The Life and Science ol Tor Bergeron

Editor's note: When a prominent scientist in our profession dies, his working notes are often inaccessible to the general scientific community and the public. In the case of Tor Bergeron, however, we are fortunate to have some of his personal notes on the ice nucleus theory. The Notes were submitted by Duncan C. Blanchard of the State Uni- versity of New York at Albany, who has also prepared an introduction describing how the Notes came about. We are also pleased that one of Bergeron's colleagues, Arnt Eliassen of the University of Oslo, has prepared a necrology of Bergeron, which appears first in the series that follows. In August, a Bergeron Memorial Lecture will be given by Helmut K. Weickmann of ERL/NOAA at the AMS Conference on Physics and Atmospheric Electricity being held in Issaquah, Wash.; the lecture will appear in the Preprint Volume.

Tor Bergeron, 1891-1977

With the death of Tor Bergeron on 13 June 1977 the science of has lost one of its most creative and original minds. Bergeron's contributions cover a wide range of sub- jects, and, to a greater extent than is realized today, his ideas are behind much of what is now considered basic meterological knowledge. He probably did more than anybody to bring order and a system to the seemingly chaotic realm of weather phenomena. Bergeron was born in 1891 in England, but he went to school in , and received his B.Sc. in Stockholm in 1916. From 1919 to 1922 he was employed as a Me- teorologist at the Swedish Weather Service. However, much of the year 1919 he spent in Bergen, assisting Vil- helm and Jack Bjerknes with their newly organized fore- casting service. Tor Bergeron with the Pluvius gage. The year before, J. Bjerknes had presented his famous cyclone model. So far, however, this model was static, mann; there he worked together with G. Swoboda and and it came to life only after two additional discoveries G. Schinze. Together with Swoboda, he published in had been made, on the basis of careful synoptic analyses: 1924 a paper that demonstrates the application of the Solberg showed that cyclones may form as small, amplify- Bergen methods in the analysis of a particular weather ing waves on a preexisting polar front, and Bergeron situation over Europe. discovered the complicated occlusion phenomenon, lead- In 1928 came the first part of Bergeron's impressive ing to the final stage in the life cycle of cyclones. Thus work, "Uber die dreidimensional verkniipfende Wetter- the cyclone model of the Bergen School was extended analyse" (in Geofysiske Publikasjoner)1 for which he from an instantaneous picture into a dynamic process. was given the Ph.D. from the University of Oslo. The Today the occlusion process is a habitual part of our paper is a tremendous effort to classify and systematize basic meteorological knowledge, and is displayed on the physical processes leading to the changing weather. every weather map. To appreciate Bergeron's achieve- It gives ideas and conclusions over a wide spectrum of ment, however, one must consider the fact that, at the subjects, although it has two main themes: air mass time, the idea was completely unknown, although me- analysis and frontogenesis. In addition, the paper con- teorologists had been looking at weather maps for more tains an early version of Bergeron's ice crystal precipita- than 50 years. tion theory. In 1922 Bergeron returned to Bergen as a Meteorologist The concept of air masses goes back to Dove in the at the Bergen Weather Service, a position he held until middle of the 19th century and was used by many other 1929. Part of this time (1923-25) he spent at the Geo- physical Institute in Leipzig, headed by Prof. L. Weick- i Now Geophysica Nowegica.

