To anyone who listens to weather forecasts, the words cold front and warm front convey at least a rudimentary picture of the state of the atmosphere. The terms, and the concepts behind them, seem to have been around since time immemorial, but in truth, they are less than a century old, 90 years to be exact. It seems odd, but the United States did not place fronts on their daily weather maps until August of 1941. In this essay, I look at fronts, not from the science behind them, but as an introduction to a group of dedicated and innovative scientists centered in Norway, know in meteorological history as the Bergen School.
The story starts with Norwegian physicist Vilhelm Bjerknes just prior to the turn of the twentieth century. Vilhelm was born in the Norwegian town of Christiania (later Kristiania and now Oslo) on 14 March 1862, the son of a professor of mathematics at the University of Christiania. As a youth, Vilhelm assisted his father in his research in the field of hydrodynamics and continued to be a part of the senior Bjerknes' research team until he had received his Master's Degree from the University of Kristiania in 1888.
With a degree and state fellowship in hand, Vilhelm took his studies in Paris, attending lectures by Jules Henri Poincaré on electrical wave diffusion. He learned there of the research conducted by Heinrich Hertz and in 1890 moved to Bonn to attend the university there. Vilhelm had the good fortune to become an assistant and collaborator to Hertz. Hertz and Bjerknes conducted studies of electrical resonance that would become influential in the development of radio.
Bjerknes returned to Norway in 1892 where he completed his doctorate in electrodynamics and shortly thereafter was appointed to a lectureship at Stockholm's School of Engineering. Three years later, he was named professor of applied mechanics and mathematical physics at the University of Stockholm. But hydrodynamics returned as Vilhelm's main research interest.
Working with the two primitive circulation theorems of absolute motion derived by British mathematician and physicist William Thomson Kelvin and German physicist Hermann Helmholtz, Bjerknes understood that these theorems provided vital concepts to the motions within the planet's two largest fluid systems, the atmosphere and oceans. From these, Bjerknes realized that atmospheric motions could be mathematically understood when hydrodynamics was combined with thermodynamics.
Bjerknes recognized that fluid motions in the atmosphere created weather patterns and that by applying the mathematical expressions of his theorems, weather prediction was, in fact, a solvable problem of mathematical physics. In a 1904 paper, he proposed that, given enough information about the current state of the atmosphere, its future state could be predicted. This was the first insight into what we now know as numerical weather prediction.
Bjerknes crossed the Atlantic in 1905 to give a lecture series on his work at the Massachusetts Institute of Technology (MIT). The purpose of the lectures was to present his work on how air masses move and how mathematics could be applied to weather prediction. He also sought funding for his research. His vision impressed representatives from the Carnegie Institution in Washington DC, and they awarded him a research associateship with an annual funding grant that continued until 1941.
In 1907, Vilhelm Bjerknes left Stockholm for the University of Kristiania as professor and chair of the Department of Applied Mechanics and Mathematical Physics. There, he began lecturing on the application of the circulation theorems to the problem of weather forecasting. Bjerknes also completed the first volume of his landmark Dynamic Meteorology and Hydrography series. In 1911, he began collaborating with another giant in geophysics, fellow Norwegian Harald Ulrik Sverdrup. The research of Bjerknes and collaborators impressed German scientists, and they offered him a professorship and the directorship of the new geophysics institute at the University of Leipzig in 1912. Bjerknes brought with him Sverdrup and Theodor Hesselberg.
Though his research work was successful at Leipzig, the outbreak of the First World War took many of Bjerknes' German researchers and students into military service. Then Hesselberg and Sverdrup left to return to Scandinavia. He replaced them with his son Jakob and Halvor Solberg, but the war continued to make life and his research difficult.
The Bergen School
Help came in the name of famed Norwegian Arctic explorer, Fridtjof Nansen who, with the support of oceanographer, Bjorn Helland-Hansen, made Bjerknes an offer to establish a geophysical institute at Bergen, Norway as part of the Bergen Museum. The Bergen Museum had been a prime institute of scientific investigation for many years. Helland-Hansen, the professor of oceanography at Bergen Museum, had given courses in ocean research for many years. With a strong science faculty in place, it was only a matter of time before a full university arose in Bergen.
