Jacob Bjerknes (1897–1975)


Jacob Aall Bonnevie Bjerknes was born in Stockholm, Sweden, in 1897. He continued a legacy of hydrodynamic application theory that began with his physics professor grandfather Carl and was extended by Jacob’s father, the theoretical physicist Vilhelm Bjerknes. From 1914 to 1916, Bjerknes attended the University of Kristiania (Oslo), where he became interested in his father’s work relating hydrodynamic theory to atmospheric motion and weather prediction. Jacob followed his father and his group of young Norwegian meteorologists to the University of Leipzig, where their research focused on the formation of heavy precipitation along ordered cloud boundaries then called “squall lines.” Through surface map analysis, Jacob discovered that these boundaries coincided with the convergence of wind fields, making the latter a potential detector and predictor of weather patterns.

Vilhelm Bjerknes returned to Norway in 1918 and founded a geophysical institute at the University of Bergen, where he organized an analysis and forecasting branch which would evolve into a weather bureau by 1919. During this period, Jacob’s research resulted in the identification of two boundary lines of convergence (one leading, one following) as an integral part of atmospheric wave development (cyclogenesis) which could progress to the formation of a low-pressure center and the southward transfer of polar air. In his classic paper “On the structure of moving cyclones” in 1919, Bjerknes introduced the concept of the “extratropical cyclone,” which became a foundation for the long-range weather forecasting envisioned by his father. The group at Bergen, now including the Swedish meteorologists Carl Gustav Rossby and Tor Bergeron, used balloon data to investigate the “squall line” boundary in three dimensions and realized that it was a thermal discontinuity of hemispheric proportions. Defining it as the “polar front” (borrowed from World War I terminology), they named the two active convergence lines of the cyclone model the “warm front” and the “cold front.” As pointed out in a key paper by Jacob and Halvor Solberg in 1922 (“The life cycle of cyclones and the polar front theory of atmospheric circulation”), the dynamics of the polar front, integrated with the cyclone model, provided the major mechanism for north-south heat transport in the atmosphere. For this and other research, Jacob was awarded the Ph.D. from the University of Kristiania in 1924.

In 1926, Jacob served as a support meteorologist for Roald Amundsen’s polar dirigible flight. In 1928 he married Hedvig Borthen. In 1931, he left his position as head of the weather service at Bergen to become professor of meteorology at the geophysical institute his father had founded. By 1933 he discovered yet another aspect of the cyclone phenomenon, the upper atmospheric wave. His preliminary formulation of the use of pressure tendency as a surface indication of cyclone development appeared in 1937.

Jacob lectured at the Massachusetts Institute of Technology during the 1933–1934 school year and emigrated to the United States in 1940 where he headed a government-sponsored meteorology annex, for weather forecasting, to the department of physics at the University of California, Los Angeles (UCLA). That same year of 1940 saw the invasion of Norway by Germany and Bjerknes’ receipt of the Symons Medal from the Royal Meteorological Society. At UCLA, Bjerknes and fellow Norwegian Jorgen Holmboe further developed the pressure tendency and the extratropical cyclone theories.

The science of meteorology entered the computer and space ages during the 1950s. Bjerknes, then head of the department of meteorology at UCLA, was an early advocate of using photography from rockets to image atmospheric weather patterns, and he would later help usher in the use of satellites for the same purpose. Bjerknes’ cyclone model was a key element in the Princeton atmospheric program used to obtain the first accurate computer-aided weather forecast in 1952. Bjerknes’ later research focused on the energy exchange of the atmosphere and oceans and, specifically, the El Niño effect.

To his friends, Bjerknes was just “Jack.” With his students, he was always conscientious in sharing his gift for simplifying the complexity of atmospheric motion. The American Geophysical Union honored Bjerknes’ pathbreaking work in meteorology in 1945 with the William Bowie Medal, its highest award, recognizing fundamental contributions to geophysics and the AGU principle of unselfish cooperation in research. Additional honors included the American Meteorological Society’s Rossby Medal (1960) and the National Medal of Science (1966). Bjerknes died in 1975.

—William J. McPeak

Lake Forest, California