Bulletin American Meteorological Society 387

Unauthenticated | Downloaded 09/28/21 05:41 PM UTC 388 Vol. 59, No. 4, April 1978 authors before the Bergen School. It was inherent in the At the International Union of Geodesy and Geophysics polar front concept; J. Bjerknes and Solberg, in their assembly in Lisbon in 1933, Bergeron presented a paper: papers from 1921 and 1922, state that the warm air "On the Physics of Cloud and ," which is south of the front originates from the subtropical high probably his most important contribution. Here he dis- pressure belt and the cold air north of the front, from cussed several conceivable mechanisms for precipitation polar latitudes. However, it was Bergeron who, in his release in and advanced his ice crystal theory. 1928 paper, first took up a systematic study of air masses, Alfred Wegener had pointed out already in 1911 that their source regions, and transformations. He arrived at in a mixed cloud at temperatures below freezing, con- a rather elaborate classification based on the four main taining both undercooled droplets and ice crystals, the types: equatorial, tropical, polar, and arctic air. In par- droplets would evaporate, their mass being transferred ticular, he was looking for quasi-conservative air mass by diffusion toward the ice crystals, which would grow properties, and as one of these (besides potential tem- correspondingly. But this process does not lead per se to perature) he introduced a measure of aerosol content release of precipitation. The important point, which based on visibility. apparently Bergeron was the first to recognize, is the In his discussion of air mass transformations, he intro- condition that the number density of ice particles must duced the important notion of air masses that are cold be much smaller than that of the droplets (but not too or warm, respectively, relative to their underlying surface small). Only then will the ice particles grow to a size and discussed their characteristic temperature structure, sufficient to release precipitation. cloud and precipitation forms, visibility, and turbulence Bergeron's paper was the beginning of a rapid de- activity. This notion was not only a valuable forecasting velopment of . It forms the basis for the tool but could also be used diagnostically, leading to methods of artificial precipitation, as well as cloud or what Bergeron termed indirect aerology: from surface fog dissipation, by means of cloud seeding. Although observations of cloud, precipitation forms, and other we know that precipitation can be released also without weather characteristics, it is possible to draw conclusions the presence of ice particles, Bergeron's ice crystal theory concerning the vertical thermal structure. still represents one of the cornerstones of cloud physics. The other important contribution contained in Ber- From 1929 to 1935 Bergeron worked as a Meteorologist geron's 1928 paper was the concept that fronts form in and consultant at the Norwegian Meteorological Insti- the vicinity of hyperbolic points in the large-scale hori- tute in Oslo. In 1936 he returned to Sweden as Senior zontal motion field, where, as a result of advection, Meteorologist and