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any region of the world with just a few clicks casting large hail. Forecasting Research research/modelling-systems/unified- of a button. Technical Report 329. Met Office: Exeter. model/weather-forecasting [accessed Hand WH, Cappelluti G. 2010. A global 20 May 2011]. hail climatology using the UK Met Office Sheridan P, Smith S, Brown A, Vosper S. References convection diagnosis procedure (CDP) 2010. A simple height-based correction for Bell S, Milton S, Wilson C, Davies T. and model analyses. Meteorol. Appl. temperature downscaling in complex ter- 2002. A New Unified Model, NWP Gazette, doi:10.1002/met.236. rain. Meteorol. Appl. 17: 329–339. pp 3–7. Hohenegger C, Schär C. 2007. Fawbush EJ, Miller RC. 1953. A method Atmospheric predictability at synoptic for forecasting hailstone size at the versus cloud-resolving scales. Bull. Am. earth’s surface. Bull. Am. Meteorol. Soc. 34: Meteorol. Soc. 88: 1783–1793. Correspondence to: Stephen Moseley 235–244. Howard T, Clark P. 2003. Improvement [email protected] Golding BW. 1995. Formulation and to the nimrod wind nowcasting scheme Getting the most out of model data performance of an algorithm for deter- over high ground. Forecasting Research © British Crown copyright, the Met Office, mining precipitation type from a rain rate Technical Report 406. Met Office: Exeter. 2011, published with the permission of the forecast, using NWP model parameters. Howard T, Clark P. 2007. Correction and Controller of HMSO and the Queen’s Printer Forecasting Research Technical Report downscaling of NWP wind speed fore- for Scotland 157. Met Office: Exeter. casts. Meteorol. Appl. 14: 105–116. DOI: 10.1002/wea.844 Hand WH. 2000. An investigation into the Met Office. 2011. Met Office Unified potential of Miller’s technique for fore- Model. http://www.metoffice.gov.uk/

Weather – October 2011, Vol. 66, No. 10 66, No. – OctoberVol. 2011, Weather The cloud chamber and CTR Wilson’s legacy to atmospheric science

Giles Harrison The cloud chamber has become the vital means for studying the ultimate particles Department of , of matter, and is responsible for a major University of Reading part of modern physics. (Bragg, 1968)… It can tell us the history of a single one Introduction and early life of these minute particles, which leaves 2011 is the centenary year of the short a trail in the cloud chamber like that left paper (Wilson, 1911) first describing the in the upper air by an aeroplane under cloud chamber, the device for visualising certain meteorological conditions. (Bragg, high-energy charged particles which earned 1969) the Scottish physicist Charles Thomas Rees Wilson was born at Crosshouse farm, (‘CTR’) Wilson the 1927 Nobel Prize for phys- Glencorse, on 14 February 1869. The family ics. His many achievements in atmospheric moved to Manchester where he enrolled, science, some of which have current rele- with his brother, to study medicine at vance, are briefly reviewed here. CTR Owens College; the brothers frequently Wilson’s lifetime of scientific research work returned to Scotland where they pursued was principally in at the then popular scientific hobby of pho- the Cavendish Laboratory, Cambridge; he tography, for example producing images of was Reader in Electrical Meteorology from clouds in the mountains. Following his Figure 1. CTR Wilson in 1889. (Plate 8 of Wilson 1918 and Jacksonian Professor from 1925 to Manchester degree Wilson obtained an (1960), reproduced with permission of the Royal 1935. However, he is immortalised in phys- entrance scholarship to Sidney Sussex Society.) ics for his invention of the cloud chamber, College, Cambridge University,2 in October because of its great significance as an early 1888 (Figure 1). He intended chiefly to and after his Cambridge scholarship he visualisation tool for particles such as cos- study physics, wary of limited employment acted as a volunteer at Ben Nevis mic rays1 (Galison, 1997). Sir Lawrence Bragg prospects from undertaking the Observatory for a fortnight in September summarised its importance: Mathematical Tripos alone. Natural land- 1894. The Observatory was frequently shrouded in cloud and mist and the optical 1Despite Wilson’s awareness of background scapes retained a powerful lure to him he phenomena of coronas and glories he wit- ionisation, cosmic rays themselves (i.e. described himself as…strongly impressed high-energy particles from space) were with the beauty of the world (Wilson, 1960), nessed provided strong motivation for his discovered by Victor Hess (1883–1964) in experiments in imitating cloud processes in balloon flights during 1911–1912; he was 2 The Sidney Sussex student club for Natural the laboratory (Wilson, 1927). He died in 276 rewarded by the 1936 Nobel Prize for Physics. Science is called the Wilson Society. Edinburgh on 15 November 1959. (a) (b) The cloud chamber and CTR Wilson The cloud chamber and CTR Weather 2011, Vol. – October 66, No. 10

Figure 2. Cloud chamber images of (a) ionisation generated by a cylindrical X-ray beam about 2mm in diameter, passing right to left and (b) magnified tracks of ions formed by X-rays. (From Plates 8 and 9 of Wilson (1912), reproduced with permission of the Royal Society.)

