Bjerknes Discovers Fronts in Atmospheric Circulation
Vilhelm Bjerknes was a pivotal figure in the early 20th century who significantly advanced the understanding of atmospheric circulation and weather prediction. He recognized that accurate weather forecasting depended on extensive data collection, linking local weather events to the broader global atmospheric movements driven by solar heat. His work was partly motivated by the need for improved weather knowledge for commercial fishing and agriculture, particularly during the restrictions of World War I. Bjerknes established observing stations in Norway and founded a school in Bergen, attracting noted meteorologists, including his son, Jacob Bjerknes.
Bjerknes was instrumental in developing a mathematical theory of weather fronts, focusing on the interactions between warm and cold air masses along defined boundaries. His research into extratropical cyclones laid the groundwork for understanding storm development and the life cycle of frontal systems, describing how air masses clash to create low-pressure areas and initiate storm behavior. His pioneering studies in the early 1920s were groundbreaking, occurring in an era without modern technology like computers or satellites, yet they set the foundation for contemporary meteorological practices and greatly improved the science of weather forecasting.
Bjerknes Discovers Fronts in Atmospheric Circulation
Date 1919-1921
Vilhelm Bjerknes developed a model of the atmosphere that emphasized the idea of “fronts,” the boundaries along which masses of warm and cold air clash and converge to produce the weather.
Locale Bergen, Norway
Key Figures
Vilhelm Bjerknes (1862-1951), Norwegian meteorologistJacob Bjerknes (1897-1975), Norwegian meteorologistCarl-Gustaf Arvid Rossby (1898-1957), Swedish American meteorologistTor Bergeron (1891-1977), Swedish meteorologist
Summary of Event
Vilhelm Bjerknes, a Norwegian geophysicist and meteorologist, knew in the early 1900’s that accurate forecasting of weather required much information. He also knew that local weather was tied to the global circulation of the atmosphere. While teaching at Stockholm University from 1895 to 1907, he proposed that movements in the atmosphere are stimulated by heat from the Sun. At the same time, these movements radiate heat as air masses rub up against one another, causing friction.
Bjerknes was motivated by the need for improved weather prediction for commercial fishing and agriculture. In part, the urgent need for better domestic food production arose from restrictions of imports and communications as a result of World War I (1914-1918). He persuaded the Norwegian government to help set up strategically located observing stations. In addition to the stations, he founded a school at Bergen that attracted meteorologists from all over the world, including his son Jacob Bjerknes; Carl-Gustaf Arvid Rossby, a Swedish American meteorologist; and Tor Bergeron, a Swedish meteorologist.
Weather types and changes, along with the interaction of moving masses of air, have been studied and noted for centuries. In the nineteenth century, Luke Howard, an English physicist, had written of northerly and southerly winds blowing alongside each other, with the colder wedging in under the warmer and the warmer gliding up over the colder and causing extensive and continued rains. In 1852, evidence had been found of a polar wind advancing under a warm, nearly saturated tropical wind and pushing it upward, producing cumulus clouds.
Vilhelm Bjerknes was a pioneer in the development of a mathematical theory of fronts and their effects. In addition, along with his son, he was the first to study extratropical cyclones and use them to forecast the weather. Extratropical cyclones are cyclones that may cross an ocean in ten days, lose most of their intensity, and then develop again into large and vigorous storms. In the years following World War I, Norwegian meteorologists had a fairly good understanding of the action in the big storms sweeping across the Atlantic. From this knowledge, Vilhelm Bjerknes theorized that the main idea in storm development is a clashing of two air masses, one warm, the other cold, along a well-defined boundary, or front.
Aside from the idea of storm fronts, his view of cyclone development produced another important idea. At the beginning of the life cycle of a storm, there is an undisturbed state in which cold and warm air masses flow side by side, separated by a front. Each air mass flows along its side of the front until some of the warmer air begins to invade the cooler air, leading to a wave disturbance. This disturbance spreads and grows, creating low-pressure areas at the tip of the wave. Air motions try to spiral into these areas, and both fronts begin to advance. The cold air generally moves faster, catching up with and moving under the lighter warm air. As the storm grows deeper, the cold front becomes more pronounced. The whole process—from the time the polar air meets the northward-flowing warm air to the point at which the area of low pressure is filled completely—is known as the “life cycle of a frontal system.” This description is based on the wave theory, which was originally developed by Vilhelm Bjerknes in 1921.
In 1919, when this work began, upper-atmosphere studies were limited by the lack of knowledge of such things as radar images, lasers, computers, and satellites. Vilhelm Bjerknes showed that the atmosphere is composed of distinct masses of air meeting at various places to produce different meteorological effects. He published the study On the Dynamics of the Circular Vortex with Applications to the Atmosphere and Atmospheric Vortex and Wave Motion in 1921.
Significance
The weather forecasting stations established by Vilhelm Bjerknes in the 1920’s were a monumental accomplishment, considering the limited amount of information and the lack of high-speed, worldwide communications. All the computations were done without the assistance of a computer or modern weather satellites to analyze and model the data. Today, these and other tools have made it possible for scientists to compile data into real-time images of current weather patterns around the globe.
Bibliography
Aguado, Edward, and James E. Burt. Understanding Weather and Climate. 3d ed. Upper Saddle River, N.J.: Prentice Hall, 2003. Meteorology textbook aimed at both science majors and non-science majors focuses on the processes that produce weather and climate. Includes many illustrations and other learning aids.
Bates, Charles C., and John F. Fuller. America’s Weather Warriors, 1814-1985. College Station: Texas A&M Press, 1986. Comprehensive history of meteorology carried out by the U.S. military. Includes maps, bibliography, and index.
Friedman, Robert Marc. Appropriating the Weather: Vilhelm Bjerknes and the Construction of a Modern Meteorology. Ithaca, N.Y.: Cornell University Press, 1989. History of modern meteorology discusses Bjerknes’s work and its significance.
Humphreys, W. J. Physics of the Air. New York: Dover, 1964. Textbook on physical meteorology for upper-level and graduate students. Although somewhat technical, still a good reference for advanced high school and lower-level college students. Well illustrated.
Lutgens, Frederick K., and Edward J. Tarbuck. The Atmosphere: An Introduction to Meteorology. 9th ed. Upper Saddle River, N.J.: Prentice Hall, 2003. Introduction to meteorology for college students is accessible to advanced upper-level high school students as well. Well illustrated. Includes glossary and index.
Neiburger, Morris, James G. Edinger, and William D. Bonner. Understanding Our Atmospheric Environment. 2d ed. San Francisco: W. H. Freeman, 1982. Textbook for college students focuses on physical explanations of atmospheric phenomena. Well illustrated. Includes a section after each chapter devoted to questions, problems, and projects.
Reiter, Elmar R. Jet Streams: How Do They Affect Our Weather? 1967. Reprint. Westport, Conn.: Greenwood Press, 1979. Excellent source on the topic for high school and lower-level college students. Presents a popular survey on how jet streams are formed, how they move, and the effects they have on weather and climate. Includes illustrations and a brief list of suggested readings.
Schaefer, Vincent J., and John A. Day. A Field Guide to the Atmosphere. Boston: Houghton Mifflin, 1981. Excellent reference source for high school and lower-level college students. Features numerous color and black-and-white photographs as well as drawings to help clarify the physical principles discussed. Includes twenty appendixes and a very good bibliography after each chapter.