Bergen Geophysical Institute today Courtesy University of Bergen, Norway
Vilhelm Bjerknes thus left Leipzig to take up the professorship at the Bergen Museum in the summer of 1917 and establish the institute. Vilhelm brought son Jakob and Halvor Solberg with him as his assistants. He also realized that he did not have the resources available to fully tackle the problem of numerical weather forecasting. He therefore turned the attention of the institute to practical weather forecasting using surface map analysis with the goal of offering agricultural forecasts during the summer. These began in the summer of 1918 with Jakob as forecaster in Bergen and Solberg in Christiania.
While his father continued work on the numerical aspects of weather prediction, Jakob continued the study of convergence lines which he had begun in Germany. Using an enhanced weather observation network across Norway, Jakob saw that the convergence lines had a characteristic pattern when associated with extratropical cyclones. Those convergence lines would later be termed fronts, so-called because of their analogy with First World War battle fronts. Jakob combined these features in the landmark paper "On the Structure of Moving Cyclones" written in the autumn of 1918 (before Jakob's 21st birthday) and formally published in 1919.
Jakob Bjerknes model of the structure of moving cyclones. Courtesy NOAA, National Weather Service (NWS) Collection
The convergence line that separated cold air from the warm to the west and north of a low pressure center, originally referred to as the squall line, was termed the cold front, and the line between the warm and cold air to the low's south and southeast, originally called the steering line, was termed the warm front. (The introduction of the frontal terms came in 1920.) Both fronts were characterized by vertically sloping frontal surfaces with cold air underneath, following research by Austrian meteorologist Max Margules.
The paper also reported that where warm air ascends along the sloping frontal surfaces, bands of clouds and precipitation form. The rising air added to the cyclone's kinetic energy, a finding in agreement with Margules's theory on cyclone formation published fifteen years earlier. Finally, the paper put forth the idea that a series, or family, of cyclones followed the same path, with the trailing cold front behind one cyclone serving as a warm front of a new cyclone farther west.
Norwegian model cyclones. Courtesy NOAA, National Weather Service (NWS) Collection
Jakob and Solberg would continue to study and refine the frontal cyclone model in the following years. In 1919, a student from Sweden Tor Bergeron joined the team. Bergeron, who would later develop the air mass classification system and elucidate the precipitation process, added to the cyclone frontal model by describing the process of occlusion. In the occlusion process, the cold front overtakes the warm front and thus lifts the warm sector to higher altitudes, eventually causing the cyclone to die.
Tor Bergeron also gave meteorology one of its greatest weather map traditions: the symbolism for depicting fronts on weather maps. In a postcard from Germany to Jacob Bjerknes, Tor sketched his shorthand method of displaying fronts, thus eliminating the then-expensive need to print weather maps in color. Previously, cold fronts were displayed as blue lines on the weather map and warm fronts by red lines. Bergeron proposed cold fronts be indicated by sharp, triangular barbs on the frontal line and warm fronts by rounded, semicircular pips. Occluded fronts would be indicated by alternating barbs and pips. By placing the elements on the side of the line pointing in the direction of the frontal movement, the map would have a dynamic visual component as well.
Solberg further added to Bjerknes' original work by describing the process whereby cyclones form on waves along the polar front. The results of this collaboration were an idealized cyclone model with a characteristic three-dimensional structure evolving over its live cycle. As Theodor Hesselberg later summed it up: "[The cyclone] is born as Solberg's initial wave on the polar front, develops into Jack Bjerknes's ideal cyclone, and finally suffers the Bergeronian occlusion death."
Norwegian model cyclone formation along the polar front. Courtesy NOAA, National Weather Service (NWS) Collection
Over the years, as work continued on the frontal cyclone model from a number of different angles, the Bergen School attracted many scientists to its staff who would reach "superstar" status within the meteorological community during the century. They included: Carl Gustaf Rossby, Sverre Petterssen, Eric Palmén, Jørgen Holmboe, and Carl Ludvig Godske. Members of the Bergen School spread the concepts developed there around the world. The school also served as a training ground for many foreign meteorologists who took the concepts home for applications and further refinements. Vilhelm Bjerknes remained with the institute until 1926 when he moved to the chair of the Department of Applied Mechanics and Mathematical Physics at the University of Oslo. He was replaced by Harald Sverdrup, a famed oceanographer and meteorologist. When Sverdrup left, the chair went to Jakob Bjerknes in 1931. The post next went to Carl Godske, when the younger Bjerknes left for the United States in 1939.