Development of the cloud call Aitken nuclei, had been removed, one ern era is dominated by digital image pro- chamber still was able to produce condensation in duction, the photographic effort once the form of drops provided a certain quite required to obtain such iconic scientific Wilson’s Nobel Lecture acknowledged the definite degree of supersaturation was images may not be widely appreciated. For influence on his experiments of the particle exceeded. (Dee and Wormell, 1963) the strikingly beautiful pictures in Figure 2, visualisation technology of the time, the condensation particle counter invented by Wilson had discovered that air always con- John Aitken.3 The operating principle of the tained nuclei on which droplets could form Aitken counter was to moisten a sample of if the water supersaturation was sufficient. atmospheric air and then cool it by rapid His interpretation of these nuclei as charged expansion. The resulting substantial super- particles (ions) was influenced by the then saturation of the air sample yielded droplet developing understanding of charged parti- condensation on whatever particles (nuclei) cles. Initially, he tried applying ultraviolet were present, and these could be counted light to generate ions but, further inspired optically. Aitken made dust measurements by an early demonstration of X-ray appara- 4 at Ben Nevis Observatory (Aitken, 1889) tus made for J. J. Thomson at the Cavendish between 1891 and 1894 on this basis, Laboratory, Cambridge, in 1896, he was immediately prior to Wilson’s visit. Wilson delighted to find that X-rays greatly increased developed Aitken’s expansion chamber the number of droplets formed under the technique to use filtered air. In a lecture necessary high supersaturation. The fact that broadcast two days after his 90th birthday the droplets formed responded to an electric Wilson described this approach: field demonstrated that the condensation nuclei were charged, i.e. that they were ions I found that when all the dust particles, as (Wilson, 1899). Figure 2 shows images of Aitken called them, or what we should now high energy particles made visible within a cloud chamber (Wilson, 1912). As the mod- 3 John Aitken (1839–1919) was a Scottish atmospheric physicist. He made detailed investigations into the properties of the 4 Sir Joseph John (J.J) Thomson was Cavendish Stevenson screen and, using his condensation Professor of Physics and a president of the instrument to detect atmospheric aerosol, he Royal Society. He received the 1906 Nobel Prize was the first to observe the formation of new in Physics for discovering the electron and Figure 3. Cloud chamber images on the glass atmospheric particles by nucleation. research on the electrical conductivity of gases. photographic plates employed. 277 a stable photographic emulsion, the correct relative changes in the electric field,10 but ions with his atmospheric electricity instru- exposure, accurate focus and successful Wilson was to improve these techniques, mentation, and visualise them by his expan- chemical processing yielding good contrast largely through work undertaken during sion technique. He continued improving the would all have been needed, quite apart University vacation at the family home in expansion technique and in 1911 he pub- from the opportune visualisation of a high- Peebles, where his brother had a medical lished the short but pivotal paper summaris- energy event. Figure 3 shows the glass nega- practice. Wilson’s notebooks (Dee and ing the application of the cloud chamber. tive plates which formed the basic Wormell, 1963) show his interest in atmos- Wilson’s notebooks (Dee and Wormell, photographic technology employed; the pheric electricity from 1898, and his appre- 1963) show continued refinements of the exposure dates are unfortunately unknown. ciation of new findings about air ionisation cloud chamber beyond 1911 with improved Furthermore, experimental scientists of generated by radioactivity and X-rays no photographs of ion condensation tracks, Wilson’s time almost always had to make doubt further underpinned his atmospheric interleaved with atmospheric electricity their own apparatus, and Wilson was no electricity instrumentation developments. research on, for example, The