later became Scientific Chief of isotherms become crowded and oriented along the di- Weather Service at the Swedish Meteorological and Hy- latation axis. This idea enabled Bergeron to determine drological Institute, a position he held until 1947. During the location of the principal frontogenetic zones be- these years he did much to improve weather analysis tween the large-scale semipermanent low and high pres- and forecasting in the weather services of Norway and sure systems and thus give an explanation of the exis- Sweden. He was also a very active member of the Com- tence of the main frontal systems on the globe. He mission of Synoptic Meteorology of the World Mete- developed this idea even more convincingly in his paper orological Organization and its predecessor, and had a "Richtlinien einer dynamischen Klimatologie" (in Me- great influence on the internationally adopted terminol- teorologische Zeitschrift, 1930). ogy and classification of clouds and hydrometeors. Of the Bergen School team, Bergeron is distinguished In 1947 Bergeron became Professor of Synoptic Mete- by his intimate knowledge of earlier meteorological ideas. orology at the University of Uppsala. He now returned His papers are filled with references and comments to to his work on cloud physics. In a series of papers, he German, British, and particularly Austrian authors and discussed the precipitation release mechanisms in various thus form a good starting point for historical studies of cloud forms: cumulonimbus, warm front clouds, and pre-Bergen meteorology in Europe. orographic clouds. He introduced the useful notion of During the winter of 1928-29 Bergeron worked at the an upper "seeder" cloud, which provides the necessary Meteorological Office in Malta, acting as a missionary for ice crystals, and a lower "feeder" cloud, which supplies the methods of the Bergen School. In 1930 and 1932 he most of the water mass to the falling particles. He dis- lectured in Moscow on the polar front meteorology and cussed the prospects of cloud seeding and came to the air mass analysis. Part two of his 1928 paper, dealing with important conclusion that the possibilities of attaining the physics of atmospheric fronts and their perturbations, a significant increase of precipitation in this way are was published in Moscow in 1934. In the preparation of particularly good in orographic clouds lacking an effi- cient release mechanism. his paper, he was assisted by his Russian student Vera Romanovskaja, who became his wife. Bergeron's lectures In 1953 Bergeron started a research project on meso- in Moscow had considerable impact on the progress of scale rainfall distribution, which he named Project meteorology in the Soviet Union. Thu9 the well-known Pluvius. Realizing that existing rainfall-measuring net- textbook by Khromov, which was translated into German works are much too coarse to reveal the detailed distri- by Swoboda (Einfuhriing in die synoptische Wetter- bution, he organized experimental rainfall networks of analyse, 1940, Springer), was to a large extent based on very high resolution in several regions in Sweden. Thus, Bergeron's lectures. in the Uppsala region, an area of 400 km2 was covered

