Hann, Julius Ferdinand von

(b. Mühlkreis, near Linz, Austria, 23 March 1839; d. Vienna, Austria, 1 October 1921)

meteorology, climatology.

Hann’s father, Joseph, was curator of the manor house in Mühlkreis; his early death in 1852 left the large family in a difficult economic situation. Hann’s mother, Anna, subsequently opened a pension for pupils of the Gymnasium in Kremsmünster, where Hann’s education began in 1853. After passing his Abitur (1860) he enrolled at the University of Vienna and studied mathematics, chemistry, and physics, then geology and paleontology under Eduard Suess and physical geography under Friedrich Simony. He passed his teaching examination in 1863 and taught at high schools in Vienna and Linz.

Hann had pursued meteorological investigations since his boyhood. In 1865 he was invited by Karl Jelinek, director of the Zentralanstalt für Meteorologie und Erdmagnetismus in Vienna, to participate in the editorship of the newly founded Zeitschrift der Oesterreichischen Gesellshchaft für Meteorologische (which merged in 1885 with the German Meteorologische Zeitschrift). In 1867 he was appointed to the staff of the Zentralanstalt. Hann received his Ph.D. at Vienna in 1868, and in the following year he delivered his Habilitationsschrift. Appointed associate professor of physical geography at the University of Vienna in 1874, he taught meteorology, climatology, and oceanography. In 1877 he succeeded Jelinek as the director of the Zentralanstalt and was appointed professor of physics. In 1897, at the age of fifty-eight, he resigned his directorship and became professor of meteorology at Graz. Partly because of poor library facilities in Graz, Hann returned in 1900 to Vienna, where he held a professorship of cosmic physics until 1910. He edited the Meteorologische Zeitschrift until 1920 and carried on his scientific work until his death.

In 1878 Hann married Luise Weismayr, daughter of a district court president. He was fully absorbed in his work, and only summer journeys to the Alps and several meteorological congresses took him from it. Hann was one of the secretaries of the first international assembly of meteorologists at Leipzig in 1872 and was a member of the International Meteorological Committee (1878–1898). He was also a member of the Vienna Academy and honorary or foreign member of academies and societies throughout the world. He was knighted in 1910.

Hann was one of the most prominent meteorologists of his generation. His importance rested less on the creation of new theoretical concepts than on his efforts to coordinate empirical and theoretical results into a coherent structure. Hann was a very competent editor, and the Meteorologische Zeitschrift became the leading meteorological journal under his guidance. He exerted great influence through his capacity to stimulate thought and through clarifying debates by commentaries and critical reviews. He contributed to almost all branches of meteorology and attracted scientists of highest ability to Vienna, including Max Margules, J. M. Perntner, Wilhelm Trabert, F M. Exner, and A. Merz.

Hann first became widely known when he entered the heated debate between H. W. Dove and the Swiss meteorologists on the origin of the warm Föhn winds occurring in the Alps (1866). He dismissed explanations based on mechanical displacement of warm air from tropical regions and demonstrated that the Föhn was produced locally when air passed over mountain ranges: moisture was removed on the windward side as a result of precipitation and the air was compressed during its descent on the lee. This thermodynamic theory included all katabatic wind phenomena in mountains. Unnoticed by meteorologists, the same explanation had briefly been offered by Helmholtz in 1865. Hann’s article a year later and his subsequent papers on adiabatic changes of state in vertically displaced dry and moist air gave new directions to research, particularly in Germanspeaking countries.

