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The Cloud Chamber and CTR Wilson's Legacy to Atmospheric CORE Metadata, citation and similar papers at core.ac.uk Provided by Central Archive at the University of Reading 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 Meteorology, 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 un der 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 atmospheric electricity 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 CTRWilson Weather – October 2011, Vol. 66, No. 10 66, No. – OctoberVol. 2011, Weather 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
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