cloud chamber and CTRWilson exception, for example developing glass- He described (Wilson, 1906) apparatus electrification, droplet charging and devel- blowing skills. As the Cavendish was relatively which not only measured the potential gra- opment of a capillary electrometer. He used near London there may well have been tech- dient in a calibrated way, but, by observing a small hut on the west side of Cambridge, nical exchange visits typical of the era (Gay, the rate of change of an alternately exposed now within the site of the New Cavendish 2008) to see the latest creations, but of course and covered electrode, determined the ver- laboratory, for his atmospheric measure- international attention also followed publica- tical current flow in fair weather.11 Wilson’s ments. One aspect of his work on disturbed tions describing the results. Wilson’s work delight at obtaining measurements of the weather atmospheric electricity is the became widely known; he demonstrated the vertical current flow remained with him all extended discussion with G. C. Simpson12 expansion apparatus to Stokes5 and Kelvin6 his life, illustrating the importance that he on the vertical charge structure of thunder- in the late 1890s, was elected a Fellow of the attached to experimental observations: storms (Williams, 2009). Royal Society in 1900 and, in 1903, met the I remember the satisfaction I had when my Curies7 during a Paris visit to see Langevin.8 work led to the fulfilment of my dream of The global circuit concept Weather – October 2011, Vol. 66, No. 10 66, No. – OctoberVol. 2011, Weather isolating a portion of the earth’s surface The origin of the potential gradient consist- Atmospheric electricity and measuring the charge upon it and the ently present in fair weather atmospheric instrumentation current flowing into it from the atmos- conditions has always remained a particular phere. (Wilson, 1960) In 1900 the Meteorological Council sought challenge to atmospheric science. Wilson Wilson’s involvement for work on atmos- The ‘Wilson apparatus’, as the Meteoro- argued that the conducting ionosphere pro- pheric electricity, which added to the logical Office subsequently referred to it, vided the connection between electrified existing demands of his expansion chamber demonstrated his immense experimental storms and the extensive fair-weather regions work and a Fellowship at Sidney Sussex. ingenuity and resourcefulness, for example elsewhere (Wilson, 1921). The presence of an Atmospheric electric field measurements in devising a guarding technique to mini- upper-atmospheric conducting layer had had been made at Kew Observatory9 since mise leakage current. He even deployed his been postulated more than 50 years earlier the 1840s by the flame probe and mechani- childhood microscope for thunderstorm (e.g. Chalmers, 1961), but was only firmly cal electrometer system, refined in the 1860s electricity measurements (Wilson, 1960). established experimentally by early radio to utilise the photographic recording meth- Evidence of the success of the Wilson appa- studies. Wilson elaborated further on this ods of Kelvin employing the water-dropper ratus lies in its longevity, as it was used in idea, for which fundamental support was potential sensor (Harrison, 2003). The water an almost identical manner until 1979 at found through comparison of the potential dropper (or ‘Kelvin electrograph’) measured Kew Observatory, having been moved to an gradient measurements made during global underground laboratory in 1931 (Harrison survey cruises by the research ship Carnegie and Ingram, 2005). Beyond its original scien- with meteorological information on the glo- 5 Sir George Stokes (1819–1903) was Lucasian tific application, data from the Wilson appa- bal distribution of (Wilson, Professor of Mathematics at Cambridge and a ratus has proved suitable for reconstructing 1929; Whipple and Scrase, 1936). A close rela- president of the Royal Society. historic data (Harrison, 2006). tionship was implied as both potential gradi- 6 Lord Kelvin (William Thomson, 1824–1907) was ent and global thunderstorm area showed a a President of the Royal Society, and made maximum at 1900 UTC and minimum around seminal contributions in thermodynamics and Lifetime research