G.M.B.Dobson (25 February 1889 - 11 March 1976)

Dobson was an experimentalist of unusual ingenuity who devoted much of his life to the observation and study of atmospheric ozone. The results were to be of great importance in leading to an understanding of the structure and circulation of the stratosphere. He came to Oxford in 1920 to take up the position of University Lecturer in Meteorology, having previously been a Captain in the Royal Flying Corps and Director of the Experimental Department at the Royal Aircraft Establishment, Farnborough, during World War 1. Together Professor F.A. Lindemann (later Lord Cherwell), who was Head of the Clarendon Laboratory, and Dobson worked on studies of meteor trails, from which they deduced that the temperature profile above the tropopause was not constant - as simple theory would predict and the name 'stratosphere' implies - but rather that there was a region where temperature increased substantially with height.

Dobson inferred correctly that the cause of the warm stratosphere was heating by the absorption of ultraviolet solar radiation by ozone, and he set out to make measurements of the amounts and their variability. He decided to measure ozone by observing its absorption in the solar ultraviolet spectrum, as Fabry and Buisson had done a few years before. Dobson's first spectrograph employed a Fabry prism, an optical wedge consisting of gelatine and carbon black between quartz plates designed by T. Merton of the Clarendon Laboratory, and a filter consisting of a mixture of chlorine and bromine vapour to cut out unwanted solar radiation at longer wavelengths. A special tank was built to ensure consistent development of the photographic plates. To measure the line intensities Dobson built a photoelectric microphotometer using a potassium photocell, the current from which was measured by an electrometer also made in the Clarendon Laboratory.

Ozone measurements

The first spectrograph was built in the summer of 1924 at Dobson's laboratory and workshop in a hut built for the purpose in the grounds of his home, Robin Wood, Boars Hill, near Oxford. Extensive measurements made from Boars Hill during 1925 established the main features of the seasonal variation of ozone, the maximum in the spring and the minimum in the autumn, and also demonstrated the close correlation between ozone amount and the meteorological conditions in the upper troposphere and lower stratosphere.

The results of the 1925 ozone measurements were of such interest that Dobson decided to make measurements at a number of locations in Europe to study the relation between ozone distribution and synoptic meteorological variables. The winter of 1925-26 was spent building five spectrographs and calibrating them at Boars Hill. Measurements were begun in mid-1926 and by the end of 1927 ozone values had been calculated from over 5000 spectra. From these the distribution of ozone relative to pressure systems and a limited indication obtained of the variation of mean ozone with latitude was obtained.

More extensive measurements of the variation with latitude were made during 1928 and 1929 by redistributing the instruments to places widely scattered over the world. Only the instruments at Oxford and Arosa in Switzerland remained at their old stations; the others were sent to Table Mountain in California, Helwan, Egypt, Kodal Kanal, India and Christchurch, New Zealand. As before, the photographic plates were returned to Oxford for development and measurement. By the end of 1929, therefore, the main feature of the variation of the ozone amount with synoptic conditions, with latitude and with season had been established.

Meanwhile Dobson's lectures in the Clarendon Laboratory attracted large audiences of undergraduates. In 1923 he was awarded the University's Johnson Memorial Prize, and in 1925 took his Oxford D.Sc. In 1926 he gave the Halley lecture on 'The uppermost regions of the Earth's atmosphere' including 'a pictorial diagram of the heights of various phenomena'. He was elected to a new Readership in Meteorology in 1927 and also appointed University Demonstrator in Physics and Chemistry. In that year he was made an Official Fellow of Merton College and became a Fellow of the Royal Society, serving on the Council from 1932 to 1933.

The development of the photoelectric spectrophotometer

The inconvenience of the photographic technique led Dobson to design and build his photoelectric spectrophotometer which enabled the relative intensity at two wavelengths to be measured directly. It was completed in 1927 or 1928, the design being remarkably advanced for its day. It is probably the first spectrophotometer to use the principle of synchronous detection of a weak signal. It used a photocell with a sodium cathode made by T. C. Keeley, who had also made the photocells used in the microphotometer which Dobson built for measuring the photographic plates. This instrument, like the Fabry spectrographs, was built by Dobson himself in his own workshop.

Work on atmospheric pollution

In the early 1930s Dobson became concerned with the study of atmospheric pollution, and from 1934 to 1950 served as Chairman of the Atmospheric Pollution Committee of the Department of Scientific and Industrial Research Under his guidance reliable methods were developed for the measurement of smoke, deposited matter and sulphur dioxide, and a detailed survey was conducted in Leicester from 1937 to 1939.

The move to Shotover

The second international conference on atmospheric ozone was held at Oxford in 1936 (the first had been in 1929 in Paris). In the following year Dobson moved into his new house, 'Watch Hill' at Shotover on the outskirts of Oxford. His laboratory there was a substantial brick building with two workrooms, one large and one small, with provision to make zenith sky observations, and a well equipped workshop. A wooden hut was built near by and was used during the International Geophysical Year (195&57) as the office for collating and plotting the data sent in by the network of spectrophotometers.

