Stratosphere

Teisserenc de Bort designed and built meteorological recording instruments and sent them aloft on balloons and kites of his own design and 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 of cable, cut a narrow but astonishing swath through Paris, where it stopped traffic, disabled a train, and 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.

Under the circumstances, de Bort suspended kite flying for a while and return to a series of upper-air experiments with free balloons. He 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. de Bort's aim was to get these all up as high as possible, often over 11 kilometers and occasionally as high as 14 kilometers. When the balloon reached its maximum altitude, the instrument pack was parachuted back to earth.

The measurement performed by James Glaisher in the 1860s and corrected by Aßmann some years later showed that the temperature should decrease by about 6 C with each kilometer of ascent. But when de Bort looked at the temperature records on flights that reached altitudes of 10 kilometers or greater, he noted that the recorded air temperatures failed to decrease with altitude. In de Bort's records, the temperature showed no such decrease between 10 and 14 kilometers.

Aßmann had measured this effect several times between 1894 and 1897 in manned balloon ascents, but he was reasonably sure that it was due to the warming of instrument packet by solar radiation. He was delighted to treat the phenomenon as another problem in instrument design. Hergesell agreed that the phenomenon was probably due to instrument warming, and so did 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.

De Bort took the investigation further. He solved all the instrumental problems he could think of. Aware of the possibility of a radiation effect, he began to fly balloons at night. When these night flight also showed a steady temperature around 11 kilometers, 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.

The problem was difficult because the effect was elusive. The zone of steady temperature moved up and down between 8 - 12 kilometers, though it was most often at 11 kilometers. It was stronger and weaker over a range of many degrees. By 1900, he had the record of 146 ascents to report to the Academia des Sciences in Paris, but he still postponed conclusions concerning his measurements of temperatures at altitudes above 10 kilometers, unsure as to the character of what he was seeing.

He worked for two more years on the problem, and in 1902, with an accumulation of 236 ascents to support 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 with varying thickness, in which the adiabatic lapse rate (of temperature with altitude) diminished to zero, usually at an altitude of 11 kilometers after which the temperature would remain constant for several kilometers. The layer was higher over high-pressure centers (anticyclones) and lower over low-pressure centers (cyclones). He suggested the problem of the general circulation of the atmosphere would have to be reconsidered, such such an isothermal layer meant the atmosphere as a whole was not in convective equilibrium, but only that part bounded above by this "isothermal zone".

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. He endorsed 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 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 an upper boundary to the region of temperature shift in a layer at about 15 kilometers, meaning that there was both a "lower inversion" and an "upper inversion".

The discovery that the atmosphere was distinctly layered, with a permanent boundary layer near 11 kilometers was momentous. It required a new 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. This therm was de Bort's, coined in 1908. The "sphere of change", of rising and descending air, of precipitation and clouds. It was the zone of weather. Its upper bound was a mobile surface, later named the "tropopause". Above that boundary surface was the "stratosphere", a zone of stable or rising temperature, without clouds, moisture, turbulence, or convective mixing. It characterized by a laminar airflow.