Bulletin - The journal of the World Meteorological Organization

He asked whether we should carry out the interview in Swedish or English; his English was indeed impeccable. He warned the Editor that he would probably get carried away at times, giving unnecessary explanations. It soon became evident that the grand old man was enjoying reminiscing over past events in the company of his good friend, and Dr Nyberg helped fill in dates and other details when Dr Ångström's memory failed him. He asked for an assurance that he would be able to see and approve the edited typescript. This subsequently proved to be rather a difficult exercise which required frequent visits and discussions with Dr Nyberg.

The interview which appears in the following pages is thus the result of considerable work by Dr Nyberg who probably played a greater role than the Editor. He merits full credit for this.

Dr Anders Ångström was born in Sweden in 1888. He was the third generation in his family to have contributed enormously to research in the field of atmospheric physics, and especially atmospheric radiation. The wavelength unit 10~8 cm was called the Ångström after his grandfather who studied the solar spectrum. His father designed the pyr-heliometer which is to this day the standard instrument for measurement of direct solar radiation. It was therefore natural that the young Ångström should follow the same line of interest. He started by working as assistant to his father who was a professor at the University of Uppsala in Sweden. He enjoyed the work, mainly because it involved meticulous observations, the handling of instruments, considerable outdoor work and the opportunity to participate in expeditions. After the death of his father, Ångström decided to see more of the world. He wrote to C. G. Abbot in the USA, inquiring whether he needed the help of a young and enthusiastic collaborator. In reply, Abbot asked him to join in an expedition to Algeria. This was 1912 when Ångström was 24 years old: During the months they worked together, the two men enjoyed excellent collaboration and friendship; Ångström was pleased to have an opportunity to test his father's radiation instruments and Abbot found in him the promise of a brilliant young scientist. When the expedition was over, Ångström went with the Abbot family to the USA and continued his studies with Professor Nichols at Cornell University. There Ångström met Dr E. Kennard and together they carried out an expedition to Mount Whitney in July 1913. Ångström's main interest was to measure the outgoing long-wave radiation with a pyrgeometer, also designed by his father. He established and calculated the effect of carbon dioxide on radiation in the atmosphere which had been neglected by previous research workers.

In 1916 Ångström obtained a doctorate from the University of Uppsala and for the next two years worked there as associate professor. In 1919 he joined the Swedish Meteorological and Hydrological Institute and immediately embarked on studying relationships between solar radiation and climatology. One of his methods for calculating global radiation from sunshine records is still used where direct radiation measurements are not frequent. In 1949 Ångström became Director of the Institute and held this office until he retired in 1955. However, he continued his scientific activities for ten more years, regularly visiting the Eppley Laboratory in the USA.

Ångström had his first contact with IMO in 1924 when he attended the International Radiation Commission. He served as chairman of the Radiation Commissions of IUGG from 1930 to 1951 and of IMO from 1936 to 1946.

In addition to his primary field of solar radiation and energy exchange at the surface of the Earth and in the lowest layers of the atmosphere, Ångström has written on climatology and the forecasting of frost and forest fires. In fact his scientific interest has encompassed many diverse practical fields. During the 35 years he worked for the Swedish Meteorological Institute he published no less than 151 scientific papers of a very high standard.

Ångström's contributions to the scientific world brought him many awards and much acclaim. In 1934 he became a member of the Swedish Forestry and Agricultural Academy, in 1946 he was made a member of the Swedish Military Sciences Academy and in 1948 he was elected member of the Swedish Royal Academy of Science. In the international arena he was awarded the IMO prize in 1962.

Ångström was very friendly, he had a good sense of humour and was a perfect example of a gentleman scientist. He retained his remarkable intellectual capacity throughout his long life.

As already mentioned, this interview took place at the Ångström's villa in Stockholm. The Editor of the WMO Bulletin wishes to thank Mrs Ångström for her excellent hospitality, and Dr Nyberg for his valuable assistance.

Bull. — Dr Ångström, you are the third generation in your family to be very well known. Your grandfather, Anders Jonas, was professor at the University of Uppsala and made outstanding contributions to atmospheric physics through his studies of the solar spectrum. The Ångström unit got its name from him. Your father, Knut Johan, in turn became professor of physics at the same university and perhaps the design of the compensation pyrheliometer was his most remarkable achievement in the field of solar radiation. How did growing up surrounded by such eminent scientists affect you?

A.Å. — As you say, both my father and my grandfather were outstanding physicists. For me, this had its bad and its good sides. On the one hand, I sensed that perhaps I was not fulfilling all the great expectations they had of me, but on the other I developed in an environment rich in experiences.