Unauthenticated | Downloaded 09/28/21 05:41 PM UTC Bulletin American Meteorological Society 389 by 150 (and sometimes as many as 350) rain gages—a cieties. From the University of Uppsala he received an quite unique network. The project revealed an astonish- Honorary Doctorate. ingly large orographic effect upon rainfall of modest Tor Bergeron was a wonderful and inspiring person, wood-covered hills not higher than 40-70 m. Bergeron with a humorous outlook. He had many interests be- explained the effect as an influence of low-level "feeder" sides his science. Languages were one, and he could ex- clouds. He continued his precipitation studies after re- press himself in English, French, German, and Scandi- tirement in 1961, and his numerous papers on the sub- navian. In addition, he spoke Spanish, Italian, and Rus- ject, of which the last will come out posthumously, sian quite well and also some Dutch, Czech, and Serbo- contain a wealth of interesting results. Croatian. He was particularly fond of music and was Naturally, Tor Bergeron received many distinctions; nearly all his life an active member of academic choirs. he was an Honorary Member of the Americal Mete- In his home were frequent performances of chamber orological Society and of the Royal Meteorological So- music with his wife at the piano. ciety, London, and from the latter he was awarded the It was always a fascinating experience to hear Symon's Gold Medal. He was a Member of the Royal Bergeron lecture, devoted to his subject, in his very Swedish Academy of Sciences, the Norwegian Academy personal style. His impact on meteorology will last for of Science and Letters, and many other scholarly so- a long time.—Arnt Eliassen

Tor Bergeron and His "Autobiographic Notes"

With the death of Tor Bergeron in Uppsala on 13 June around the turn of the century, anticipated many of the 1977, we have lost yet another of the great pioneers in discoveries of modern cloud physics. Three months later, modern meteorology. In 1919 he was a member of the Bergeron wrote back. In a fascinating letter he expressed Bergen School, that tiny but immensely creative group his feelings about discovery, including the role played of young meteorologists who, under the leadership of by subjectivity. In one of his German papers he said he Vilhelm and , introduced the new con- wrote "Vor lauter Objektivitat kein Fortschritt." 1 cepts of frontal theory to the world of meteorology. But Included with his letter was a long account of the ob- Bergeron's major contributions were not limited to servations and ideas that led up to that day in Lisbon synoptic-dynamic meteorology. He is equally well known in 1933 when he presented the paper that outlined his to cloud physicists for his discovery of the importance of "ice nucleus theory." He had indeed, while walking ice crystals in the initiation of precipitation in super- through the woods many years before, made a chance cooled clouds. This landmark discovery was presented in observation that enabled him to make that brilliant con- a classic paper at a meeting of the International Union ceptual leap to the role of ice crystals in the formation of Geodesy and Geophysics in Lisbon in 1933, and pub- of precipitation. I urged Bergeron to publish this ac- lished two years later in the proceedings of the meeting count. A short time later he decided to submit a paper (Bergeron, 1935). to the Henri Dessens Memorial Issue of the Journal de I first read this paper many years ago. In 1971, in a Recherches Atmospheriques. Published in French (Berge- seminar I was teaching on the history of cloud physics, I ron, 1972), it was a somewhat modified version of the introduced it to the students. We discussed its significance account he had sent me. I continued to urge him to and the new ideas it injected into the mainstream of publish it in English, pointing out that most English- cloud physics of the 1930s. I had been telling the stu- speaking students and scientists of the present genera- dents that although a great many ideas and discoveries tion have little or no command of any other language. follow chance observations, we could not conclude that He said he would expand the account, giving more anyone who made these observations would necessarily details of the early observations, and submit it to the make the discovery. A receptive mind is required, one BULLETIN. In 1974 he wrote and said that due to "... a that will look at the observations in an entirely new number of adversities . . ." the work was going slowly. way. Nobel Laureate Albert Szent-Gyorgyi put it very I never heard from him again. well when he said that discoveries are made by those Tor Bergeron's "Autobiographic Notes" are a delight who . . see what everybody else has seen, and think to read. He tells the story of meteorological research in what nobody else has thought." a bygone era and of the men who laid the foundations I was curious about the story behind Bergeron's dis- of modern meteorology. In these Notes we meet among covery. Did he make some chance observations and, if so, others V. and J. Bjerknes, Rossby, Wegener, Weickmann, what caused him to ". . . think what nobody else had Koppen, and Langmuir. The comments about Bentley thought"? In March 1971 I wrote to him and asked these obviously were a result of the paper I had sent to questions. I told him of my interest in the history of Bergeron. I have kept the editing to a minimum, making ideas in cloud physics and sent him a reprint of the first minor changes in only a few sentences for the sake of of my papers (Blanchard, 1970, 1972, 1973) on the work clarity. I have not changed his style of writing. What of Wilson Bentley, a self-educated Vermont farmer who, you are about to read is vintage Bergeron, with his well- known emphasis of the human side of the story, and the 0003-0007 / 78 / 0389-0390$05.00 © 1978 American Meteorological Society i From pure objectivity, no progress.

