Dobson, Gordon Miller Bourne
DOBSON, GORDON MILLER BOURNE
(b. Windermere, Cumbria, United Kingdom, 25 February 1889;
d. Oxford, United Kingdom, 10 March 1976), atmospheric ozone, atmospheric physics, atmospheric chemistry, meteorology.
Dobson made the first systematic measurements of stratospheric ozone in the 1920s and 1930s. He elucidated how its behavior varied with latitude and season, using equipment that he developed.
Education and War Years . Gordon Dobson was born the youngest of four children (sisters Alice and Kate, brother Harry). His father, Thomas Dobson, was a medical practitioner in Windermere; his mother was Marianne Bourne. The family had a large house close to the lake. He was educated at Sedbergh School then at Gonville and Caius College, Cambridge University, from which he graduated in 1910 with a first-class BA in natural sciences. During his final year he studied geophysics and came across work by George Chrystal on seiches (oscillations in the water level) in Scottish lochs. This prompted him to build a simple chart recorder, which he used at his family boathouse on Lake Windermere. The lake is 18 kilometers long and displays various modes of oscillation that Dobson was able to isolate and successfully compare with elementary theory. The results were published in Nature in 1911.
Dobson had shown a clear bent for experimental physics from his time at school, having built a Tesla coil and a Wimshurst machine from rudimentary materials, and his later skill as an instrument designer and builder was already evident in a photograph of his seiche recorder.
His Nature paper led to a job offer from (William) Napier Shaw, director of the British Meteorological Office, to work at Kew Observatory, jointly funded by the Meteorological Office and Gonville and Caius College. While at Kew he worked with Charles Thomas Rees Wilson (on vertical wind velocity measurement) and William Henry Dines (assisting with balloon flights) amongst others, and he had a short spell running the Eskdalemuir magnetic observatory. In 1913, he was appointed meteorological advisor to the new military Central Flying School on Salisbury Plain (a 1917 paper gives his position as captain in the Royal Flying Corps) where he began pioneering work measuring the vertical wind profile. This led to a close association with the fluid dynamicist Geoffrey Ingram Taylor, who had predicted the wind behavior in the boundary layer theoretically.
At the outbreak of war in 1914, his duties changed to instrument design, for example overseeing the development of the Aldis signaling lamp, leading to his appointment in 1916 as director of the Experimental Department of the Royal Aircraft Establishment (RAE), Farnborough. This carried a wide remit given his youth, including oversight of areas well outside his expertise. At a time when science was thought to have little relevance to the war effort, the RAE was a powerhouse for invention: Thus Dobson was fortunate to have found himself there working with a group of very able scientists and mathematicians. One of these was Frederick Alexander Lindemann, who was later to become Lord Cherwell, scientific advisor to and close confidant of Sir Winston Churchill during World War II. Together Dobson and Lindemann worked on projects such as one of the first automatic pilots and balloon cable cutters for aircraft. In 1919, Lindemann took the post of head of the Oxford University Physics Department (the Clarendon Laboratory) and quickly initiated a period of several decades of growth and success. Dobson was recruited as meteorology demonstrator in 1920.
Temperature and Ozone . Together Dobson and Lindemann undertook pioneering analysis (later described by Richard Goody as brilliant) of the height at which meteors burn up, leading to the conclusion that the atmospheric density above the 50-kilometer region was 100–1,000 times larger than previously assumed. This implied that temperature must increase substantially at lower levels, whereas currently it was assumed to be constant above the limit of balloon measurements (25 km). This was later corroborated by analysis of long-range sound propagation patterns, and with sounding rockets in the late 1940s. They proposed that the increase was caused by solar radiative heating of ozone that had been formed by ultraviolet dissociation of oxygen, which has proved to be correct.
This discovery led Dobson to develop methods to measure atmospheric ozone distribution. Work by Alfred Cornu, Walter Hartley, Alfred Fowler, and Robert Strutt had demonstrated ozone features in solar ultraviolet spectra. In 1921, Charles Fabry and Henri Buisson had determined the equivalent thickness of the atmospheric ozone layer by comparing the ultraviolet intensity in the Sun’s direct beam at a single ozone absorbing wavelength with the Sun at two or more different zenith angles. They allowed for the effects of scattering by air molecules but were unable to correct for any aerosol absorption. Dobson and Douglas Neill Harrison improved this technique. By employing two nearby wavelengths with differing ozone absorptions and by assuming the same aerosol absorption in each, they were able to mainly eliminate aerosol contributions. Several pairs were normally used.
This work was performed with a spectrophotometer that Dobson designed and built, which employed a Féry prism, and a cell containing a mixture of chlorine and
bromine that transmits in the ultraviolet but blocks stronger and longer wavelengths. The cell enabled the instrument to be smaller than Fabry and Buisson’s double monochromator, and portable. A photographic plate recorded the spectrum, and a neutral density wedge was arranged to give a known linear gradation perpendicular to the wavelength direction (so that features produced a signature whose size on the emulsion was proportional to their intensity). Development of the plate required a consistent procedure and a purpose-designed development tank. He built probably the first electronic densitometer to measure the degree of absorption on the plates, employing photocells developed elsewhere in the Clarendon. He wrote a book about the photographic aspects in 1926.