As Dora B. Grimes of NOAA Central Library pointed out in her paper Bjerknes like Father like Son, the lives and careers of Vilhelm and Jakob Bjerknes had many parallels. Both were sons of renowned physicists, and both delayed their own education and personal research interests to willingly support their father's research. Both organized institutes of learning and research. And both had their scientific work disrupted by world war. But whereas Vilhelm left Leipzig during the First World War to return to his native Norway, Jakob left Norway on the eve of the outbreak of World War II for the United States.
Jacob's work on frontal theory and cyclones garnered much attention across Europe and North America during the 1920s and 1930s. Many meteorologists visited Bergen to learn more about the subject. He also travelled extensively, lecturing on meteorology in England, Switzerland, the Netherlands, Germany, Canada and the United States. When Jakob and his family were touring the US in 1939 on an eight-month lecture series, the European war broke out. Bjerknes thus decided to remain in the US rather than return to Norway, which had been invaded by Germany.
Bjerknes/Solberg cyclone model. Courtesy NOAA, National Weather Service (NWS) Collection
With the threatening situation in Europe and Asia, the US military understood that air power would be crucial in any coming conflict and to support an air force, the country needed a considerable number of meteorologists. With his knowledge of weather analysis techniques (and, I am sure, his familiarity with European weather situations), the US military asked Jakob, soon to be known as Jack, to organize a training school for air corps weather officers. Jack selected the University of California at Los Angeles (UCLA) as the location of the school. This location, according to his wife, was chosen for its proximity to the Scripps Oceanographic Institution in La Jolla because Jack felt oceanographers' input would be important.
Jack was therefore appointed to the faculty of UCLA as professor of meteorology and head of the Section of Meteorology in the Department of Physics. He invited his old colleague Jørgen Holmboe, who had been with Rossby at MIT, to join him. Following the war, Jack, like his father before him, would establish a meteorological research center. In 1945, the Department of Meteorology came into existence at UCLA with Jack as chairman. The work of the department would focus first on the problem of cyclone growth along an upper level wave and on major research on the general circulation of the atmosphere. Bjerknes and colleagues conducted extensive studies of the upper atmosphere and the jet stream following the war. (Bomber pilots had found the high-altitude jet stream affected their mission flights.) In 1952, he used photographs taken from high-altitude research rockets in his research, work that would foreshadow the use of satellites in meteorological analysis.
By the end of the 1950s, Jack Bjerknes renewed his interest in the interaction between the atmosphere and sea, looking first at the role of the Gulf Stream on weather in the North Atlantic Ocean and then turning his attention to the Pacific Ocean and an interesting local Peruvian phenomenon known as El Niņo. He eventually realized that El Niņo worked larger scale than the Peruvian coast and would link it to the Southern Oscillation and global weather. Jack continued this research until his death in 1975.
The Front Advances
The concept of fronts and frontal association with cyclones seems so intuitive today to those studying large-scale weather patterns. But the meteorological community did not jump into the full acceptance of frontal analysis as quickly as one might imagine. Like any other paradigm shift, it was resisted by many of the old school, and thus it took time before it was fully accepted into common weather analysis and forecasting.
The prime example of such resistance can be found in the reaction of the United States Weather Bureau. Though the Bjerknes' concepts of weather analysis and forecasting were published in the Bureau's journal Monthly Weather Review in 1919 and received funding for their work from the Carnegie Institute of Washington for over 30 years, their research findings were not fully adopted into routine weather analyses by the US Weather Bureau until the late 1930s.
The US Navy was among the first US agencies to take an interest in the work of the Bergen School. They had set up an independent aerological service to support naval aviation in the later years of the First World War. In 1922 the Navy sent Lieutenant Francis Reichelderfer, one of a handful of officers still working in meteorology following the war, to direct the Navy's aerological service in Washington DC. Reichelderfer had been sent copies of the papers outlining air mass and frontal analysis, and impressed by the concepts, he had been using them in his forecasting. By 1925 Reichelderfer had made the Norwegian techniques standard practice throughout the Navy.