Unauthenticated | Downloaded 09/28/21 05:41 PM UTC 390 Vol. 59, No. 4, April 1978 copious use of italics. Bergeron had no references in the pioneer in cloud physics that so impressed Bergeron. Notes. I added a few where I thought they were neces- Here then is the story of a great discovery made acci- sary. I hope readers will forgive me for adding my Bent- dentally by a young man whose puzzlement over some ley papers to the reference list. They may want to read commonplace observations led him to . . think what for themselves what it was about America's little-known nobody else has thought."—Duncan C. Blanchard.

Some Autobiographic Notes in Connection with the Ice Nucleus Theory of Precipitation Release

The wise Solon said "There is nothing new under the Bentley's ideas about the origin of rain. A pity that Sun." Thus, I freely admit that in formulating the above Hann in 1904 (or I in 1917/18) did not really get theory I drew quite a lot on Alfred Wegener's ideas acquainted with Bentley's farsighted ideas! That might about the "intensive condensation" in cumulonimbus have speeded up the development of cloud physics. One clouds. (See, for example, pages 81 and 289 in his book might here see an example of one of my pet notions Thermodynamik der Atmosphare (Wegener, 1911).) I regarding the history of ideas and the development of had read it in 1917-18 before my first visit to Bergen in science (see Bergeron, 1959, p. 455, first paragraph of 1919. Other forerunners in this field (Guilbert, Dauzere, right column). Too little but also too much knowledge etc.) were unknown to me until the discussion after my may prevent making important discoveries. Hann knew lecture in Lisbon in 1933. too much meteorology, whereas Bentley (like Durand- As a schoolboy in 1908, living at a public school in Greville in 1890) probably did not know enough physics the countryside, I started to keep a detailed weather log. or dynamics. This task was pursued with even greater zest from 1909 To return to my own problem. What I had read in on, when for the first time I spent the summer in the Wegener's book evidently lay latent in my mind, when Swedish mountains at Ann (63°20'N, 12°30'E; 560 m in February 1922, just before emigrating to Norway for altitude). We have had our summer house there for the a 13 years' stay there, I spent a couple of weeks for past 27 years, and I still keep a weather log when we are recreation at a health resort at an altitude of 430 m there! (1400 ft) on a hill near Oslo. On that hill we were often From the very beginning I devoted my main interest in a supercooled stratus layer, and when walking on a to the clouds, hydrometeors, and visibility. This hobby narrow road in the fir forest, parallel to the contours of was furthered by the many oreigenic 1 clouds and also the hillside, I noticed that the "fog" did not enter the cloud systems (Wehrle's and Schereschewsky's work only "road tunnel" at, say, —5° to —10°C, but did enter it appeared in 1919, but I saw such "systems" too without when the temperature was >0°C. The profile of the using that name) and the very wide vistas from that fine road, trees, and fog for the two temperature regimes is lookout point (see, for example, some of my cloud shown in Fig. 1. photos in the International Cloud Atlas of 1956). Thanks My tentative explanation came immediately. At to the "stage wings" offered by the different mountain — 10°C the rime-clad branches of the firs along the ridges, I could spot the movement of distant and windward side of the road by diffusion transport "filtered showers and keep track of different precipitating cloud away" so much of the water vapor in the air (because at layers and their height. — 10°C the saturation vapor pressure over ice is less than Thus, it became natural to ponder over the fact that that over water) that the fog droplets partly evaporated some thick clouds did not rain, whereas thinner ones (some droplets were, of course, also captured directly by did\ This was also J. von Hann's question already in the "netting" of fir needles). At temperatures >0°C the 1900 (quoted by Bergeron, 1935, pp. 159-160). fog was seen everywhere between the trees and filled the In my copy of J. von Hann's Lehrbuch (3rd edition, whole "road-tunnel." So striking was the difference that 1915) you will, in fact, find Bentley's paper from 1904, it could not only be due to the mechanical capture of "Studies of Raindrops and Raindrop Phenomena" the droplets. Why should, indeed, the latter effect ap- quoted on pp. 306-307 (i.e., 2-3 pages after the above parently fail utterly with temperatures above freezing? quotation from Hann). Hann, however, only reproduces When I came to Bergen in May 1922 I got immersed a table on raindrop sizes from Bentley, but nothing of in my new duties there, and from July 1922 to July i Bergeron did not give the meaning of this word in these 1923 I was an acting chief of the Weather Forecasting Notes but did in the French version (Bergeron, 1972). He Center, while J. Bjerknes spent a year in Zurich writing said that Wehrle and Schereschewsky coined the term in his doctor's thesis. Soon after his return to Bergen I took 1920. He went on to say that in the meteorological sense it a year and a half leave for studies with Ludwig Weick- means produced because of mountains. The old word mann, Sr., in Leipzig. His son Helmut was then 8 years orographic, which describes mountains, is purely a geo- graphical term. old, and we have remained great friends ever since.

