ASSMANN, RICHARD

(b. Magdeburg, Germany, 13 April 1845; d. Giessen, Germany, 28 May 1918),medicine, meteorology, aerological measurements, discovery of the stratosphere.

Assmann Assmann

Assmann contributed significantly to the exploration of the free (high altitude) atmosphere and to observational techniques in meteorology between 1880 and 1910. He conducted human scientific ascents with balloons for about ten years and became one of the discoverers of the stratosphere in 1902. His cloud observations on mountaintops led to the clarification of the nature of cloud droplets. He invented the aspirated psychrometer, a radiation-shielded instrument to measure simultaneously the temperature and humidity of air. Following his suggestion, sounding balloons continued to be made from rubber in the early 2000s. The summit of his career was the 1905 inauguration of the meteorological observatory at Lindenberg, southeast of Berlin, whose first director he became.

From Medicine to Meteorology. Assmann was the first of three children of the leather maker and town councillor Adolph Assmann and Dorothea, née Burkhard. Having finished school in his native town, Assmann matriculated for medicine at the University of Breslau in 1865 and continued these studies in Berlin, interrupted by two years of military service. In 1869, he took his doctor’s degree in medicine with a thesis titled Hemophilia. A year later he received his license to practice medicine and served as a sergeant first class and surgeon in the Prussian army in the Franco-Prussian War (1870–1871). Directly after the war he settled as a physician in the small town of Freienwalde on the river Oder. In 1871, he married Johanna Andrée, and six years later their only child was born. Here in his spare time he started to study meteorological phenomena and set up a small private observatory in the tower of a war memorial. The observatory was equipped with self-registering instruments built by Assmann. During this period he visited established meteorological offices including the Deutsche Seewarte (German Hydrographical Institute) in Hamburg, where he was introduced to the noted climatologist Wladimir Köppen, with whom he corresponded nearly his whole life.

In 1879, Assmann moved back to his native town of Magdeburg, hoping to become medical superintendent of the municipal hospital. Failing to receive this appointment, he happened to meet a former classmate, Alexander Faber, the owner and editor of the local newspaper Magdeburgische Zeitung, who wanted to establish a meteorological station in order to supply weather reports for his newspaper. Assmann agreed to become head of this station, which was situated in a 34-meter-high tower specially built for this purpose on the grounds of the newspaper print shop. The station opened on 1 November 1880 and issued its first daily weather map on 12 December, just a few days after the publication of the world’s first newspaper weather map in London. Financial support provided by Faber enabled Assmann to leave the medical practice and become a full-time meteorologist. In 1881, he founded the Society for Agricultural Meteorology in Magdeburg, which soon established a network of more than 250 observing stations in central Germany. In the following year he founded Monatsschrift für praktische Witterungskunde (Monthly Paper for Practical Meteorology). Renamed Das Wetter (The Weather) two years later, the journal turned into a publication for popular meteorology. The journal was intended to promote widespread interest in meteorology among people whose livelihood depended on the weather, including farmers, gardeners, and foresters. With this, Assmann hoped to increase the number of diligent weather observers. Assmann served as its chief editor until the end of his life. In 1904, he founded and thereafter edited together with Hugo Herge-sell the scientific journal Beiträge zur Physik der freien Atmosphäre (Contributions to the Physics of the Free Atmosphere). In contrast to Das Wetter, the Beiträge was established to serve for the exchange of information among scientists working on the exploration of the free atmosphere (i.e., the upper troposphere and the stratosphere). It was devoted especially to the physical processes occurring in these atmospheric layers.

Cloud Physics and Aspirated Psychrometer. In November 1884, Assmann made investigations in cloud physics on top of the Brocken, the highest peak (1,141 meters) of the Harz Mountains in central Germany. Using a microscope he definitively settled the question of whether cloud particles are droplets or bubbles in favor of the first. In the following years he also investigated rime, hoarfrost, and snow crystals. In 1885, he earned an additional degree (Habilitation), this time from Halle University, by presenting a thesis on the thunderstorms in Central Germany. On 30 October of that year he gave his inaugural lecture at Halle as an unpaid university lecturer (Privatdozent).

In 1886, Assmann became a civil servant at the Prussian Royal Meteorological Institute in Grünau, near Berlin, and headed the department on thunderstorms and extraordinary phenomena. The director of this institute was Wilhelm von Bezold, who also was the first professor of meteorology at Berlin University beginning in 1885. Here Assmann made his famous instrumental invention: the aspirated psychrometer. Aiming to construct a thermometer that was not influenced by shortwave radiation from the sun, he suspended two mercury-in-glass thermometers in a chromium-plated, highly polished frame. The bulbs of the thermometers were surrounded by two coaxial metal tubes through which the air was drawn by means of a clockwork-driven fan at the top of the central hollow column. The bulb of one of the thermometers was covered by wet muslin. The evaporating moisture from the muslin kept this thermometer at a lower temperature than the other one. As evaporation increases in proportion to the

dryness of the air, the temperature difference between the two thermometers could be converted into values for air humidity. The principle of Assmann’s psychrometer is still used today, with platinum resistance thermometers substituting for the older mercury thermometers.