To teach these techniques, Reichelderfer convinced the Navy to establish the first US post-graduate program in meteorology to train aerological officers. The course was initially established at Harvard but subsequently moved to MIT in 1928. Reichelderfer further recommended that Carl-Gustaf Rossby, a member of the Bergen School, be invited to organize and direct the MIT effort. The MIT program soon became the first Department of Meteorology in the US. In 1939, Sverre Petterssen would become its chairman. Among its first class were Howard Orville who commanded the Navy weather service from 1940 to 1950 and Randolph Williams who became known as the "Father of the Air Weather Service" for the US air corps. The first civilian student (in the second class) was Horace Byers, renowned cloud physicist and long-time chair of the University of Chicago meteorology department (also established by Rossby).
When weather maps drawn and analyzed at the MIT aerological school did not agree with those done by the naval analysts, the new head of the Navy weather service Lt Charles Maguire wanted to know why. He convinced the Navy to send Reichelderfer to Bergen to learn more in 1931. There he became friends with Jakob Bjerknes who assisted him in analyzing some of the American weather maps of abnormal situation. Tor Bergeron and Sverre Petterssen would help in the analyses.
What Reichelderfer discovered was that the Norwegians had advanced the fundamentals far beyond what he had introduced to the navy forecasters years earlier. He wrote: "There certainly is something fundamental and of value in the new principles vs. the old. It is a chance of a lifetime to develop those as applying to the US and to use them to improve forecast service."
While the Navy adopted the new Bergen techniques in 1925, the US Weather Bureau forecasters dug in their heels against any form of the Norwegian analyses. At the time, the Weather Bureau believed forecasting could only be taught by years of learning from a master forecaster; it was more an art of pattern or geometric recognition with some statistics thrown in than a science based on the concepts and equations of physics. Many ignored or put down the concepts of air mass and frontal analysis of the Bergen School.
The Weather Bureau Forecast Office, Washington DC, 1926 Courtesy NOAA, National Weather Service (NWS) Collection
Until the 1920s, the US Weather Bureau's prime mandates were to support the agricultural industry and Great Lakes and coastal shipping. With the emergence of the American airline industry and the need for aerological forecasts, Congress made the Bureau responsible for supporting air navigation in the US and "above the high seas." The airlines, at the recommendation of their pilots and staff meteorologists, wanted Bureau forecast and analysis products to explicitly depict fronts and air masses. When Weather Bureau Chief Charles F. Marvin resisted such changes, the industry complained strongly to President Franklin Roosevelt, who asked his Science Advisory Board to look into the matter.
Weather Bureau meteorologist preparing pilot briefing board at San Francisco. Courtesy NOAA, National Weather Service (NWS) Collection
The Board's recommendations included adoption of the Bergen techniques and the replacement of Marvin. As a result, Willis Ray Gregg, an aerological meteorologist, was appointed new Bureau Chief in 1934. Gregg appreciated the Bergen concepts and set about incorporating them into the daily analysis. Though he brought in three MIT instructors Horace Byers, Harry Wexler, and Steven Lichtblau to teach Bureau forecasters the new techniques, the old guard forecasters strongly resisted the changes. In September 1938 Gregg unexpectedly died of a heart attack. In choosing his successor, the head of the US Civil Aeronautics Authority warned, the Bureau must pick someone from outside its bureaucracy with strong credentials in aviation weather. Otherwise, they would form their own weather service.
The search committee led by Secretary of Agriculture Henry Wallace selected Francis Reichelderfer. Reichelderfer pushed the initiatives concerning frontal and air mass analyses begun by Gregg. He reorganized the Bureau and began hiring meteorologists trained in the Norwegian methods while providing in-house training for those currently on staff.
The transition to air mass and frontal analyses was completed 1 August 1941 when the first Daily Weather Map series map was issued with fronts and air masses depicted on it.
Vilhelm Bjerknes lived until 1951 by which time the meteorological concepts arising from the Leipzig Institute and the Bergen School had become firmly entrenched in weather forecasting. He summed up the contribution thus:
"During 50 years meteorologists all over the world looked at weather maps without discovering their most important feature. I only gave the right kind of maps to the right young men, and soon they discovered the wrinkles in the face of the Weather."
Of course, the work of the Norwegian meteorologists was but the beginning of the story. Their research and their personalities opened the doors for improvements and insights over the 90 years since Jakob first crafted the techniques of frontal analysis. The meteorological institutes and university departments created by the Bjerknes and their colleagues but the beginning of today's synoptic weather analysis.
Weather map courtesy NOAA Central Library Data Imaging Project. Picture of J. Bjerknes courtesy Eugene M. Rasmusson, University of Maryland.
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