Convinced that progress in science was possible only through extensive use of observational material, Hann made it his lifelong duty to bring rapidly accumulating meteorological data into precise and consistent forms. His work therefore formed the basis for many theories. of far-reaching consequence in this respect were his investigations of cyclones and anticyclones. An early theory, based principally on surface observations, attributed the initial fall of pressure and the driving force of cyclones to rising columns of warm air and the release of latent heat during condensation of water vapor. Anticyclones, being relatively cool, were regarded as regions of descending cold air. Hann criticized this so-called convective theory, supported by William Ferrel, C. M. Guldberg, Henrik Mohn, Helmholtz, and others, as early as 1874. His most substantial objections were based on observations made on the mountains of Europe and the United States (1876). He demonstrated that up to the height of the mountains anticyclones were, on the average, warmer than cyclones except for a shallow surface layer. These temperature conditions contradicted the thermally induced independent circulation between cyclones and anticyclones supposed in the convective theory. Hann’s observations received wide attention after 1890, when he corroborated them with new European data. Investigators in the United States, however, obtained different results. This discrepancy resulted from the statistical nature of the studies of Hann and others, which did not discriminate between the relatively cold mature cyclones and slow-moving warm anticyclones common in Europe and the initially warm-core young cyclones and migratory cold anticyclones frequently experienced in the United States. These differences were largely clarified by S. Hanszlik’s studies on anticyclones in 1909 and the model of the thermal structure of cyclones developed by the Norwegian school of meteorologists after 1918.

From the beginning Hann had regarded cyclones and anticyclones as essential components of the general circulation of the atmosphere. His observational results convinced him that warm, deep anticyclones were produced by large-scale subsidence of the upper midlatitude westerlies. He suggested that extratropical cyclones originated as eddies in the westerlies, their kinetic energy deriving ultimately from the basic north-south temperature gradient (1879, 1890). Influenced by Helmholtz’s vortex theory, Hann proposed that cyclones tended to develop in preexisting low-pressure areas by conversion of potential energy available in the horizontal pressure distribution into kinetic energy (1877, 1880). Latent heat of condensation and local temperature differences appeared to him of only secondary importance for the development and maintenance of cyclones.

Hann was a driving force in the establishment of mountain observatories because he realized the importance of aerological data. He studied upper-air data from many different aspects. One of his lifelong interests was the daily variation of meteorological elements, requiring harmonic analysis of high-altitude and low-altitude observations. He computed the harmonic coefficients of the diurnal oscillations (and their seasonal variations) of pressure, temperature, and wind for more than 100 stations throughout the world. In 1886 Hann produced conclusive evidence of the dominant and universal character of the twelve-hour solar pressure oscillation first suggested by Johann von Lamont in 1862. For many years Hann’s publications, together with the results of C. A. Angot (1889), formed the basic material for theoretical studies on the cause of this oscillation. Hann presented details of a terdiurnal solar oscillation in 1917.

Hann also studied diurnal pressure and temperature variations in relation to the formation of mountain and valley winds. Two wind systems were required in his theory (1879), a thermal slope wind and a larger circulation between plain and mountain valley resulting from thermally induced horizontal pressure gradients. The development of this theory culminated in the work of A. Wagner in 1932.

The collection of long, homogeneous data series enabled Hann to make careful statistical studies of correlations between pressure, temperature, and precipitation anomaly patterns of distant regions and their causal relations (1904). He recognized future possibilities of such investigations for long-range forecasting, yet in his own time he considered practical weather forecasting a goal not scientific enough to merit interest.

Hann regarded his investigations in meteorology as inseparable from his climatological work. He viewed the statistical treatment of data as an indispensable basis for the knowledge of atmospheric motions and their explanation by physical laws, as well as for the knowledge of climatic conditions in the geographical sense. He developed valuable tabulation and statistical techniques in climatology and was instrumental in making accessible data series from remote places. He produced the first comprehensive climatologies of the tropics and the polar regions. Hann’s classification of climates was based on geographical features and the variation of meteorological elements, primarily temperature and humidity. Still in use, this geographical-statistical approach has been supplemented by considerations of atmospheric dynamics.

One of Hann’s greatest services to meteorology was the publication of two comprehensive treatises, Handbuch der Klimatologie (1883), the standard work for half a century, and Lehrbuch der Meteorologie (1901). Hann was never very active in organizational and administrative matters but, significantly, he insisted on the establishment of chairs of meteorology at all Austrian universities.