themes 0400 UTC, no matter where the Carnegie was electrical engineering. He developed methods Familiarity with the current flowing vertically located at the time of the measurements. for continuous electric field recordings, in the atmosphere in fair weather proved Wilson’s global atmospheric electrical cir- concluding that the sensitivity of atmospheric central to Wilson’s later unifying ideas in cuit concept (Figure 4) argues that charge electrical changes might prove useful in atmospheric electricity. The Wilson appara- weather forecasting. separated by shower clouds and thunder- tus’ combination of potential gradient and 7 storms is transferred through the iono- Marie Curie (1867–1934) and Pierre Curie conduction current measurements allowed sphere to fair weather regions. Fair weather (1859–1906) shared the 1903 Nobel Prize in simple calculation of the electrical conduc- Physics with Henri Becquerel for their work on vertical ion flow between the ionosphere tivity of air from Ohm’s Law. Air’s electrical radioactivity; Marie Curie won the 1911 Nobel and the surface, which Wilson so effectively conductivity is almost exclusively due to the Prize in Chemistry. observed, is directly linked to the surface ions it contains. Hence Wilson could uniquely 8 Paul Langevin (1872–1946) is known for his potential gradient. Subsequent findings measure the concentration of atmospheric work on piezoelectricity and relativity, but also studied air ions. 12 Sir George Simpson (1878–1965) was the 10 The vertical atmospheric electric field E is, by 9 Kew Observatory in Richmond, London, was z meteorologist on Scott’s ill-fated Antarctic convention, usually recorded as the potential the principal site for meteorological develop- Expedition, longest-serving director of the Met gradient , where = – . ments and observations, originally established F F Ez Office (1920–1938) and, in 1940, president of as an astronomical observatory in 1769 by 11 Construction of a simplified modern version the Royal Meteorological Society. His atmos- George III. It was closed in 1980, but meteoro- of the Wilson apparatus is described by Bennett pheric electricity work included direct logical measurements continue nearby at an and Harrison (2007) at http://arxiv.org/abs/ balloon-soundings of thunderstorms using a 278 automatic weather station in Kew Gardens. physics/0701280. specialised sensor (the alti-electrograph). IONOSPHERE Gay H. 2008. Technical assistance in the world of London science, 1850–1900. Notes Rec. R. Soc. Lond. 62: 51–75. Harrison RG. 2003. Twentieth century Wilson The cloud chamber and CTR atmospheric electrical measurements at the observatories of Kew, Eskdalemuir and Lerwick. Weather 58: 11–19. disturbed weather fair weather semi-fair weather Harrison RG. 2006. Urban smoke con- centrations at Kew, London, 1898–2004. Atmos. Environ. 40(18): 3327–3332. doi:10.1016/j.atmosenv.2006.01.042 Harrison RG, Ingram WJ. 2005. Air-earth current measurements at Kew, London, 1909–1979. Atmos. Res. 76: 49–64. + + + + + + Mach DM, Blakeslee RJ, Bateman MG. 2011. Global electric circuit implications Weather 2011, Vol. – October 66, No. 10 of combined aircraft storm electric current measurements and satellite-based diurnal conduction lightning statistics. J. Geophys. Res. 116: SURFACE current D05201. doi:10.1029/2010JD014462 Nicoll KA, Harrison RG. 2010. Experimental determination of layer Figure 4. The global atmospheric electrical circuit. Charge separated in cloud edge charging from cosmic ray disturbed weather regions (thunderstorms and shower clouds) flows ionisation. Geophys. Res. Lett. 37: L13802. upwards to the conductive lower ionosphere and downwards to the doi:10.1029/2010GL043605 surface (through lightning, precipitation and cloud discharge). The circuit is Whipple FJW, Scrase FJ. 1936. Point completed by a small vertical conduction current in distant fair weather Discharge in the Electric Field of the Earth, Meteorological Office of Geo physical regions and the Earth’s surface. Where the vertical conduction current Memoirs, Vol. 68. HMSO: London. passes through semi-fair weather regions containing extensive layer clouds, Williams ER. 2009. C.T.R. Wilson versus charging is associated with the upper and lower horizontal cloud