BERGERON, TOR HAROLD PER- Stimulus for This Was the Observation on the Strainmeter, CIVAL (B

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particular area showed that they all occurred on a com- OTHER SOURCES mon structure, which he took to be a large inclined fault. Goodstein, Judith R. “Waves in the Earth: Seismology Comes to In 1952 he extended this concept to zones of deep earth- Southern California.” Historical Studies in the Physical and quakes all around the Pacific Ocean, making many geolo- Biological Sciences 14 (1984): 201–230. gists aware of these large and deep structures; when plate Press, Frank. “Victor Hugo Benioff.” Biographical Memoirs, vol. tectonics explained these regions of deep seismicity as 43. Washington, DC: National Academy of Sciences, 1978. locations of subduction, they came to be called Benioff zones. While Benioff used his strain-release methodology Duncan Carr Agnew to display other patterns of earthquake occurrence, this approach was not much pursued by other seismologists. Benioff, in his instrumental work in the 1950s, continued to pursue higher sensitivity at longer periods. One BERGERON, TOR HAROLD PER- stimulus for this was the observation on the strainmeter, CIVAL (b. Godstone, Surrey, England, 15 August following a great earthquake in 1952, of signals with 1891; d. Uppsala, Sweden, 13 June 1977), synoptic mete- about a one-hour period, which could be interpreted as orology, cloud and precipitation physics, weather forecasting. free vibrations of the whole Earth, a phenomenon known Bergeron was one of the principal scientists in the from theory but never observed. Benioff improved the Bergen School of Meteorology, which transformed this performance of his strainmeter, and built new instruments science by introducing a new conceptual foundation for in quieter locations in California and (as part of the Inter- understanding and predicting weather. While developing national Geophysical Year in 1957–1958) in Peru. When innovative methods of forecasting, the Bergen scientists the largest earthquake of the twentieth century occurred in Chile in 1960, these instruments gave clear records of established the notion of weather fronts and elaborated a free vibrations at many frequencies, inaugurating a new new model of extratropical cyclones that accounted for branch of seismology. For his accomplishments Benioff their birth, growth, and decay. Bergeron is credited with was elected to the National Academy of Sciences in 1953, discovering the occlusion process, which marks the final and received two awards, the Arthur L. Day Medal of the stage in the life cycle of an extratropical cyclone. Bergeron Geological Society of America in 1957 and the William also contributed to cloud physics, most notably the Bowie Medal of the American Geophysical Union in description of the Bergeron-Findeisen process by which 1965. precipitation forms inside a cloud containing both ice crystals and water droplets. Benioff had a lifelong interest in acoustics and music, which led him to develop novel musical instruments and to experiment with listening to sped-up seismograms to The Early Years. Bergeron was born in England to see what the ear might detect. He put this interest to more Swedish parents Armand Bergeron and Hilda Stawe. direct use during World War II, when he and his engineer- Much later, evidence came to light in Sweden that Berg- ing staff worked on radar and acoustics for the Submarine eron was one of several illegitimate children born to a rad- Signal Company. ical Stockholm intellectual couple who were also owners Benioff married Alice Silverman in 1929; they had of a prominent newspaper. Bergeron, as with the other three children and divorced in 1953, after which he mar- children, was given to a well-chosen family abroad, with ried Mildred Lent, with whom he had one child. money provided for his education in Sweden. His mother knew Nils Ekholm, director of the Swedish Meteorologi- cal Institute (SMI), which proved valuable for the young BIBLIOGRAPHY Bergeron. After receiving his BSc from the University of WORKS BY BENIOFF Stockholm in 1916, Bergeron spent the summers taking “Seismic Evidence for the Fault Origin of Oceanic Deeps.” observations of visibility at different locations around Bulletin of the Geological Society of America 60 (1949): Sweden and returning to SMI in Stockholm during the 1837–1856. autumn to complete his research. He found that changes “Earthquakes and Rock Creep.” Bulletin of the Seismological in visibility seemed to be related to wind-shift lines (what Society of America 41 (1951): 31–62. would be later called fronts). On 1 January 1919, Berg- “Earthquake Seismographs and Associated Instruments.” In Advances in Geophysics, vol. 2. New York: Academic Press, eron received the title of “extra assistant meteorologist” at 1955. the reorganized SMI, later called the Swedish Meteorolog- With Frank Press and Stewart W. Smith. “Excitation of the Free ical and Hydrological Institute (SMHI). Within a few Oscillations of the Earth by Earthquakes.” Journal of months, the tiny core of the incipient Bergen School, Geophysical Research 66 (1961): 605–619. father and son Vilhelm and Jacob Bjerknes and Halvor