The first spectra were obtained in September 1924, and the results were so encouraging that a year later he began to build five more instruments partly funded by the Royal Society. These were placed at various European locations; the Smithsonian Institution funded another that was placed in Chile. Observations began in July 1926, with the photographic plates being returned to Oxford for processing. Dobson maintained the enthusiasm of the observers by mailing back results, annotated weather maps, etc. More than five thousand plates were measured during this campaign.
The instruments were then redeployed worldwide to California (Table Mountain Observatory), Egypt (Helwan), India (Kodaikanal), New Zealand (Christchurch), Switzerland (Arosa), plus Oxford. By the end of 1929, Dobson and his associates had established the main seasonal and latitudinal behavior of column ozone, apart from at high latitudes. A strong correlation with weather systems was found in both campaigns, and was later known to be due to the column mass of the stratosphere varying with tropopause height.
During this campaign, Dobson was working on a very much more advanced design that replaced photographic emulsion by photocells and could therefore benefit from current and future detector advances. It employed a rotary chopper and a double Féry monochromator (i.e., two in tandem) but in an unconventional configuration, to reduce stray light. Moveable neutral density wedges were used in a null arrangement that directly found the intensity ratio for two wavelengths.
The first observation considered publishable was made in July 1930. Measurements could then be made much more quickly (a few minutes) and the analysis performed on-site without sending the plates to Oxford. The technique was much more sensitive, and opened the way to making measurements with the Sun obscured by cloud.
It also enabled Umkehr measurements to be made: Paul Götz had recently shown that measurement of the intensity ratios for scattered light using a zenith view at several different wavelengths would allow a height-resolved profile to be obtained. This showed that the ozone concentration peaked at about 25 kilometers, half that of earlier results by others using views of the Sun at angles near the horizon. Balloon flights by Erich and Victor Regener confirmed this lower altitude, which was later known to be correct. Umkehr (a term Götz suggested to describe the way the graphs turned back at low sun angles) measurements opened up a new dimension which Dobson continued to explore throughout his life. The instrument was too complex for Dobson to manufacture in quantity himself, and a commercial instrument maker started limited production in about 1932.
Water Vapor . The outbreak of World War II forced a national change in scientific activity. While his Clarendon colleagues moved to radar and isotope separation work, Dobson was asked to help understand the formation of aircraft condensation trails, which were unpredictable and causing many losses because they made aircraft very prominent (a small dot of an aircraft can leave behind a huge trail that couldn't be missed). Existing humidity sensors were completely inadequate for the low dew points existing at high altitudes, and Dobson designed and built a new frost point hygrometer for aircraft use. The Meteorological Office provided Alan W. Brewer to lead the measurement program. By 1942, Dobson and Brewer had completed extensive measurements and achieved a good understanding of quite subtle behavior. Using a Flying Fortress they were able to reach above 11 kilometers altitude in 1943, and found that above the tropopause (then at 9 km) the frost point fell rapidly while the temperature rose.
Dobson presented the 1945 Royal Society Bakerian Lecture and showed clear evidence, based on thirteen ascents, of a much drier stratosphere than was expected. No firm explanations were proposed. He suggested that air might be entering the stratosphere via the tropics where the tropopause was very cold and thereby being freeze-dried, but he still assumed the classical picture of a stationary stratosphere in radiative balance, whereby the incoming and outgoing radiation were thought to balance each other, with no significant vertical motion to cause adiabatic heating or cooling. Brewer was recruited to Oxford in 1948 as meteorology lecturer, and in 1949, Brewer published a seminal paper proposing a circulation where air rises in the tropics, and falls over the poles in a circular fashion that would largely explain the water vapor observations, but departs a long way from radiative balance. Dobson seems to have been unconvinced by this theory (on the grounds that angular momentum would not be conserved) until further evidence mounted up by 1957. Not until the 1980s was the Brewer-Dobson circulation, as it came to be called, generally accepted.
Later Work . Dobson undertook work on the formation of ice particles in the 1950s, building cloud chambers which his research students used, but his main effort was concentrated on ozone measurement, both improving the spectrophotometer design (e.g., fitting photomultipliers, with greater sensitivity and enabling use of shorter wavelengths), and supporting the worldwide network that he had built up (forty-four Dobson spectrometers by 1956, and as of 2005 the primary standard for ozone column measurements with about one hundred active stations). He was assisted by Sir Charles Normand, who had retired from the Indian Meteorological Service, and by C. D. Walshaw. The International Geophysical Year (1957) brought new results, including the observation that the Antarctic seasonal cycle was very different from that of the Arctic.
Dobson was made a DSc of Oxford in 1924, based on a submission of fourteen scientific papers, and elected as a Fellow of the Royal Society in 1927, being awarded their Rumford Medal in 1942. In 1938, he was awarded the Royal Meteorological Society Symons Medal. He was appointed a professor in 1945, and was president of the Royal Meteorological Society from 1947 to 1949. He was award the CBE honor in 1951 and retired in 1956. He was president of the newly founded International Ozone Commission between 1948 and 1960, and the commission provided important funding to support the observing network. Between 1934 and 1950, he was chair of the U.K. government Atmospheric Pollution Committee and undertook some research in this area.