Aerology. In 1892, Assmann’s department at the meteorological institute was expanded to include measuring devices. In 1886, he also joined the Deutscher Verein zur Förderung der Luftschiffahrt zu Berlin (German Association for the Advancement of Airship Aeronautics in Berlin), which had been founded in 1881. He took over the planning of balloon ascents for scientific purposes promoted by this association. Human scientific aviation began on 1 March 1893 with the ascent of the balloon Humboldt in the presence of the German emperor. During the following six years the association carried out sixty-five ascents. The balloons were filled with a mixture of hydrogen and methane and carried open baskets for the passengers. For higher ascents oxygen was available for the passengers through pipes from steel bottles. The instrumentation consisted of a mercury barometer, a hair hygrometer, Assmann’s aspirated psychrometer, and a simple radiation measurement device (black bulb thermometer). Different observers conducted the ascents; the most frequent traveler was the meteorologist Reinhard Süring. During the forty-fourth ascent on 4 December 1894 the record height of 9,155 meters was reached by the balloon Phönix during a courageous solo voyage by Arthur Berson, Assmann’s closest coworker. Assmann himself was on board during three ascents. In addition to the manned balloons Assmann also operated unmanned registration balloons, some of which reached heights well above 10,000 meters during their ten ascents. The aerological program was completed by twenty-one ascents of tethered balloons, two of them carrying human aeronauts.

Initially, balloon ascents were conducted from different places in and around Berlin. Following Assmann’s strong suggestions, an aeronautical observatory was established in 1899 as a fourth department of the Meteorological Institute, with Assmann as head. The following year balloon operations were centralized on an army target range near Tegel, northwest of Berlin. Assmann promoted regular launches of instrumented kites and kite-balloons—means of meteorological observation equal or even superior to free-floating balloons. The results of these soundings were published daily in newspapers and, from 1903, in the official weather bulletin of the Deutsche Seewarte in Hamburg.

The Discovery of the Stratosphere. By 1894, several unmanned balloons carrying self-registering instruments had reached—as became obvious later—the stably stratified air layer above the troposphere that is now named the stratosphere. Since no one had anticipated the existence of such a layer, Assmann had always been very cautious with respect to the temperature readings which indicated the presence of a so-called upper inversion. On 7 July 1894, the registering balloon Cirrus reached an isothermal layer, later called the tropopause. This ascent was one of ten made by Assmann between 1894 and 1897. Five ascents supplied readings from heights between 11.7 and 21.8 kilometers. The thermometer readings showed a cessation of the usual temperature decrease with height at about fifteen kilometers. Assmann, however, suspected that his instruments had been adversely affected by radiation. His doubts were finally removed by the measurements made during a balloon ascent up to 10.5 kilometers conducted by Arthur Berson and Reinhard Süring on 31 July 1901. The results confirmed the readings of the unmanned balloon missions. From 1896 on, similar registration balloon ascents were organized by the French meteorologist Teisserenc de Bort in Trappes near Versailles. In 1902, both scientists, who were friends and had long exchanged scientific results, made public their results nearly simultaneously and made their common, though independent, discoveries public. Teisserenc de Bort spoke to the Paris Academy of Science on 28 April and Assmann to the Berlin Academy of Science on 1 May.

The most important contribution by Assmann to high altitude exploration was the introduction of the closed balloon of india rubber in place of the open-mouthed balloon of varnished paper, that Teisserenc de Bort had employed. Rigid paper balloons lost their lifting force with increasing height, while elastic rubber balloons expanded with increasing height and decreasing pressure and thus guaranteed a more or less constant lifting force, upward velocity, and—very important—a constant ventilation of the meteorological instruments. The introduction of rubber balloons made possible ascents to over 20 kilometers. When a balloon burst and fell to the ground, the instrument registers could be evaluated if the balloon was recovered. From 1900 to 1913, a total of 317 rubber balloons were launched under the guidance of Assmann, of which only 16 could not be recovered.

In 1908 (most probably at the 29 September meeting of the German Meteorological Society in Hamburg), Teisserenc de Bort coined the terms troposphere and stratosphere. The intermediate layer, the tropopause, was named in 1926 by the English meteorologist Sir William Napier Shaw in his Manual of Meteorology. Shaw called the identification of the tropopause and the stratosphere the “most surprising discovery in the whole history of meteorology.”

The Aeronautical Observatory at Lindenberg. The aeronautical observatory in Tegel proved to be too close to the expanding city of Berlin. Occasionally, broken ropes from failed kite ascents fell on electric and telephone lines and on the cables of the streetcar lines that extended into the outskirts of Berlin. By 1902, the army gave notice that the observatory in Tegel would be closed. A more suitable location for the observatory was found about one hundred kilometers southeast of Berlin on a little hill near the village of Lindenberg. New buildings were erected, and on 16 October 1905, the observatory was inaugurated by the Prussian king and German emperor Wilhelm II. The ceremony was also attended by Prince Albert I of Monaco, who had made his yacht available for balloon ascents to explore the free atmosphere over the sea and who took part in the data evaluation. Assmann’s work was deemed important enough that the observatory became an independent scientific institution instead of part of the Prussian Meteorological Institute.