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CiStr Ci A Str A Str Ni Warm air Ni Cold air Cold air

Cold air

W ar m

front Cold air Cold air

Warm air front (warm sector)

Cold

CiStr ACu (lent) A Str ca Ni Ni 9km Cold air Warm air Warm air Cold air Ni

ca 70km ca 200km ca 300km ca 500km

Figure 1. Schematic of the extratropical cyclone model proposed by the Bergen School of Meteorology.

Solberg, recruited Bergeron to Bergen, Norway, to join a weather prediction service, directing a number of enthusi- new weather forecasting service. astic young scientists in developing new forecasting practices based on insight into physical processes. The impact The Bergen School and the Occlusion Process. In 1917, of the work performed in Bergen, combined with the the Bergen Museum (precursor of Bergen University) incubation of several high-quality scientists, had an called Vilhelm Bjerknes to a new professorship in meteor- immense impact internationally on the burgeoning scien- ology. Bjerknes had been working in Leipzig on a research tific field of meteorology. program for creating an exact physics of the atmosphere Much of the earliest work in Bergen focused on and ocean. In contrast, meteorologists at the time pre- understanding the structure of extratropical cyclones, dicted weather primarily by often inaccurate empirical storms outside the tropics responsible for most of the rules of thumb and statistical insight. Upon coming to weather in the midlatitudes (not violent tropical storms Bergen in 1918, Bjerknes organized an experimental like hurricanes). Based on the first summer’s forecasting,

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Jacob Bjerknes proposed in November 1918 a new model for these disturbances which accounted for their asym- metric distribution of precipitation (Figure 1). The basic structure was described as a counterclockwise swirl of air around the low-pressure center. Warm air advancing from the south rose up over cold air retreating northward on ab the east side of the low center. The boundary between the two was ultimately called a warm front. On the southwest side of the low center, dense cold air advancing from the north lifted the warm air, forming a boundary later called a cold front. The recently ended World War I inspired using the word front to describe battle lines of advancing and retreating air masses. Bergeron would later suggest the symbols that came to be used for cold and warm fronts cd (lines with filled triangles and semicircles, respectively) on a postcard to Jacob Bjerknes on 8 January 1924. During the fall of 1919, Bergeron noticed that the cold front at times seemed to catch up to and overtake the warm front, what he dubbed sammenklapping (roughly “coming together” or “closing up”). He intuited that the cold front probably rode aloft over the warm front, but he remained puzzled over the nature and significance of this finding. It was not clear whether sammenklapping entailed ef an evolutionary component of extratropical cyclones or simply a local geographical effect. Furthermore, Jacob Bjerknes resisted changes to his model. While in Stockholm and Bergen, Bergeron returned on occasion to this baffling phenomenon. International efforts to increase the amount and frequency of weather data enabled Bergeron to bring into clearer focus the cyclone’s structure. He arrived at a convincing three- dimensional representation by which a cold front and gh warm front merged, resulting in the previously sand- wiched warm air being lifted aloft. Without access to the Figure 2. Schematic life cycle of the extratropical cyclone model warm air fueling the storm, such a cyclone would weaken. proposed by the Bergen School of Meteorology with Bergeron’s Eventually, Bergeron used the term occlusion for this occluded front shown in panels e and f. Dashed lines represent process, and the resulting boundary between the two cold surface fronts; arrows represent streamlines of the flow. air masses was called an occluded front. By 1922, he con- vinced Jacob Bjerknes of the importance of this process to the evolution of extratropical cyclones. This discovery, was a perfectionist, oftentimes not completing publica- along with Solberg’s concept of cyclone families, changed tions for want of further analysis. And whereas Solberg the Bergen cyclone model from a static conceptualization and the Bjerkneses accepted the need to simplify when (Figure 1) into one that featured the entire life cycle of presenting the new findings, Bergeron aimed to depict all birth, maturity, and death (Figure 2). Forecasters and the- the new insights in their full complexity. By temperament oreticians now had a model to help them understand the and principle, he could not easily collaborate with the processes affecting storm intensification and decay. others in writing what he considered a much too hastily Occlusion is a seminal feature of the classic 1922 prepared publication. Although he was not a coauthor, his paper by Jacob Bjerknes and Halvor Solberg, “Life Cycle Bergen colleagues always gave him full credit in discover- of Cyclones and the Polar Front Theory of Atmospheric ing occlusion, the capstone of the Bergen school’s early Circulation,” yet Bergeron was not a coauthor. At the achievements. time, Bergeron was in Stockholm where he was preoccu- pied with other tasks, including preparation of a supple- Indirect Aerology and Air-Mass Analysis. To convince mental manuscript, which never was completed. Bergeron others of the reality and importance of fronts, the Bergen

