The Swedish meteorologist Tor Bergeron was born in England but returned to Sweden with his family at a young age and completed his education there. After receiving his bachelor's degree in 1916, he joined the Swedish Weather Service as a meteorologist. Bergeron spent much of 1919 helping Vilhelm and Jacob Bjerknes establish their new forecasting program in Bergen. Jacob Bjerknes had introduced his now-famous cyclone model in 1918, but the complete model—including the life cycle of the cyclone—depended upon Bergeron's critical discovery of the occlusion process that leads to the decline and death of the cyclone. (In simplest terms, an occluded front develops when a cyclone's cold and warm fronts collide.) Bergeron returned to Bergen in 1922, becoming a full-time meteorologist with the Bergen Weather Service, a position he held intermittently until 1929. From 1923 to 1925 he worked at the Leipzig Geophysical Institute collaborating with German researchers who were applying Bergen School analysis techniques to European weather scenarios.
Returning to Norway, Bergeron completed his doctoral dissertation in 1928. Although earlier scientists, including Jacob Bjerknes, had addressed the concept of air masses and their relationship to weather patterns, Bergeron was the first to make a systematic study of air masses, the source regions that spawned them, and ways they were subsequently transformed as they moved. Classifying air masses as equatorial, tropical, polar, and arctic, he looked for the characteristics (for example, temperature and humidity) that tended to be retained by the air masses even after they left their source regions. These characteristics—termed quasi-conservative—allowed meteorologists to track air masses around the world. Bergeron described how the transformation of the air masses resulted in distinctive cloud and precipitation patterns, temperature structure, visibility, and turbulence activity. So consistent were these results that they could be used to draw conclusions about the vertical distribution of air temperature and to predict the weather. In the decade before the widespread use of radiosonde equipment, Bergeron's methods were especially important for aviation forecasting. This work explained the existence of the world's main frontal development zones and provided a method for determining the location of cold and warm frontal zones on weather maps.
After completing his doctorate, Bergeron became a "missionary" for the Bergen School. He spent a year at the Meteorological Office on the island of Malta in the Mediterranean, providing training on Bergen School methods. He then spent two years in Moscow lecturing on air mass analysis and polar front theory. There Bergeron met his future wife, Vera Romanovskaja, who served as one of his student assistants. Bergeron greatly influenced the development of Soviet meteorology. His lectures provided the basis for a textbook used for training Russian meteorology students in meteorological theory.
Returning to Norway, Bergeron served as a meteorologist and consultant to the Norwegian Meteorological Institute in Oslo. While there, he worked on the famous ice crystal precipitation paper described earlier. Moving to Sweden in 1936, he became a senior meteorologist and later the scientific chief of the Swedish Meteorological and Hydrological Institute. He held the latter position until 1947, when he became professor of synoptic meteorology at the University of Uppsala, a position he held until his retirement in 1961. At Uppsala he started Project Pluvius—an effort to determine the influence of topography on the distribution of precipitation. Realizing that rain gauge networks were generally too coarse to determine the geographic variation of rainfall accurately, he directed the installation of 350 rain gauges in a 154-square-mile (400-km2) region surrounding Uppsala. The resulting data showed that even modest 131- to 230-foot (40- to 70-m) hills could induce a surprising increase in rainfall as a result of orographic lifting. Bergeron continued his research even after retirement, and several of his works on the subject were published after his death.
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