Alfred Wegener

by Mott T. Greene

  The great advantage of kite flying, even though kites never reached such impressive altitudes as the free balloons, was the virtual certainty of the return of the instruments. Moreover, using the sort of winch that Wegener had access to at Lindenberg, it was possible to control very accurately the rate of ascent and descent of the instruments. There was often a significant lag time (upward of forty-five seconds) before a recording instrument could respond to a change in temperature or humidity. Therefore, a slow, controlled ascent, with the ability to pause the instruments briefly at any altitude, gave a much more accurately detailed record of atmospheric conditions.

  On days when there was not enough wind to take the kites aloft (winds of less than 6–7 meters per second, or about 15 miles per hour), Wegener and the other technical assistants and helpers sent aloft, using the same cable and winch arrangement, large rubber balloons (Gummiballon) of Aßmann’s design, twice as tall as a man and filled with 20–30 cubic meters (706–1059 cubic feet) of hydrogen generated with an apparatus at the station. These balloons flew well only when the wind speed was less than 3 or 4 meters per second (about 7 miles per hour), so there were days when no flying could be done at all—too much wind for a balloon, not enough wind for a kite. Moreover, as anyone knows who has been in an airplane or watched clouds drift by on a sultry summer day, winds shift velocity and direction rapidly with altitude, and it is possible to have still air at the ground and a respectable breeze only a few hundred meters up—so the flight plans for the balloons were often frustrated in this way as well.

  There were other alternatives: sounding balloons could be released (without instruments) and followed visually to very great altitudes, in order to study shifts in wind velocity and direction. Meteorologists, on partly cloudy days, had for many years studied winds aloft with theodolites using the direction and velocity of clouds as the data, and using triangulation from observers far apart to find the altitudes. But sounding balloons allowed observations of motions in clear air as well. Aßmann had worked out a system of underinflating the balloons to keep them from rising too rapidly. Using the theory of the rate of expansion of hydrogen gas with altitude and the corresponding resistance of the rubber balloon fabric (a theory developed by Hugo Hergesell in 1903), Aßmann was able to calculate how fast a balloon was rising at each altitude.10 A balloon rising at 1 meter per second (2 miles per hour) at an altitude of 6 kilometers (4 miles) would be rising at 8 meters per second (18 miles per hour) at 15 kilometers (9 miles) and 8.5 meters per second (19 miles per hour) at 20 kilometers (12 miles).11 One could theoretically use these numbers during a flight to calculate the air pressure at various altitudes by plotting the calculated velocity of ascent against the actual, though the main reason for sending up these balloons was to observe the wind speed and direction.

  In 1905–1906 the Lindenberg scientific staff attempted thirty-nine sounding balloon ascents.12 Observers (usually two or more) followed the balloons with theodolites of various designs, and they constantly tested new ones—work in which Wegener took a vigorous and active role. Balloon theodolites are compact telescopes capable of extremely precise angular measurements and are set on mountings that swivel both up and down and from side to side (technically, swivel in both elevation and azimuth). They are fitted with an eyepiece with a right-angle prism so that the observer always looked horizontally into the instrument no matter what its orientation or inclination. As one observer followed the balloon with the theodolite, the other observer used a graphing apparatus to mark the elevation and direction of the balloon at intervals of several seconds. Using two pairs of observers, the angular data could give the actual flight path of the balloon.

  Armed with these instruments, the Lindenberg staff studied every aspect of the atmosphere in the first few kilometers above the surface: atmospheric layering, winds, temperature and humidity, vertical atmospheric motions, cosmic radiation, polarization of light, atmospheric electricity, atmospheric particulates, cloud types, and photographic documentation of atmospheric phenomena of all kinds. The emphasis was definitely not on development of new theory; it was on testing theories by observation and data collection, and on the refinement of observation by improved instrument design and employment.