A.Å. — My father was a genius as a designer of instruments, and after obtaining my Bachelor's degree in 1909 I was able to assist him in the calibration of his radiation instruments, and thus I entered a research area which attracted me more and more.

A.Å. — After my father's death in March 1910, I embarked on a thesis which was published in 1912 under the title Uber die Reflexion von Flussigkeiten in ultrarotem Spektrum. This also dealt with radiation. However, I soon got the impression that, with my father's death, interest in radiation problems had diminished in the university, which I suppose was only natural. This and certain other factors induced me to try to supplement my education outside Sweden. Some time earlier my father had exchanged views (partly controversial) with Dr C. G. Abbot, chief of the Astrophysical Observatory of the Smithsonian Institution in Washington, D.C., as well as with Professor E. L. Nichols. I therefore wrote to both Abbot and Nichols. Giving as references my relatively modest contributions within their fields of research, I asked whether I could continue my studies at their respective institutions. Almost immediately I got replies from both of them.

A.Å. — Yes. Abbot asked me if I would be willing to take part as assistant in his planned expedition to Algeria. The purpose was to determine the so-called solar constant and its possible variations. Nichols offered me a fellowship in physics at his institution in the Rockefeller Hall at Cornell University. Both offers included salary and fellowship benefits which amply covered my expenses. I accepted both with gratitude. Abbot's offer seemed to give an excellent opportunity for me to use my father's radiation instruments, the pyrheliometer and the instrument for measuring outgoing nocturnal radiation to which I had already given the name pyrgeometer. Abbot's reply was meet me bassour Algeria may 20.

A.Å. — Abbot was about 40 years old and I was 24. He had once been assistant to Langley, whose method of determining the solar constant through radiation measurements and simultaneous analysis of its spectrum we were going to apply every clear day for four months. I could not avoid the impression that our first meeting was perhaps tinged with mutual disappointment. I had expected to meet an elegant elderly gentleman surrounded by a staff of assistants. Abbot had certainly not expected that the 'little European' would arrive dressed for tennis and carrying racket and balls. However, during the first two weeks there was a favourable development in our relationship. Abbot was that type of American who could do practically anything. He helped me build a small house and assisted Mrs Abbot with the housekeeping. With my help, he transported all the instruments to the observation post 60 m above the village.

A.Å. — The 'little European' demonstrated that he was an interested and entirely capable observer who carefully followed the important time schedule. My task was to read off the silver disk pyrheliometer which Abbot had designed. Abbot reserved for himself the more demanding work of handling the automatic spectrobolometer. We got up at 5 a.m., and between the four daily observation times (which started at 7 a.m.) we had plenty of time to discuss all sorts of problems.

Many years later it became clear to me that the Abbot instrument must have given values three or four per cent too high owing to the fact that the opening through which the radiation was collected was larger than the apparent disk of the sun, so that the instrument also picked up some diffuse sky radiation. When I was given access to new investigations by Darno and Linke on diffuse sky radiation and its dependence on the position of the sun and on atmospheric turbidity, I could deduce the order of magnitude of the error caused by this factor.

Bull. — Did you conclude that the solar constant was actually constant, or that any variation was too small to be determined?

A.Å. — The method of determining the solar constant used by Abbot, and by both Langley and Knut Ångström before him, required a series of measurements to be made of solar radiation at the surface of the Earth after absorption by the atmosphere. Simultaneous recordings had to be made of the radiation in practically all parts of the solar spectrum. Atmospheric absorption in the various spectral ranges was determined using the spectrobolometer, and in this way the solar constant could be calculated assuming absorption to be constant during the 3-4 hour measurement period. The solar constant was the sum of the constants which could be computed for various spectral ranges. Abbot considered that he could determine both the solar constant and its possible variations. Only many years later I came to the conclusion that the method required that both the solar constant and the scattering factor remain constant during the four hours used for measurement. As a matter of fact, we tried to get an idea of solar constant variations by using a method which assumed that it was constant during a six-hour series of observations. This was one of the reasons which made me sceptical of the rather small variations which had been found by several investigators. Also variations in the atmospheric turbidity were not completely eliminated.

Bull. — The measurements of outgoing terrestrial radiation were perhaps your main interest during your stay in Algeria?

A.Å. — Yes. I got excellent results using my father's instrument, the pyrgeometer, for measuring nocturnal outgoing radiation in the dry air of the high altitude station. I realized immediately that my measurements there would be an excellent way of studying the properties of the atmosphere and of identifying those gases which contribute to the fact that the outgoing radiation is only about 15 per cent of the value it could be if there were no atmosphere. This is, of course, due to the re-radiation of the atmosphere.