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spect), but it never came off. Through our British col- league, Capt. C. K. M. Douglas, I knew, however, that altostratus clouds always contain crystals, and that was enough for me for the time being. G. Swoboda and I had become great friends while writing the book Wellen und Wirbel . . . together in 1924 (Bergeron and Swoboda, 1924). On returning to Bergen in 1925 I found that J. Bjerknes was then not at all interested in aerology. After finding that the Nor- wegian salary of a meteorologist was extremely low, Swoboda and I got in contact with P. Scherescliewsky (the friend and collaborator of P. Wehrle in Paris). Scherescliewsky, who really was an engineer, had be- come "inspecteur des mines" in Saarbriicken, the capital of the disputed Saar-region. He planned to organize a Weather Service in Saarbriicken, and we were willing to go there together, provided he could give us a rea- sonable salary and a daily weather flight also for scien- tific purposes, not least for sounding the precipitating clouds. The thing was almost settled when Scherescliew- sky at the end of 1926 suddenly married, moved to the FIG. 1. Distribution of the stratus observed in February United States, and seemingly forgot all about it! (To my 1922 at an altitude of 470 m in the forest on a hill near Oslo. great amazement Scherescliewsky, after 44 years silence, The distribution is shown for two different temperatures. turned up in my home last year. We had a very nice The shaded area represents the stratus layer. This figure (from Bergeron, 1972) is used here with the permission of evening, but I never spoke about Saarbriicken.) This Journal de Recherches Atmospheriques. incident was only mentioned to show that since 1922 I was always on the lookout for the precipitation release mechanism. Only a year and a half later, J. Bjerknes got Even after returning to Bergen in 1925 the "front and very keen on aerology, thanks to Jaumotte and Palmen air mass synoptics" and routine work in the Weather (but at Bergen there still was none). Service took all my time, and there was no chance to write about ice crystals. I hadn't forgotten them, though, Since A. Wegener plays an important role in this story, and when observing the clouds, I collected more and let me go back in time even a few years more. In Sep- more evidence as to their role in cumuliform and strati- tember 1923 I was, as aforesaid, on my way to study form clouds. Thereby I gradually became convinced with L. Weickmann in Leipzig. This was the disastrous that the transition from cumulus to cumulonimbus, or fall when the German Mark "fell" to a tenth of its rather the start of fallstreaks from them, always came value every two weeks, and at last a small scrap of paper soon after the glaciation of the tops, and that the latter bore the imprint "Eine Billion Mark" (1012). Thanks to was a necessary condition for the former—what I believe Vilhelm Bjerknes I was invited to stay one or two days 2 the philosophers call to take "post" for "propter." underways in Alfred Wegener's home, in Hamburg. A 1 then also generalized from the icing of cumulus tops charming family! The old W. Koppen, who was the to the icing and precipitation release in stratiform father of Wegener's wife, Else, lived upstairs, so I had clouds. It was a significant observation that even low the privilege of meeting him, too, and also the brother, and thin cloud layers often give some precipitation in Kurt Wegener. There I also met Wegener's later col- winter but not (in daytime) in summer when the freez- laborator in Greenland, Dr. Georgi, a very fine man. He ing level lies too high. still lives in a kind of farmhouse outside Hamburg, and During my year and a half in Germany I spent a few we still correspond! weeks in May of 1924 with H. C. Cannegieter at the Alfred Wegener states in the foreword to his Thermo- Meteorological Institute in Holland. I was then specially dynamik der Atmosphdre (which in reality had only one keen on finding out what was really going on in the chapter on that subject, but to my knowledge is the first altostratus-nimbostratus cloud system, and not least attempt at a real cloud physics) that he planned a second about its precipitation release. To that end, Cannegieter, volume on these subjects, but it was never realized. First who in 1911 had organized the first regular meteorolog- came World War I, and then he became engrossed in his ical airplane flights in Europe (or in the world?), let exploring work in Greenland, and at last his untimely me ascend into such a cloud deck with the then famous death in 1930. That causes one to think. . . ! pilot Bakkenes at Scesterberg. We had bad luck, though, In 1927, when settled again at Bergen, I put the ideas and did not reach high enough. Cannegieter also planned about the role of ice crystals in clouds before my chief, a balloon flight with me (he was a veteran in that re- Dr. Hesselberg, when he visited Bergen. I recall as if 2 Bergeron is referring to the expression used in logic, it were yesterday how he said very kindly, "Quite inter- "post hoc, ergo propter hoc." esting," but nothing more. Nevertheless, when starting