  Discovery of the Stratosphere

  Richard Aßmann himself was not a theorist, nor did he intend to be; he had made his reputation as an organizer, observer, editor, and instrument designer. He did, however, make observations of tremendous theoretical significance. One of these had been the determination in November 1884 that clouds are made up of microscopic droplets (not bubbles) and that these droplets do not have dust particles as condensation nuclei. He had determined this on a mountain outside Berlin, the Brocken, where for several days he had worked in a cloud layer about 50 meters (164 feet) thick just below the summit, with a microscope of about 200 times magnification, collecting the droplets on glass slides, studying them under oblique illumination, and measuring them with a micrometer. This series of several hundred careful measurements answered unambiguously a question that had been in the meteorological literature for many years and in so doing inaugurated the study of the microphysics of clouds.13

  Aßmann’s other great triumph (and also his greatest near miss) was his role in the discovery of the stratosphere. This event was of immense significance for meteorology and inspiring to Wegener, who just missed taking part in it: Aßmann in Berlin and Teisserenc de Bort in France performed the work, between 1900 and 1903. Much of Wegener’s own work in the next decade was an outfall of this discovery, which dominated the research program at Lindenberg during his year there in 1905–1906 and for years thereafter. The work leading up to this remarkable finding, as well as the problems it created, led the Russian historian of meteorology A. Kh. Khrgian to declare, “No other chapter in the history of meteorology contains as much of the element of drama as the chapter we are coming to now. Heroic, very dangerous flights, difficult observations, remarkable and unexpected discoveries … and the constant attention of the press and general public, were all part of the period.”14

  The discovery of the stratosphere is one of the most influential discoveries in modern physics, as well as one of the most overlooked. It ranks with the theory of radioactive transmutation as an example of fastidious, delicate experiments, many times repeated, leading a scientist (or scientists) to reject a universally accepted theoretical principle in favor of an empirical result. In the case of radioactivity, the theoretical and laboratory work of Ernest Rutherford (1871–1937) and Frederick Soddy (1877–1956) on the elements uranium and thorium led them to the unbelievable but inescapable conclusion that they were witnessing the active transmutation of one element into another, thereby disproving one of the most fundamental principles of physics: the immutability of elemental species of matter. This led within a few years to the discovery that elements, rather than distinct substances, were simply different ways of organizing the real fundamental stuff of the universe—protons and electrons.15 Within three years it also led to a means of determining the age of a rock by comparison of the ratios of uranium and lead they contained, understanding the lead to be a decay product of uranium.16 This revolutionized both geology and evolutionary biology almost instantaneously, showing that Earth was billions, not millions, of years old, and that there was, after all, sufficient time for even the slowest and most gradual sorts of changes to shape the world into the form in which we see it.

  The discovery of the stratosphere, occurring the same year as the work of Rutherford and Soddy (1902), was epochal and far-reaching in its own way, leading to profound changes in meteorology, oceanography, and geophysics, all within a decade. Aßmann had a role in the discovery, but it was not in him to take the leap of judgment or faith that would have let him enjoy sole credit for uncovering this aspect of the natural world. That distinction belongs to Teisserenc de Bort, working from his private laboratory near Paris.

  Teisserenc de Bort, like Aßmann and Hugo Hergesell, designed and built meteo
rological recording instruments and sent them aloft on balloons and kites, also of his own design or modification. On 9 September 1899, he had come briefly into the public eye when one of his kite flights—a large kite carrying a meteorograph and ten other “helping-kites” supporting the tethering cable—broke free and, trailing 7 kilometers (4 miles) of cable, cut a narrow but astonishing swath through Paris, where it stopped traffic, disabled a train, and then “cut off all telegraphic communication with Rennes on the day when all France and most of the rest of the world were anxiously awaiting the result of the famous Dreyfus court-martial at that city.”17

  Under the circumstances, it seemed wise to Teisserenc de Bort to suspend kite flying for a while and return to a series of upper-air experiments he had begun in 1898 with free balloons. He used balloons of several designs but preferred working with those made of kerosened paper. These were filled with hydrogen and trailed a (very short) cable holding the meteorograph and a bag of sand ballast, which dribbled out at a steady rate to control the balloon’s rate of ascent. Teisserenc de Bort’s aim was to get these all up as high as possible—often over 11 kilometers (7 miles), and occasionally as high as 14 kilometers (9 miles). When the balloon reached its maximum altitude, the instrument pack was parachuted back to earth.