Bull. — In this connexion, you made a special study of the effect of water vapour and carbon dioxide?

A.Å. — Yes. I studied these factors and deduced a theoretical explanation for this strong re-radiation. From my measurements, I could extrapolate down to zero water vapour content, and I then found a considerable residual term. Emden had made a great contribution with his theoretical computations of re-radiation by the atmosphere, but he had neglected the effect of carbon dioxide. The size of my residual term corresponded well with the expected value.

A.Å. — I went over to America with the Abbot family, and then from the autumn of 1912, I studied with Professor Nichols at Cornell University at Ithaca in New York State. I started experimental work at the Rockefeller Hall with spectral measurements of infra-red reflexion from a group of substances in aqueous solution.

A.Å. — No. Not to the extent I felt necessary. During the Algerian expedition Abbot had himself become very interested in my observations and the influence of different atmospheric factors on outgoing radiation. He had previously made measurements of solar radiation on Mount Whitney in California, 4420 m above sea-level, and he offered to apply for support from the Smithsonian Institution for an expedition during the summer of 1913 to measure outgoing radiation and find its dependence on various factors such as altitude. I gratefully accepted this offer, and he went on to make arrangements for support by staff of the astronomical observatory at Pomona College.

Bull. — Did you carry out the work on Mount Whitney by yourself?

The work was favoured throughout by good weather, and when I went back to Sweden, I took with me some good basic material for my doctorate thesis which I eventually defended in 1916.

Bull. — Your thesis was much appreciated and the dissertation led to your appointment as Docent or assistant professor. What did you do back in Sweden?

A.Å. — After my return, I supplemented my earlier results in several ways. I measured atmospheric radiation during the solar eclipse of 20 August 1914, and measured radiation during the arctic night at Abisko in northern Sweden. When I was appointed Docent at the University, I also became assistant at the Meteorological Institute. I succeeded in interesting a young colleague in my work and we published a paper on the dependence of outgoing radiation on clouds, a subject to which I had already given some attention during my expedition in California.

A.Å. — No. It was at that time that the Meteorologiska Centralanstalten in Stockholm was reorganized and combined with the Hydrografiska Byran to form the Statens Meteorologiska och Hydrografiska Anstalt. This became effective on 1 January 1919 and was placed under the leadership of Axel Wallen. From that date I became a state-employed meteorologist. The position offered numerous advantages. I had a many-sided contact with competent administrators and an efficient support staff. As leader, Wallen was something of a humanist as well as being a scientific investigator. J. W. Sandstrom had a natural gift for research; although he had no academic diplomas he nevertheless made a career as assistant to Vilhelm Bjerknes.

Bull. — Could you continue your research work in the field of radiation in your new post?

A.Å. — Yes. There were many contributing factors which stimulated me in this work, not least the personal interest that Wallen showed in me. One of my first tasks was to organize solar and sky radiation observations in Stockholm using the recording pyranometer which I had designed. This pyranometer model was later widely used, being marketed by the Eppley Laboratory. I used the results of my pyranometer measurements to derive the relationship between sunshine duration (recorded with simple autographic instruments) and total incoming radiation (from sun and sky). This important climatological factor could then be estimated for many places in the country from the relatively simple sunshine recorders.

Bull. — Although it was in the field of radiation that you made your most important contribution to meteorology, you also looked into several other problems?

A.Å. — Through my close contact with my colleagues, and especially with my chief, Axel Wallen, I was involved in work connected with frost, soil temperature, forest-fire warnings, evaporation from lakes, the use of pilot balloons and the efficiency of weather forecasts. I also introduced a new definition of atmospheric turbidity.

Bull. — You were also involved at the start of systematic observations of the chemical composition of pollutants in precipitation, which later led to the establishment of the international network of background air-pollution monitoring stations organized by WMO.

A.Å. — In this work I co-operated with Professor H. Egner in Uppsala, and we could use the regular meteorological observers of the Institute. With a colleague I published a study on the nitrogen content of precipitation. Subsequently, interest has focused on the sulphur content.