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methods of air-mass analysis to maturity. Indirect aerology

0° C enabled the Bergen meteorologists to achieve more accurate and detailed predictions, as well as to gain insight into the nature of fronts, cyclones, and air masses. For Berg- eron as well as other members of the Bergen school, synoptic meteorology, or the study of the state of the atmosphere at a specific moment in time on scales with a range of several hundred kilometers, was a legitimate means for winning new knowledge of the atmosphere, an equal partner to theoretical and mathematical study.

؊10° C The Bergeron-Findeisen Process. When Bergeron returned to Bergen in 1922, he stopped for several weeks at a health resort at Voksenkollen, a hill north of Oslo often enclosed by fog. Bergeron noted that when the temperature was well below freezing the roads through the forest were clear of fog. When the temperature was above freezing, however, the fog would extend down to the ground. Bergeron recognized that the saturation water- vapor pressure over water is higher than that over ice at Figure 3. Sketch of the conditions (top) not favoring and temperatures below freezing. Thus, he surmised that dif- (bottom) favoring the Bergeron–Findeisen process. Dotted areas fusion of water vapor from evaporating supercooled liq- represent clouds composed of liquid water droplets. uid-water droplets in the fog to frost growing on the trees might have been occurring to disperse the fog. Although Alfred Wegener had already argued in 1911 that such meteorologists needed to create systematic methods to growth was possible in a cloud with both ice and water reproduce them reliably in daily forecasting work. droplets, Bergeron was the first to recognize that this Although all members of the emerging Bergen school con- growth of the ice crystals to precipitation-sized particles at tributed towards this goal, Bergeron played a crucial role the expense of the supercooled liquid-water droplets could in establishing innovative forecasting practices. lead to precipitation. These ideas would be briefly devel- Bergeron collaborated with Swede and close friend oped in his doctoral thesis in 1928, and presented more Ernst Calwagen to bring greater clarity to an ever-growing fully in 1933. Coupled with experimental confirmation number of new but elusive phenomena emerging from the by the German Walter Findeisen in 1938, this process of analysis of weather maps. To achieve this goal, they sought forming precipitation in a cloud possessing both ice crys- to refine and systematize the Bergen group’s innovative tals and supercooled liquid-water droplets was eventually methods for observing and analyzing weather. They called the Bergeron-Findeisen process (sometimes called brought to maturity a method of indirect aerology, to iden- the Wegener-Bergeron-Findeisen process). This discovery tify fronts and track the life history of large homogeneous promoted the subsequent growth of cloud physics as a bodies of air called air masses. At a time when direct meas- vital subdiscipline, not the least by providing a means to urement of the atmosphere through weather balloons and dissipate fog and a physical mechanism for precipitation kites above the surface (aerology) was not available for enhancement through cloud seeding. daily forecasting, this method combined observation of the clouds and sky overhead with analyses of phenomena Apostle of the Bergen School. More than any other mem- plotted on the weather map to envision the physical ber of the Bergen School, Bergeron conducted the processes occurring in a three-dimensional atmosphere. detailed case studies, lectures, and travel needed to With the help of the Norwegian military air forces Calwa- develop grassroots support abroad for the Bergen School gen began to supplement the indirect aerological methods concepts and methods. His role as apostle was facilitated with direct vertical measurements in the atmosphere. by his linguistic talents: he spoke seven languages and While taking observations on 10 August 1925, Calwagen knew some of three others. was killed along with the pilot after their airplane fell While employed during the early and mid-1920s by apart in midair. Calwagen’s death deeply affected Berg- the Norwegian Meteorological Institute, Bergeron spent eron—he never flew again. Bergeron took over his friend’s time in Leipzig, working with Gustav Swoboda to demon- work, integrated it with his own, and helped bring the strate the applicability of the Bergen School concepts in