  When Teisserenc de Bort looked at the temperature records on flights that reached altitudes of 10 kilometers (6 miles) or greater, he noted that the recorded air temperatures failed to decrease with altitude, though theory predicted that they should. The measurement of the lapse rate of temperature with altitude, performed by James Glaisher (1809–1903) in the 1860s and corrected by Aßmann (with much fanfare) some years later, showed that the temperature should decrease by about 6°C (11°F) with each kilometer of ascent. If one followed the assumption of William Thompson (1824–1907) that the atmosphere was in convective equilibrium (and everyone did), one expected the temperature to decrease indefinitely with altitude until it approached the zero of temperature on the Kelvin scale (−273°C [−460°F]). In Teisserenc de Bort’s records, however, the temperature showed no such decrease between 10 and 14 kilometers (6 and 9 miles).

  Aßmann had measured the effect several times between 1894 and 1897 in manned balloon ascents, but he was reasonably sure that it was due to the warming of the instrument packet by solar radiation, and he was delighted to treat the phenomenon as another problem in instrument design—leading to an improved, ventilated housing for his thermograph. Hergesell agreed that the phenomenon was probably due to instrument warming, and so did Teisserenc de Bort; all of them tried various ways to shield their thermometers from reflecting or absorbing surfaces on the instrument packets that contained them, in order to keep the thermometers from false readings.18

  Teisserenc de Bort took the investigation further. He solved all the instrumental problems he could think of. No longer could the strange temperature recordings be blamed on frozen ink in the recording pens—his pens scribed directly onto smoked paper without the use of ink. Neither could the thermometer reading be blamed on warming by reradiation from the thermograph itself, since he had boxed the thermograph with cork and moved his thermometer outside the housing: he had never attributed the effect to direct solar radiation, but to reradiation from the instruments, from Earth itself, or from clouds. The thermometer fluid could not be a problem because there was none—his thermometer was a curved spring of two strips of dissimilar metals that expanded and contracted at different rates, moving the recording pen directly. Aware of the possibility of a radiation effect, he began to fly balloons at night. When these night flights also showed a steady temperature at around 11 kilometers (7 miles), he began a systematic program to see whether there was a seasonal variation—the “radiation effect” should be smaller in the winter than the summer if this were the answer to the subtle but persistent temperature anomalies.19

  Drawing of Teisserenc de Bort’s modified balloon meteorograph. The thermometer (B) was composed of two dissimilar metals that expanded differently with changes in temperature, attached to a stylus (J) that recorded data directly by scratching the smoked paper on the revolving drum. Air pressure was measured by the expansion and contraction of the “Bourdon tube” (A), attached to its own scribing apparatus (I). Teisserenc de Bort had moved the thermometer outside the housing to eliminate the possibility of heating within the box of the apparatus.

  The problem was difficult because the effect was elusive—the zone of steady temperature moved up and down between 8 and 12 kilometers (5 and 7 miles), though it was most often at 11 kilometers (6.8 miles), and it was stronger and weaker over a range of many degrees. It was clear that the predicted lapse rate was not consistent in what he came to call the “isothermal zone.” By 1900 he had the record of 146 ascents to report to the Académie des Sciences in Paris, but he still postponed discussion or conclusions concerning his ample measurements of temperatures at altitudes above 10 kilometers (6 miles), unsure as to the character of what he was seeing.20