Unauthenticated | Downloaded 09/28/21 05:41 PM UTC 392 Vol. 59, No. 4, April 1978 later in 1927 to write my doctor's thesis, these ideas came release in stratus clouds. In the United States, H. Byers to form a short section (No. 9) of "Wetteranalyse, 1" was quite positive, stating that inland outside the (Bergeron, 1928). It was written in German (and thus tropics, where big nuclei are rare and small ones very hardly read in the United States or England) and printed numerous, the ice nuclei were indispensable. Lastly, as in September 1928. That year, therefore, is the real year you know, Findeisen (1938) in Germany completed the of birth of that theory, and not 1933 or 1935 as generally theory by useful quantitative measurements and theoret- quoted. W. Bentley could, in fact, have lived to see how ical calculations. his ideas, unknowingly, were followed up by another At this point, one might also ask why I did not draw meteorologist. When later J. Bjerknes, , the obvious conclusion that here was a method by which and I, thanks to the understanding and generosity of one coidd modify supercooled clouds and produce rain our "boss" Hesselberg, were chosen to represent Norway artificially and then embark on that task? I did, in fact, at the UGGI meeting at Lisbon in 1933, I at last sat draw that conclusion, but it was not followed by action. down trying to present the ice nucleus theory in a more The main reason was that I did not know of any complete, detailed, and convincing shape, with the efficient method of producing or introducing ice nuclei known result (Bergeron, 1935). on a large scale in the clouds. The only thing I could 1 hope to be excused for having underestimated the think of was to seed with ordinary snow or pulverized role and importance of rain release in warm clouds, since ice from airplanes, and I realized that even if every I then hardly had seen any weather or climate south small ice or snow grain grew to a big snowflake, the total of 50°N (except the winter of 1928/29 on Malta). One snowfall or rain amount would be insignificant. Again, may also make allowance for a young person's enthu- this is an example of the rule that there is always an siasm and zest to advocate a new idea that he regards optimum of knowledge that may open the gateway to new as sound. knowledge. I knew not enough physics and hadn't read Volmer's (1939) Phasenlehre. Irving Langmuir, Vincent Schaefer, and/or Bernard Vonnegut evidently had, and it was really a treat when, thanks to Carl Rossby, my Naturally, the ice nucleus theory was judged and re- wife and I visited Langmuir in Schenectady on the 20tli ceived very differently, depending on the disposition and of April 1947, and he devoted half a day to us. I wrote personal experiences of my colleagues, the climate they a report on that visit that was sent to Rossby in Chicago. lived in, etc. Sir Gilbert Walker (quoted in the pub- It was filed there as "classified"(!). (I might have a copy lished discussion of my Lisbon paper), as a "tropical of it somewhere.) man," was definitely against it, and so were others, at least to begin with. W. Peppier, on the other hand, had some good aerologic evidence in favor of it. When visit- Anyhow, as often happens in science, the ice nucleus ing England in 1949, for the first time since 1923 (I was theory, as we know, proved to be better and more fruit- boru there, but left England at the age of four), we met ful than the partly doubtful or incorrect observations, two nice young assistants with Sir David Brunt; their and the somewhat daring conclusions on which it was names were Ludlam and Mason. Frank Ludlam was founded. It was maybe a case of "the good guess," or call rather frank and told me that icing of cumulonimbus it intuition! But in order for an idea to be accepted, one tops often was rather a by-product of the release process. must evidently believe in it and fight for it. Young B. J. Mason was definitely more modest and be- Uppsala in July 1971 lieved that ice nuclei were decisive for the precipitation (Signed) Tor Bergeron

References 3 comme declencheurs de precipitation un cinquantenaire. J. Rech. Atmos., 6, 49-53. Bentley, W. A., 1904: Studies of raindrops and raindrop , and G. Swoboda, 1924: Wellen and Wirbel an einer phenomena. Mon. Wea. Rev., 32, 450-456. quasistationaren Grenzflciche ilber Europa. Veroff. Geoph. Bergeron, T., 1928: Uber die dreidimensional verkniipfende Inst. Leipzig 2, Series 3, No. 2. Wetteranalyse, 1. Geophys. Norv., 5 (6). Blanchard, D. C., 1970: Wilson Bentley, The Snowflake Man. , 1935: On the physics of cloud and precipitation. Pro- Weatherwise, 23, 260-269. ceedings of the 5th General Assembly of the UGGI, Lisbon, , 1972: Bentley and Lenard: Pioneers in cloud physics. vol. 2, pp. 156-175. Amer. Sci., 60, 746-749. , 1959: Methods in scientific weather analysis and fore- , 1973: Wilson Bentley, Pioneer in snowflake photo- casting: An outline in the history of ideas and hints at a micrography. Photogr. Appl. Sci. Tech. Med., 8, 26-28, program. The Atmosphere and Sea in Motion, Rockefeller 39-41. Institute Press, New York, pp. 440-474. Findeisen, W., 1938: Die kolloidmeteorologischen Vorgange , 1972: L'origine de la theorie des noyaux de glace bei der Niederschagsbildung. Meteor. Z., 55, 121-133. Volmer, M., 1939: Kinetik der phasenbildung. Steinkopff, Dresden. 3 These references include those mentioned in the intro- Wegener, A., 1911: Thermodynamik der Atmosphare. Barth, ductory note. Leipzig. •

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