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an analysis of a weather event over Europe. “Wellen und development of the international terminology and classi- Wirbel an einer quasistationären Grenzfläche über fication of clouds and precipitation. Europa” [Waves and vortices at a quasi-stationary frontal surface over Europe] (1924) was the first detailed publica- Uppsala University. By the time World War II ended, tion to use the methods developed in Bergen. Bergeron Sweden had a great shortage of meteorologists, especially wrote part 1 of his “Über die dreidimensional for its rapidly growing aviation interests. Practical weather verknüpfende Wetteranalyse” [Three-dimensionally com- forecasting classes were not offered at the universities, and bining synoptic analysis] in 1928, for which he received a an official report indicated the need for a professorship in doctoral degree from the University of Oslo. That paper this subject. Although efforts were underway to create covered many disparate topics, including air-mass analysis such a professorship for him in Stockholm, in 1947 Berg- and frontogenesis (the process of forming and strengthen- eron instead became professor and head of the Depart- ing a front). Bergeron recognized that fronts were found ment of Synoptic Meteorology at Uppsala University. in regions where the horizontal flow was confluent. What Despite Bergeron’s reputation, finding students was diffi- had been considered an uninteresting singularity in the field of flow—the neutral point or col—held the key for cult, especially after Carl-Gustaf Rossby established an understanding where and when fronts form. Bergeron active department at Stockholm. Nevertheless, Bergeron showed that such flows tended to occur between the semi- persisted in lecturing and writing on the principles of permanent highs and lows, explaining the climatological meteorology. locations of fronts around the world. Slowly he returned to the topic of cloud physics. In After completing his doctorate, Bergeron traveled to 1949, Bergeron posited that ice crystals could fall from Malta and the Soviet Union to lecture on the Bergen high-altitude clouds into liquid-water clouds below. Such School methods. Part 2 of his doctoral thesis on fronts and a seeder-feeder process could enhance precipitation at the their perturbations was published in Russian in 1934. But ground. He also wrote about the feasibility of artificially his overly critical demands prevented him from publish- stimulating the production of precipitation from a synop- ing his own research and completing several books. Still, tic and cloud-physics perspective. he produced several texts that played critical roles in the In 1953, Bergeron started Project Pluvius, a research diffusion of the Bergen meteorology. Although many of program designed to understand precipitation better by Bergeron’s papers remained unpublished into the early establishing high-resolution surface rainfall networks. 2000s, his popular lecture notes served as foundations of Among its rich research results, Project Pluvius showed major textbooks on synoptic meteorology written by his that a modest elevation of only 40 to 70 meters could pro- colleagues in Russian, English, and German. duce orographic precipitation enhancement. During the In 1935, Bergeron failed in his bid to be appointed last decades of his life, Bergeron wrote many articles and professor of meteorology at Uppsala University. Bergeron lectures on the history of meteorology. He seems in part and his many supporters from abroad could not overcome to have been drawn to history after his unsuccessful bid local bias against synoptic meteorology, which was consid- for a professorship in Uppsala in the mid-1930s. History ered inferior to laboratory-based research, as well as a provided a way to set the record straight as to his own and long-standing resentment against Bjerknes and the “Nor- others’ contributions to the Bergen School, which is not wegian” achievements with which he was intimately asso- always clear from the publications. He also sought to show ciated. Bergeron returned to Stockholm in 1936, initially what the Bergen School actually did accomplish, since the as a meteorologist, but eventually as scientific chief of originality of its contributions was, in part, denied by SMHI. He began giving lectures and conducting labora- some German, Austrian, and Swedish meteorologists. His tory exercises in weather-map-analysis techniques based on the Bergen methods. These proved popular, although most important contribution from this time, “Weather at times his perfectionist goals in map analysis and in the Forecasting: Methods in Scientific Weather Analysis and wording of predictions created tension between him and Forecasting. An Outline in the History of Ideas and Hints his meteorological colleagues working with him at SMHI. at a Program” (1959) offers a highly personal, but insight- Legend has it that he insisted on the exclusive use of a par- ful, essay outlining the development of modern forecast- ticular brand of colored pencils if accurate weather maps ing as the result of improvements in observations, were to be drawn. Within two years, however, the quality analytical tools, and models of atmospheric structures. of Swedish forecasting had significantly improved. Often Bergeron retired in 1961, but continued to work on Pro- meteorologists came from abroad to Bergeron for train- ject Pluvius, spending his time traveling and lecturing ing. During this time, Bergeron also served on the Com- worldwide, including several trips to the United States. mission of Synoptic Meteorology of the World He died in 1977 of pancreatic cancer, the last of the orig- Meteorological Organization and was influential in the inal Bergen School meteorologists to do so.