  He worked for two more years on the problem, and in 1902, with an accumulation of 236 ascents to buttress his argument, he made the plunge and publicly rejected the prevailing theory of temperature decrease with altitude. He asserted the existence of an isothermal zone of varying thickness, in which the adiabatic lapse rate (of temperature with altitude) diminished to zero, usually at an altitude of 11 kilometers (7 miles), after which the temperature would remain constant for several kilometers. The layer was higher over anticyclones (high-pressure centers) and lower over cyclones (low-pressure centers). In consequence, he suggested, the problem of the general circulation of the atmosphere would have to be reconsidered, since such an isothermal layer meant that the atmosphere as a whole was not in convective equilibrium, but only that part bounded above by this “isothermal zone.”21

  It was an astounding and transforming discovery, though it remained far from the public eye. Aßmann realized immediately what he had missed and rushed to claim a share of it for himself, based on the analysis of six balloon ascents at Berlin in 1901; if he could not get ahead of the discovery, he could at least show that he had the relevant data himself even before Teisserenc de Bort’s announcement. He endorsed Teisserenc de Bort’s results but modified them by asserting that his own superior rubber-balloon technology, with a controlled rate of ascent and more sensitive recorders (both true claims), could prove that this was not simply an isothermal zone but a true “inversion zone”: the temperature not only stopped decreasing in this region but actually increased for several kilometers before decreasing again. Moreover, he thought he could detect (he was right) an upper boundary to the region of temperature shift in a layer at about 15 kilometers (9 miles), meaning that there was both a “lower inversion” and an “upper inversion.”22

  The discovery that the atmosphere was distinctly layered, with a permanent boundary layer near 11 kilometers (7 miles), was momentous. It required an entirely new approach to the study of atmospheric circulation and the energetics of storms, as well as a new (but by no means well-understood) picture of the vertical structure of the atmosphere. In the new picture there was, between the surface of Earth and the altitude where the lapse of temperature diminished to zero, a shell of turbulent air, soon named the “troposphere.” The term was Teisserenc de Bort’s, coined in 1908, and pleasing to the classically trained ear. It was the “sphere of change,” of rising and descending air, of precipitation and clouds; it was the zone of weather. Its upper boundary was a mobile surface, later named the “tropopause” (this was Aßmann’s “upper inversion”), and above that boundary surface was the “stratosphere,” a zone of stable or rising temperature without clouds, moisture, turbulence, or convective mixing and characterized by a laminar (flat and circumglobal) airflow: the atmosphere’s weather zone had thus a ceiling as well as a floor. Storm systems might now be seen as coherent masses of air trapped between the defining boundaries of Earth’s surface below and the tropopause above; that the latter was invisible made it no less real than the
former—it was a true barrier and a boundary surface.

  These events provided for Wegener and others of his generation an unparalleled opportunity. It meant that the atmosphere had to be “remapped” from pole to equator and through its full vertical height. No one knew how the tropopause changed with latitude, or how it differed over land and over the oceans, or even why it was there: was it a matter of temperature and pressure alone, or was it the result of a chemical differentiation of the atmosphere with altitude? A number of major theoretical predictions had been overthrown, and the real structure and dynamics of the atmosphere would now be decided by intensive field research—most of it in distant (polar or equatorial) locations. The bounded layers—no one knew how many—in the upper reaches of the atmosphere might even hold the key to the motion of storms and their sources of energy (the grail of nineteenth-century meteorology). In any case, meteorology now had a goal in which weather prediction formed only a subsidiary part: this was the development of a comprehensive physics of the atmosphere.

  Wegener’s Scientific Work at Lindenberg

  For Wegener, the discovery of the stratosphere was a momentous change—something big enough and far-reaching enough to make it seem that he had come into the science just as it was reaching its “modern form”—not its culmination, the sad fate of the planetary astronomy of his day, but that moment in the life of meteorology when it had moved past some threshold, some barrier, beyond which lay an ample freedom to change the overall structure of the science. Most scientists can point to such an event in their own training and early work, something that galvanized their community, generated new research opportunities, opened new vistas.