The observatory conducted systematic monitoring of the vertical distribution of wind, temperature, and humidity in the free atmosphere. Such information was necessary—apart from scientific needs—for the rapidly evolving field of aeronautics. In the first thirty years of the observatory, more than 25,000 vertical soundings of the troposphere and the lower stratosphere were performed with specially designed instruments. In 1910, based on the experience from such measurements, Assmann wrote a memorandum, “On the establishment of an aeronautical weather service relying on aerological observations,” in which he proposed a network of pilot balloon stations for Germany and an alert service for aeronauts. This network, organized by Assmann and having its central office in Lindenberg, started its operations in 1911 with twenty-five pilot balloon stations and six hundred thunderstorm reporting points at post and telegraph offices.

On 1 October 1914, Assmann retired and was succeeded by Hugo Hergesell, the former president of the International Commission for Scientific Aeronautics (founded 1896) and professor in Strassburg since 1900. The observatory continue to exist in the early 2000s, one of two remaining observatories of the German Weather Service, still devoted to the monitoring of vertical profiles of atmospheric parameters. That the observatory kept its tasks nearly unchanged for over a century demonstrates the essential soundness of Assmann’s scientific vision. After retirement, Assmann moved to Giessen and lectured at the university there. In 1915, he published a book on the Royal Prussian Aeronautical Observatory Lindenberg, in which he summarized his scientific achievements. He died in Giessen sixteen months after his wife.

Awards. In 1903, Assmann, together with Berson, was awarded the second Buys-Ballot medal by the Royal Netherlands Academy of Arts and Sciences. This prize is given once every ten years to the scientist who has made outstanding contributions to the development of meteorology. On 16 October 2005, during the centennial ceremony, the name of the Meteorological Observatory in Lindenberg was supplemented by the term “Richard Assmann Observatory.”

BIBLIOGRAPHY

Personal notes on Richard Assmann’s life written by his daughter, Helene Assmann, are kept in the private ownership of the Assmann family. A large collection of letters written between Assmann and Wladimir Köppen from 1882 to 1914 is in the archives of the University Library in Graz, Austria.

WORKS BY ASSMANN

“Mikroskopische Beobachtung der Wolken-Elemente auf dem Brocken.” Meteorologische Zeitschrift2 (1885): 41–47. On the nature of cloud droplets.

“Eine neue Methode zur Ermittlung der wahren Lufttemperatur.” Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften zu Berlin 46 (1887): 505–515. First publication on the invention of the aspired psychrometer.

Das Aspirations-Psychrometer: Ein Apparat zur Bestimmung der wahren Temperatur und Feuchtigkeit der Luft. Abhandlungen des Preussischen Meteorologischen Instituts Vol. 1, No. 5, 117–270. Berlin: A. Ascher, 1892. Complete description of Assmann’s aspiration psychrometer.

With Arthur Joseph Stanislaus Berson. Wissenschaftliche Luftfahrten. 3 vols. Braunschweig: F. Vieweg und Sohn, 1899–1900. Covers the scientific balloon ascents between 1893 and 1899.

“Über die Existenz eines wärmeren Luftstromes in der Höhe von 10 bis 15 km.” Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften zu Berlin 24 (1902): 495–504. Minutes of the session at the Berlin Academy of Sciences in which Assmann reported on the discovery of the stratosphere.

Das Königlich Preussische Aeronautische Observatorium Lindenberg. Braunschweig: Vieweg, 1915.

OTHER SOURCES

Bernhardt, Karl-Heinz. “Zur Erforschung der Atmosphäre mit dem Freiballon—die Berliner wissenschaftlichen Luftfahrten (1888–1899).”Dahlemer Archivgespräche 6 (2000): 52–82. Overview of scientific balloon ascents in Berlin from 1888 to 1899.

Hoinka, Klaus P. “The Tropopause: Discovery, Definition, and Demarcation.” Meteorologische Zeitschrift 6, n.s., (1997): 281–303. Extensive discussion of the role of Teisserenc de Bort and Assmann in the discovery of the stratosphere.

Labitzke, Karin G., and Harry van Loon. The Stratosphere:Phenomena, History, and Relevance. Berlin: Springer-Verlag, 1999. First chapter on the history of the discovery of the stratosphere.

Peppler, A. “Richard Assmann.” Das Wetter 35 (1918): 70–79.Obituary giving a detailed portrayal of his life, in the popular magazine he edited.

Steinhagen, Hans. Der Wettermann. Neuenhagen: Findling Verlag, 2005. An extensive biography of Assmann, partly based on letters written by Assmann to Wladimir Köppen between 1882 and 1913. The book was written on the occasion of the centennial of the Lindenberg observatory.

Stefan Emeis