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BIBLIOGRAPHY BERGMANN, ERNST DAVID (b. Karl- A complete bibliography for Bergeron can be found in Liljequist, sruhe, Germany, 18 October 1903; d. Jerusalem, Israel, 6 1981a. His correspondence and unpublished manuscripts are April 1975), organic chemistry, education, science policy in housed in the Department of Meteorology, Uppsala University. Israel. Bergmann, a German-Jewish chemist, was forced out WORKS BY BERGERON of his position in Berlin under Nazi laws and emigrated to “Wellen und Wirbel an einer quasistationären Grenzfläche über Mandate Palestine in 1934 to become the first director of Europa.Analyse der Wetterepoche 9.–14. Oktober 1923.” [Waves and vortices at a quasistationary frontal surface over what was later the Weizmann Institute of Science. He sub- Europe. Analysis of the Weather Epoch 9-14 October 1923.]. sequently aided the British scientific effort during World Veröffentlichungen des Geophysikalischen Instituts der War II. He is best known for leading the development of Universität Leipzig, series 2, 3 (1924): 62–172. science in the fledgling State of Israel, including the inau- Über die dreidimensional verknüpfende Wetteranalyse, I. Teil. guration of its nuclear research program. Bergmann typi- (Three-dimensionally combining synoptic analysis, part 1.) fied the close involvement of scientists in Israel with Geofysiske Publikasjone 5. Oslo: Grøndahl & Sons political and defense matters. His research interests in boktrykkeri, I kommission hos Cammermeyers boghandel, organic chemistry were extensive. He introduced Ger- 1928. man-style chemical research and teaching at the Weiz- Trechmerno-Svjaznyj Sinopticeskij Analiz, I-II. (Three- mann Institute of Science and then at the Hebrew Dimensionally Combining Synoptic Analysis.) Moscow, University of Jerusalem. 1934. “Methods in Scientific Weather Analysis and Forecasting: An Early Life in Berlin. Bergmann was born into a strongly Outline in the History of Ideas and Hints at a Program.” In Zionist-leaning German family. In 1908, his father, Rabbi The Atmosphere and the Sea in Motion: Scientific Contributions Judah Bergmann, accepted a post in Berlin and moved to the Rossby Memorial Volume, edited by Bert Bolin. New York: Rockefeller Institute Press, 1959. there with his mother, Hedwig Rosenzweig Bergmann, and his brothers, Artur and Felix. Both brothers later held “Some Autobiographic Notes in Connection with the Ice Nucleus Theory of Precipitation Release.” Bulletin of the influential positions in Israel, the latter as a pharmacolo- American Meteorological Society 59 (April 1978): 390–392. gist. Ernst