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Tor Harold Percival Bergeron (1891–1977)

Basic Facts

  • Born: 15 August 1891, Godstone, Surrey, England
  • Died: 13 June 1977, Uppsala, Sweden (pancreatic cancer)
  • Nationality: Swedish (born in England to Swedish parents)
  • Fields: Synoptic meteorology, cloud physics, precipitation mechanisms

Family and Background

Parents: Armand Bergeron and Hilda Stawe. Bergeron was one of several illegitimate children born to “a radical Stockholm intellectual couple who were also owners of a prominent newspaper.” He was placed with a well-chosen family abroad, with educational funding provided. His mother knew Nils Ekholm, director of the Swedish Meteorological Institute, a connection that would later benefit his career entry into meteorology.

Education

  • BSc, University of Stockholm (1916)
  • PhD, University of Oslo (1928) – thesis: Uber die dreidimensional verknupfende Wetteranalyse (“On Three-Dimensionally Combining Synoptic Analysis”)
    • Famous anecdote: Bergeron tried to hold the night-express train from Bergen to Oslo in order to get extra time for making the very last corrections to his doctoral manuscript before sending it to the printer in Oslo – a perfectionist to the core.

Career Timeline

  • 1916–1918: After BSc, spent summers taking visibility observations at various Swedish locations, returning to the Swedish Meteorological Institute (SMI) in autumn for research.
  • 1 January 1919: Appointed “extra assistant meteorologist” at the reorganized Swedish Meteorological Institute.
  • 1919: Recruited by Vilhelm Bjerknes, Jacob Bjerknes, and Halvor Solberg to Bergen within months. Helped establish the new forecasting program there.
  • 1919–mid-1920s: Core member of the Bergen School of Meteorology, working at the Norwegian Meteorological Institute.
  • 1924: Collaborated with Ernst Calwagen on indirect aerology and air-mass analysis methods.
  • 1925: Calwagen died in an airplane accident (10 August 1925); Bergeron never flew again after this tragedy.
  • 1928: Completed doctoral thesis at University of Oslo.
  • 1930s: Lectured internationally (Malta, Soviet Union) as “apostle of the Bergen School.”
  • 1935: Failed bid for professorship in Sweden (local bias against synoptic meteorology and resentment of Norwegian achievements in the field).
  • 1936 onward: Scientific chief at SMHI (Swedish Meteorological and Hydrological Institute), Stockholm.
  • 1947–1961: Professor and head of the Department of Synoptic Meteorology, Uppsala University.
  • 1953: Started Project Pluvius, studying precipitation through high-resolution surface rainfall networks.
  • 1961: Retired but continued Project Pluvius work, traveling and lecturing worldwide.
  • 1977: Died of pancreatic cancer – “the last of the original Bergen School meteorologists to do so.”

Major Scientific Contributions

1. Discovery of Occlusion (1919–1922)

In autumn 1919, Bergeron noticed that cold fronts appeared to “catch up to and overtake the warm front,” a process he dubbed sammenklapping (roughly “coming together”). By 1922, he had convinced Jacob Bjerknes of this process’s evolutionary importance. The landmark 1922 paper by Bjerknes and Solberg, “Life Cycle of Cyclones and the Polar Front Theory of Atmospheric Circulation,” featured the occlusion concept, though Bergeron was not listed as co-author. Colleagues always credited him fully for the discovery.

2. The Walk in the Woods – Wegener-Bergeron-Findeisen Process (1922)

In 1922, while staying at a health resort at Voksenkollen (430 m above sea level), a hill north of Oslo often enclosed by fog, Bergeron made a crucial observation:

  • When the temperature was well below freezing, forest roads were clear of fog but trees were covered in frost.
  • When the temperature was above freezing, fog extended all the way to the ground.

Bergeron deduced that the saturation water-vapor pressure over water is higher than that over ice at temperatures below freezing. In mixed-phase clouds, water vapor diffuses from supercooled liquid droplets to ice crystals, causing the ice crystals to grow at the expense of the droplets. Although Alfred Wegener had already argued in 1911 that such growth was theoretically possible, Bergeron was the first to recognize that this mechanism could grow ice crystals to precipitation-sized particles, thus explaining how rain forms.