BIBLIOGRAPHY

I. Orginal Works. Hann’s comprehensive treatises on climatology and meteorology, containing extensive historical references, form a good guide to his contributions to science: Die Erde als Ganzes, ihre Atmosphäre und Hydrosphäre (Prague, 1872; rev. eds., 1875, 1881, 1886, 1897); Handbuch der Klimatologie, 3 vols., in the series Bibliothek geographischer Handbücher, F. Ratzel, ed. (Stuttgart, 1883; rev. eds., 1897, 1908, 1932), trans. into English by C. de Waard (New York, 1903); Atlas der Meteorologie (Gotha, 1887); Lehrbuch der Meteorologie (Leipzig, 1901; rev. eds., 1906, 1915, 1926, 1937–1951), eds. from 1915 with assistance of R. Süring.

Most of Hann’s more than 1,000 other contributions, including numerous climatological notices, are scattered throughout Meteorologische Zeitschrift and the publications of the Academy of Sciences in Vienna. Over 300 are listed in the Royal Society’s Catalogue of Scientific Papers: III, 162; VII, 902–903; X, 131–134; XV, 619–623. A large selection of Hann’s publications is also listed in Poggendorff, III, 582–583: IV, 580–581; V, 493–494. Some of Hann’s more important papers (most of them mentioned in the text) published in the Zeitschrift der Oesterreichischen Geseltschaft für Meteorologie are “Zur Frage über den Ursprung des Föhn,” 1 (1866), 257–263; “Die Gesetze der Tempcratur-Änderung in aufsteigenden Luftströmen und einiger wichtigsten meteorologischen Folgerungen aus densdben,” 9 (1874), 321–329, 337–349, English trans. by C Abbe in “The Mechanics of the Earth’s Atmosphere,” in Report of the Board of Regents of the Smithsonian Institution (1877), 397–419; “Ueber das Luftdruck-Maximum vom 23. Jänner bis 3, Februar 1876 nebst Bemerkungen über die Luftdruck-Maxima im Allgemeinen,” 11 (1876), 129–135; “Bemerkungen über die Entstehung der Zyklonen,” 12 (1877), 308–313, and 15 (1880), 313–321; “Einige Bemerkungen zur Lehre von den allgemeinen atmosphärischen Strömungen,” 14 (1879), 33–41; and “Zur Theorie der Berg- und Talwinde,” ibid., 444–448.

Some important publications in the Denkschriften der Wiener Akademie der Wissenschaften, Math.-naturwiss. Kl., are “Untersuchungen über die tägliche Oscillation des Barometers,” 55 (1889), 49–121; “Das Luftdruck-Maximum von November 1889 in Mitteleuropa,” 57 (1890), 401–424; “Weitere Untersuchungen über die tägliche Oscillation des Barometers,” 59 (1892), 297–356; “Der tägliche Gang der Temperatur in der inneren Tropenzone,” 78 (1905), 249–366; “Der tägliche Gang der Temperatur in der äusseren Tropenzone,” 80 (1907), 317–404, and 81 (1907), 21–113; and “Untersuchungen über die tägliche Oscillation des Barometers. Die dritteltägige (achtstündige) Luftdruckschwankung,” 95 (1917), 1–64.

Some of Hann’s papers in the Sitzungsberichte der Wiener Akademie der Wissenschaften, Math.-naturwiss. Kl, Abt. IIa (after 1888), are “Die Wärmeabnahme mit der Höhe an der Erdoberfläche und ihre jährliche Periode,” 61 (1870), 65–81; “Bemerkungen zur tägliehen Oscillation des Barometers,” 93 (1886), 981–994; and “Die Anomalien der Witterung auf Island in dem Zeitraume 1851 bis 1900 und deren Beziehungen zu den gleichzeitigen Witterungsanomalien in Nord-Westeuropa,” 113 (1904), 183–269.

See also “Die Verteilung des Luftdruckes über Mittelund Südeuropa,” in Geographische Abhandlungen, 2 , no. 2 (1887).

II Secondary Literature. Obituaries are in Meteorologtsche Zeitschrift, 38 (1921), 321–327; Deutsches biographisches Jahrbuch, III (Stuttgart, 1921), 118–122; Bolletino della R. Società geografica italiana, 5th ser., 10–11 (1921), 5–11; Wetter, 38 (1921), 161–168; Nature, 108 (1921), 249–251; Mitteilungen der Geographischen Gesellschaft in Wien, 64 (1922), 121–131; and Naturwissenschaften, 10 (1922), 49–52.

Gisela Kutzbach