A.Å. — For several years following the First World War, international scientific cooperation continued to suffer from the effects of the war. The International Meteorological Organization had previously been an effective and necessary forum for international co-operation, and it was some satisfaction that after the war the IMO had been re-established by congresses in Paris and London. However, it was most regrettable that representatives of the Central Powers were excluded. The Central Powers at that time comprised Austria, Bulgaria, Germany, Hungary, Italy, Poland, Turkey and Yugoslavia. This vitiated any universal procedures and also prevented the meteorological community at large from benefiting from the rich experiences which the Central Powers had acquired, for example in aerology. The chief of the upper-air observatory at Lindenberg (near Berlin), G. Hergesell, was a highly esteemed scientist and an effective organizer. He seized the reins and convened a meeting at Lindenberg in July 1921 with meteorologists from Germany, Austria and neutral countries. I was the Swedish delegate. A proposal was made to form a union to fill up IMO gaps. The questions which were dealt with first concerned weather services, especially the exchange of observations by wireless and the measurement of radiation. Thanks to this meeting, I made useful contacts with many other meteorologists working in my own field, such as R. Suring, chief of the observatory at Potsdam, the Austrians Schmidt and Exner, and, above all, Hergesell himself who later gave me valuable help by organizing two nocturnal balloon ascents for measuring outgoing radiation. In 1923, I accompanied Wallen to the IMO conference in Utrecht (where there were no representatives of the Central Powers). I thus made personal contact with outstanding personalities in meteorological research and organisations in both groups of countries. I was elected secretary of the reconstituted radiation commission, and later became its president.

Bull. — What was the status of meteorology in Sweden when you entered the Institute?

A.Å. — An interesting event in the history of Swedish meteorology was the establishment in 1756 of a meteorological station at the Stockholm Observatory, where observations have been recorded continuously ever since. In 1858 a network of about 20 stations all over the country was established by the Academy of Science, and in 1873 the independent Meteorological Institute was created.

Bull. — I see that you are a member of the Swedish Agricultural Academy and also the Academy of Science and the Military Academy. Could you say something about your involvement in these academies?

A.Å. — Several of my papers are related to climatology, and many touch on interesting applications, such as frost forecasts and forest-fire warnings. These were of interest to the Academy of Agriculture and Forestry. The army was also very interested in forest fires since it was involved in fighting them. A good forecast could be extremely useful. It was in conjunction with this work that my name was proposed as a member of the Swedish Military Academy. In the Academy of Science a new class for geophysics was established in 1948, and I was one of the first elected members.

Bull. — The period when you were Director of the SMHI coincided with a time of great progress in the science of meteorology. The computer was invented and the use of numerical techniques in weather forecasting was introduced. What sort of repercussions did these have on the activities of the Institute? A.Å. — As you are well aware, Professor C.-G. Rossby was one of the originators of numerical weather prediction techniques. The SMHI offered him a special post as adviser. In spite of certain routine formalities which caused some delay, we caught up very quickly and numerical weather prediction was soon introduced at the Institute. Our experience increased and in a few years we were able to organize training seminars for meteorologists from several other European countries. Bull. — I understand that you continued your scientific activities even after your retirement. What was your major contribution during that period?

		Dr Ångström with Mr W. R. Dyer, then chief of the WMO Technical Assistance Unit, during a visit to Iceland in August 1956 to survey requirements for instrumental and climatological work

A.Å. — From 1956 for more than ten years I made annual visits to the Eppley Laboratory in Newport, USA. My work had two main objectives, first the development of new instruments and the improvement of existing ones (such as the electrical compensation pyrheliometer), and second the introduction of empirical formulas, based on theoretical considerations, which could be applied to regular meteorological measurements. Some of my work related to the standardization of the characteristics of coloured glass filters, the derivation of the relationship between illumination and solar energy, and the dependence of the circumsolar sky radiation upon atmospheric turbidity parameters. Subsequent studies were mainly concerned with the determination of the solar constant and the extraterrestrial spectral flux component and the associated scattering properties of the intervening atmosphere as revealed by observations from high-flying aircraft.

Bull. — In your opinion, what should be the present and future emphasis in meteorological activities? In other words, what should be the objective of the science of meteorology?

A.Å. — That is a difficult question to answer. I think the answer would differ from one meteorologist to the next, depending on his background and interests. I would probably be biased myself towards the kind of scientific activities I have been carrying out all my life. However, there are certain fields in which surely all meteorologists would like to see increased emphasis. The world is faced with a continuously increasing population. The shortage of food is threatening the future of mankind. Our natural environment is being destroyed by human activities. Any action to alleviate these problems is more than welcome. Therefore, I feel it is natural that more emphasis should be put on application of meteorology to the economic development of the nations. A major problem these days is the shortage of fossil fuels, and as alternatives the use of solar, wind and thermal energy comes immediately to mind, and these are all strongly weather dependent.