He developed these ideas in his 1928 doctoral thesis and presented them more fully at the 1933 International Union of Geodesy and Geophysics (IUGG) meeting in Lisbon, Portugal. German researcher Walter Findeisen experimentally confirmed the process in 1938. The mechanism is now known as the Wegener-Bergeron-Findeisen (WBF) process.

3. Air Mass Classification (1928–1930)

Bergeron’s 1928 doctoral thesis covered air-mass analysis and frontogenesis. By 1930, he had established the classic classification system identifying air masses by their source regions:

  • Polar (P) vs. Tropical (T) (temperature)
  • Continental (c) vs. Maritime (m) (moisture)
  • Plus Arctic (A) and Equatorial (E) categories

This Bergeron classification became the most widely accepted form of air mass classification and remains foundational for synoptic meteorology.

4. Front Symbols (1924)

On 8 January 1924, Bergeron sent a postcard to Jacob Bjerknes suggesting the symbols that came to be universally used for cold and warm fronts – lines with filled triangles and semicircles, respectively.

5. Three-Dimensional Synoptic Analysis

Bergeron recognized that “fronts were found in regions where the horizontal flow was confluent,” identifying confluent flow patterns between semipermanent pressure systems as key to understanding global frontal locations.

6. Seeder-Feeder Process (1949)

Proposed that ice crystals falling from high-altitude clouds into liquid-water clouds below could enhance precipitation at the ground.

7. Project Pluvius (1953 onward)

Used high-resolution surface rainfall networks to study precipitation, discovering that even a modest elevation of 40 to 70 meters could produce orographic precipitation enhancement.

Personal Characteristics and Anecdotes

  • Perfectionist: Often delayed publications seeking further analysis. Colleagues noted he aimed “to depict all the new insights in their full complexity.”
  • Colored pencils: Legend claims he “insisted on the exclusive use of a particular brand of colored pencils if accurate weather maps were to be drawn.”
  • Polyglot: Spoke seven languages and knew some of three others, which facilitated his role as international ambassador for Bergen School methods.
  • Never flew again after his collaborator Ernst Calwagen died in an airplane accident in 1925.
  • Defender of the Bergen School: In later years wrote extensively defending the Bergen School’s contributions against skeptics from German, Austrian, and Swedish meteorological traditions.
  • His lecture notes from international tours became foundations for major synoptic meteorology textbooks in Russian, English, and German.

Awards and Honors

  • Symons Gold Medal (1949) – Royal Meteorological Society
  • International Meteorological Organization Prize (1966) – World Meteorological Organization
  • Member, Commission of Synoptic Meteorology of the World Meteorological Organization

Key Publications

  • “Wellen und Wirbel an einer quasistationaren Grenzflache uber Europa” (1924)
  • Uber die dreidimensional verknupfende Wetteranalyse, I. Teil (1928) – doctoral thesis
  • Trechmerno-Svjaznyj Sinopticeskij Analiz, I-II (1934, Russian)
  • “Methods in Scientific Weather Analysis and Forecasting: An Outline in the History of Ideas and Hints at a Program” (1959)
  • “Some Autobiographic Notes in Connection with the Ice Nucleus Theory of Precipitation Release” (1978, posthumous)

Connections to Other People

  • Vilhelm Bjerknes: Recruited Bergeron to Bergen in 1919; Bergeron’s mentor and the founder of the Bergen School.
  • Jacob Bjerknes: Close collaborator; Bergeron convinced him of the occlusion concept and invented the front symbols used in the Bjerknes-Solberg papers.
  • Halvor Solberg: Fellow Bergen School member who also recruited Bergeron; co-author of the 1922 cyclone life cycle framework that incorporated Bergeron’s occlusion idea.
  • Carl-Gustaf Rossby: Fellow Bergen School alumnus; later a rival of sorts when Rossby’s Stockholm department drew students away from Bergeron’s Uppsala program.
  • Sverre Petterssen: Petterssen attended Bergeron’s lectures in Bergen and was so impressed by his 1922 storm analysis that he joined the Bergen School in 1923.
  • Walter Findeisen: German researcher who experimentally confirmed Bergeron’s ice-crystal theory of precipitation in 1938.
  • Ernst Calwagen: Close collaborator on indirect aerology; his death in a 1925 plane crash deeply affected Bergeron.
  • Nils Ekholm: Director of the Swedish Meteorological Institute, known to Bergeron’s mother, which facilitated his entry into meteorology.

Sources