soc., 24, pp. 1± 30 Galindo Estrada, I. G. and Fournier D Albe, E. M. (1960) The use of the Campbell± Stokes sunshine recorder as an integrating actinometer. Q. J. R. Meteorol. Soc., 86, pp. 270± 272 Hatch, D. J. (1981) Sunshine at Kew Observatory, 1881± J. Meteorol, UK, 6, pp. 101± 113 Houghton, J. T., Jenkins, G. J. and Ephraums, J. J. (Eds.) (1990) Climate change: The IPCC scientific assessment. Cambridge University Press, Cambridge Houghton, J. T., Meira Filho, L. G., Callender, B. A., Harris, N., Kattenburg, A. and Maskell, K. (Eds.) (1996) Climatic change 1995: The science of climate change. Cambridge University Press, Cambridge Kaiser, D. P. (2001) Assessing observed temperature and cloud amount trends for China over the last half of the twentieth century: What can the sunshine duration record tell us? In: Proceedings of the 12th Symposium on Global Change and Climate Variations, 14± 18 January 2001, American Meteorological Society, Albuquerque, New Mexico, pp. 283± 284 Linacre, E. (1992) Climate data and resources. Routledge, London Martinez, J. A., Tena, F., Onrubia, J. E. and De la Rubia, J. (1984) The historical evolution of the Angstrom formula and its modifications. Agric. For. Meteorol., 33, pp. 108± 128 Middleton, W. E. K. (1966) Invention of the meteorological instruments. Library of Congress Catalog Card Number , John Hopkins Press, Baltimore Morawska-Horawska, M. (1985) Cloudiness and sunshine in Cracow, 1861± 1980, and its contemporary tendencies. J. Climatol., 5, pp. 633± 642 Roscoe, H. E. and Stewart, B. (1875) On the heat of sunshine at London during the twenty-four years 1855 to 1874, as registered by Campbell s method. Proc. R. Soc., 23, pp. 578± 582 Scott, R. H. (1885) On the measurement of sunshine. Q. J. R. Meteorol. Soc., 11, pp. 205± 216 Stanhill, G. (1998a) Estimation of direct solar beam irradiance from measurements of the duration of bright sunshine. Int. J. Climatol., 18, pp. 347± 354 Ð Ð (1998b) Long-term trends in, and spatial varia-

9 tion of, solar irradiances in Ireland. Int. J. Climatol., 18, pp. 1015± 1030 Stanhill, G. and Cohen, S. (2001) Global dimming: a review of the evidence for a widespread and significant reduction in global radiation with discussion of its probable causes and possible agricultural consequences. Agric. For. Meteorol., 107, pp. 255± 278 Stokes, G. G. (1880) Description of the card supporter for sunshine recorders adopted at the Meteorological Office. Q. J. R. Meteorol. Soc., 6, pp. 83± 94 Wheeler, D. (2001) Factors governing sunshine in south-west Iberia: A review of western Europe s sunniest region. Weather, 56, pp. 189± 197 Wilby, R. L. and Tomlinson, O. J. (2000) The `Sunday Effect and weekly cycles of winter weather in the UK. Weather, 55, pp Wilson, D. B. (1976) Catalogue of the manuscript collection of Sir George Gabriel Stokes. Cambridge University Library World Meteorological Organization (1997) Measurement of sunshine duration. In: Guide to meteorological instruments and methods of observation, 6th edition, World Meteorological Organization, Geneva, Chapter 8 World Radiation Data Center (1992) Catalogue of solar radiation data for the period 1 January± 31 December World Radiation Data Center, Voeikov Main Geophysical Observatory, St. Petersburg Correspondence to: Gerald Stanhill, Institute of Soil, Water and Environmental Sciences, PO Box 6, Bet Dagan, Israel gerald@agri.gov.il # Royal Meteorological Society, doi: /wea