David studied chemistry at the University of OTHER SOURCES Berlin, where in 1924 he began research for his doctoral Blanchard, Duncan C. “Tor Bergeron and His ‘Autobiographic degree under the supervision of Wilhelm Schlenk. His Notes.’” Bulletin of the American Meteorological Society 59 work involved investigations on polycyclic aromatic com- (April 1978): 389–390. pounds, of great interest to the chemistry of synthetic Bjerknes, Jacob, and Halvor Solberg. “Meteorological dyestuffs and, increasingly, in cancer studies. In 1927, Conditions for the Formation of Rain.” Geofysiske Bergmann was awarded his doctorate and in the following Publikasjoner 2, no. 3 (1921): 3–60. year was appointed Privatdozent (lecturer) at the univer- ———. “Life Cycle of Cyclones and the Polar Front Theory of sity’s chemical institute. In 1928, he married the chemist Atmospheric Circulation.” Geofysiske Publikasjoner 3, no. 1 Ottilie Blum, a research assistant in the institute. By then, (1922): 3–18. his outstanding scientific capabilities were widely recog- Eliassen, Arnt. “Tor Bergeron 1891–1977.” Bulletin of the nized. In 1929, chemistry Nobel Laureate Richard Will- American Meteorological Society 59 (April 1978): 387–389. stätter proposed that Bergmann become his successor to Friedman, Robert Marc. Appropriating the Weather: Vilhelm the chair of chemistry at the ETH (Eidgenössische Tech- Bjerknes and the Construction of a Modern Meteorology. Ithaca: nische Hochschule, or Federal Institute of Technology) in Cornell University Press, 1989. Zurich; however, the more renowned Leopold Ruzicka Liljequist, Gosta H. “Tor Bergeron: A Biography.” Pure and was appointed instead. With Schlenk, Bergmann in 1932 Applied Geophysics 119 (1981a): 409–442. published the first volume of a textbook on chemistry, Liljequist, Gosta H., ed. Weather and Weather Maps: A Volume Ausführliches Lehrbuch der Organischen Chemie, and was Dedicated to the Memory of Tor Bergeron (15.8.1891– the leading candidate for a vacant chair at the Technische 13.6.1977). Basel: Birkhäuser, 1981b. Hochschule in Berlin. Following passage of the anti- Schwerdtfeger, Werner. “Comments on Tor Bergeron’s Semitic Law for the Restoration of the Professional Civil Contributions to Synoptic Meteorology.” Pure and Applied Service on 7 April 1933 under the Nazi regime, Bergmann Geophysics 119 (1981): 501–509. was not appointed to the Berlin position. On the contrary, he was dismissed from his post as research assistant and Robert Marc Friedman lost his venia legendi—his right to teach at a German uni- David M. Schultz versity.

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