Dobson married Winifred Duncombe Rimer in 1914, and they had a daughter (Kathleen) and two sons (Desmond and Robert). A major outside interest was sheep farming, and their move in 1937 to a large new house on Shotover Hill on the outskirts of Oxford, with extensive grounds, enabled him to pursue this hobby as well as gardening, particularly of fruit and vegetables. He also inherited three farms in Cumbria from his father.
A necessary feature of his new house was an uninterrupted view to the south to permit ozone observation. He built a new observatory and workshop, both of which were featured in an article in the Oxford Times newspaper (1957). He was one of the last of a long series of scientists to undertake world-class research in their own facilities using partly their own funding (Winifred had a private income; Dobson had a significant inheritance); indeed, much of his own and his students’ research was conducted from home rather than at the university. Before electricity was connected in the early 1920s students would cycle several kilometers to his house from central Oxford with lead-acid batteries to supply the equipment.
His other interests included music (he played the piano and violin), and he was a warden at St. Aldates Anglican Church, Oxford. Winifred died in 1952, and in 1954, he married Olive Mary Bacon who survived him. His last paper was written in 1973. A day after making an ozone observation he suffered a stroke, from which he died six weeks later on 10 March 1976.
After his death Dobson’s name became well known even to the general public because of widespread concern about ozone depletion, and also the use of the Dobson Unit to describe the ozone column amount. In 2004, the International Ozone Commission established the quadrennial Dobson Award to a young scientist for the most significant contribution to ozone research in the previous four years.
The Dobson Unit is defined as the column amount of ozone given in units of 0.01 millimeter at 1 bar pressure and 0° Celsius. That is, if the total quantity of ozone (between the surface and space) is brought to a single layer of pure ozone at 1 bar pressure and 0° Celsius, the measurement in Dobson Units is the vertical thickness of that layer in units of 0.01 millimeter. A typical quantity is 300 Dobson Units, that is, a 3-millimeter thick equivalent layer and could correspond to an actual layer of ozone in the stratosphere 10 kilometers thick at a mean pressure of 0.01 bar and at –50° C with one atmospheric molecule in 300,000 being ozone.
WORKS BY DOBSON
“Seiches in Windermere.” Nature 86 (1911): 278–279. His first geophysical paper.
With Frederick A. Lindemann. “A Theory of Meteors and the Density and Temperature of the Outer Atmosphere to which It Leads.” Proceedings of the Royal Society of London, Series A, 102 (1922): 411–437. See also next item.
With Frederick A. Lindemann. “A Note on the Temperature of the Air at Great Heights.” Proceedings of the Royal Society of London, Series A, 103 (1923): 339–342.
Photographic Photometry. Oxford: Clarendon Press, 1926.
The Uppermost Regions of the Earth’s Atmosphere: Being the Halley Lecture Delivered on 5 May, 1926. Oxford: Clarendon Press, 1926.
With Douglas N. Harrison and J. Lawrence. “Observations of the Amount of Ozone in the Earth’s Atmosphere and Its Relation to Other Geophysical Conditions. Part III.” Proceedings of the Royal Society of London, Series A, 122 (1929): 456–486.
With Alan W. Brewer and B. M. Cwilong. “Bakerian Lecture: Meteorology of the Lower Stratosphere.” Proceedings of the Royal Society of London, Series A Mathematical and Physical Sciences 185 (1946): 144–175.
Exploring the Atmosphere. Oxford: Clarendon Press, 1963, 2nd ed. (much revised), 1968.
“Forty Years’ Research on Atmospheric Ozone at Oxford: A History.” Applied Optics 7 (1968): 387–405. Also available from http://ao.osa.org/abstract.cfm?id=15127&CFID= 34822189&CFTOKEN=38993172.
“The Laminated Structure of the Ozone in the Atmosphere.” Quarterly Journal of the Royal Meteorological Society 99 (1973): 599–607.
Brewer, Alan. “The Stratospheric Circulation: A Personal History.” SPARC Newsletter, 15 July 2000. Available from http://www.aero.jussieu.fr/ιsparc/News15/15_Norton.html
Brönnimann, Stefan, Johannes Staehelin, S. F. G. Farmer, et al. “Total Ozone Observations prior to the IGY. I: A History.” Quarterly Journal of the Royal Meteorological Society 129 (July 2003): 2797–2817. Also available from http://sinus.unibe.ch/broenn/QJRMS_1.pdf
Fort, Adrian. Prof: The Life of Frederick Lindemann. London: Jonathan Cape, 2003.
Houghton, John T., and C. Desmond Walshaw. “Gordon Miller Bourne Dobson.” Biographical Memoirs of Fellows of the Royal Society 23 (1977): 40–57. This contains a complete bibliography.
“One of the Very Last Private Laboratories.” The Oxford Times, 8 February 1957.
Walshaw, C. Desmond. “G. M. B. Dobson—The Man and His Work.” Planetary and Space Science 37 (1989): 1485–1507.