Stratospheric water vapour

During World War II Dobson directed his attention to the humidity of the stratosphere. The work on water vapour began because the Meteorological Office had a problem with forecasting the conditions and heights when aircraft make condensation trails. He looked at the dew-point, or frost-point, method of measuring humidity and designed the frost-point hygrometer. A. W. Brewer was appointed to be the Meteorological Officer at Boscombe Down. He was responsible for flying these instruments and also did a lot of the design work. The first flights, involving working unpressurised in the stratosphere, showed that the region was unexpectedly dry. The meteorologists of the day took a long time to be convinced by the new data, having expected the air in the stratosphere to be saturated.

As a result of the study of contrails and the development of hygrometry Dobson became interested in the mechanism of freezing of water drops. Several of his students built cloud chambers in which water drops could be formed and their temperature of freezing noted. Their work confirmed the large effect on freezing temperature of different- amounts of impurity and showed that pure water drops do not freeze spontaneously until a temperature of -40 deg C is reached.

Post-war work on ozone

After World War II, international ozone work became organised under the International Ozone Commission which was set up in 1948 at the Oslo meeting of the International Union of Geodesy and Geophysics. The first Secretary was Sir Charles Normand who in 1947 had joined Dobson in Oxford after retiring as Director of the Indian Meteorological Service.

R. H. Kay, working with Dobson in the Clarendon Laboratory in 1952, adapted a chemical technique for making in situ measurements of ozone from an aircraft. Kay also played a large part, together with Normand and Dobson, in making various improvements to the spectrophotometer, including the fitting of photomultipliers which had become commercially available after the war and which greatly increased the sensitivity, the introduction of three more wavelength pairs for observations, and the development of improved methods of adjustment and calibration.

At the end of 1956, no less than 44 Dobson spectrometers were distributed throughout the world. The International Geophysical Year in 1956 brought a large increase in the number of ozone instruments required All new instruments came to Oxford for final calibration and comparison with No.1. The most interesting result which came out of the I.G.Y. measurements was the discovery of the annual variation of ozone at Halley Bay in Antarctica showing a sudden rise in November - very different behaviour from the northern hemisphere.

The Dobson Ozone Spectrometer is still in use world-wide with a network of over 100 active instruments. 'Number One' is now in the London Science Museum.

Growth of the department at Oxford

In 1945 the University passed a Decree conferring upon Dobson the title of Professor. Alan Brewer came to a new Lectureship in Meteorology at Oxford in 1948. Dobson had made forecasts for the Central Flying School, using 20 observations per day, but claimed they had been 'absolutely hopeless', and he wanted somebody with more experience than himself to teach forecasting in the University. Together they continued their classic study of atmospheric water vapour. The 'Brewer-Dobson Circulation' is generally accepted as the breakthrough in understanding why the stratosphere is so remarkably dry. Its details are still a major subject of study to this day, world-wide.

Dobson retired from his University Readership in 1950 and from his University Demonstratorship in 1956, but work on atmospheric ozone continued all through his retirement years. His last paper was written in 1973, 62 years after his first one, and his last observation of atmospheric ozone was made the day before he had the stroke from which he died six weeks later.

Further information

Larger photograph of G.M.B.Dobson believed to have been taken by Dr A. Dziewulska-Losiowa, a collaborator from Poland.

Dobson wrote a detailed account of his ozone work up to 1968, and published it in Applied Optics, 387-405, vol. 7 no 3, 1968. After his death a detailed biography was written by J.T. Houghton and C.D. Walshaw in 'Gordon Miller Bourne Dobson', Biographical Memoirs of the Royal Society, volume 23, 41-57 (1997). This included a list of publications reproduced here, and the papers that he submitted to Oxford University for his DSc degree in 1923/4 have been marked with PDF markers. A shorter biography, 'G.M.B.Dobson - the man and his work', was written by C.D.Walshaw and appears in Planetary and Space Science, 37, 1485-1507 (1989).

Dobson was awarded the prestigious Symons Gold Medal of the Royal Meteorological Society in 1938. The President of the Society made the award and gave an address outlining Dobson's achievements which shows how he was regarded at the time. A photographic portrait also appeared in the journal. He was later President himself (1947-1949).

Two articles have appeared in the Oxford Times newspaper:
'One of the Very Last Private Laboratories', 8 February 1957, upper half and lower half.
'Prof Ozone and his garden-shed', 19 May 1995, upper half and lower half.

The instruction manual for the production version of the photoelectric instrument is given here. Its date is unknown. It refers to a paper in the Proceedings of the Physical Society of London, Volume 43, Part 3, 1931, which can be seen on the Institute of Physics Web site. The IOP give a citation list, including several from very recent scientific work. There is one from 1943 concerned with scanning X-ray images which shows how Dobson's pioneering work on film scanning techniques that he needed for his photographic spectrometer benefitted other fields.