edges. G.C. Simpson: fifty years of controversy in atmospheric electricity. Atmos. Res. have extensively corroborated the concept it is a tribute to his painstaking reasoning 91(2–4): 259–271. of the global circuit, making it a useful and wonderful experimental ingenuity that Wilson CTR. 1899. On the condensation nuclei produced by in gases by the action explanatory framework for a wide range of both his principal scientific achievements of Rontgen rays, Uranium rays, ultraviolet phenomena (Aplin et al., 2008), indicating a still influence physics education and atmos- light and other agents. Philos. Trans. A R. CTR legacy far beyond the use of the cloud pheric electricity research. Soc. Lond. 192: 403–453. chamber in contemporary research. Indeed, Wilson CTR. 1906. On the measurement the current flowing in the global circuit is Acknowledgement of the earth-air current and on the origin referred to in modern literature as the of atmospheric electricity. Proc. Camb. Andrew Wilson provided the glass plates Philos. Soc. 13: 363–382. Wilson current (Mach et al., 2011). from his grandfather’s cloud chamber pho- Wilson CTR. 1911. On a method of making tographs shown in Figure 3. I am also grate- visible the paths of ionising particles through Legacy ful to Alan Watson, Michael Rycroft and a gas. Proc. R. Soc. Lond. A 85: 285–288. Motivated by his early intentions to imitate Karen Aplin for their insights. Wilson CTR. 1912. On an expansion appa- ratus for making visible the tracks of ionis- nature, Wilson originally sought to link the ing particles in gases and some results behaviour of ions with atmospheric cloud obtained by its use. Proc. R. Soc. Lond. A formation (Galison, 1997). Despite the References 87(595): 277–292. immense applicability of the cloud chamber Aitken J. 1889. Dust particles in the atmo- Wilson CTR. 1921. Investigations on light- for visualisation, the mimetic approach itself sphere at Ben Nevis Observatory. Nature ning discharges and on the electric field of proved ultimately unsuccessful as the cloud 40: 350–351. thunderstorms. Philos. Trans. A R. Soc. Lond. 221: 73–155. chamber conditions needed differed vastly Aplin KL, Harrison RG, Rycroft MJ. 2008. Investigating Earth’s atmospheric electric- from those in the lower atmosphere: natural Wilson CTR. 1927. On the cloud method ity: a role model for planetary studies. of making visible ions and the tracks of cloud droplets therefore cannot form on cos- Space. Sci. Rev. 137: 11–27. doi:10.1007/ ionising particles. Nobel Lecture, 12th mic ray ions as they do in the cloud chamber. s11214-008-9372-x December 1927. Where there may yet be a closer link between Bennett AJ, Harrison RG. 2007. A simple Wilson CTR. 1929. Some thundercloud fair weather clouds and ions is at the cloud- atmospheric electrical instrument for edu- problems. J. Franklin Inst. 208: 1–12. cational use. Adv. Geosci. 13: 11–15. air boundaries of extensive stratiform clouds, Wilson CTR. 1960. Reminiscences of my where edge-charging is observed from the Bragg L. 1968. The white coated worker. early years. Notes Rec. R. Soc. Lond. 14(2): Punch 255: 352–354. global circuit current flow (Nicoll and 163–173. Harrison, 2010). Wilson’s broad interpreta- Bragg L. 1969. What makes a scientist? Proc. R. Inst. 42: 397–410. tion of natural phenomena, firmly rooted in Chalmers JA. 1961. The first suggestion laboratory and field experimentation, there- of an ionosphere. J. Atmos. Terr. Phys. 26: fore remains inspirational in the understand- 219–221. Correspondence to: Giles Harrison, ing of fundamental atmospheric processes. Dee PI, Wormell TW. 1963. An index to Department of Meteorology, In summary, CTR Wilson’s visualisation C.T.R.Wilson’s Laboratory records and University of Reading, techniques for particle physics concerned notebooks in the library of the Royal Earley Gate, PO Box 243 Reading, RG6 6BB Society. Notes Rec. R. Soc. 18(1): 54–66. microscopic cloud processes, whereas his [email protected] synthesis of atmospheric electricity unrav- Galison P. 1997. Image and Logic: A Material Culture of Microphysics. University © Royal Meteorological Society, 2011 elled invisible atmospheric properties on a of Chicago Press: London. global scale. Half a century after his death, DOI: 10.1002/wea.830 279