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temperature had only extended far enough in altitude to show a general temperature decline with hints that it might be approaching a constant value. It was generally assumed either that the temperature was constant above these altitudes or that, above this isothermal layer, the temperature contin- ued to decrease.
A more palatable option than an increase in temperature at high altitudes was the idea of a “hydrogen atmosphere.” It made perfect sense that lighter gases in the atmosphere would rise and heavier gases would sink. The upper atmos- phere should be rich in hydrogen7. Since the sound speed in hydrogen is much higher than in air, this wonderfully plausi- ble hydrogen atmosphere should refract sound downward from extreme altitudes.
The hydrogen atmosphere was doomed, though. In 1918 F. J. W. Whipple wrote, “The beautiful theory of von dem Borne that explains the existence of the outer zone of audi- bility by the refraction of the sound-rays on reaching the regions in which hydrogen is the principal constituent of the atmosphere...has been severely criticized on the ground that the transmission of sufficient energy through such a diffuse medium back into the lower atmosphere was impossible. [Also] the tendency of the outer zone of audibility to lie on one side of the source is a recognized difficulty in the way of this theory. Obviously it can offer no explanation of the shift- ing of the zone from season to season.”
Furthermore, the kinetic theory of gases was reaching maturity and Sir James Jeans showed that the molecular velocities in hydrogen were high enough that hydrogen would escape rapidly into space instead of accumulating at the top of the atmosphere.
Whipple did not realize that the hydrogen atmosphere itself was untenable. Nonetheless, he did propose an alternate solution: “In the absence of any other hypothesis, it may be suggested that the winds at great heights, say 20 km or more, may be sufficiently strong and sufficiently regular to cause the observed effects. It is known that wind-speed generally reaches a maximum at the base of the stratosphere and falls off rapidly above that limit, but it is possible that there is a régime of much stronger winds at higher levels and that they blow with the regularity of the Monsoons.” As it turns out, he was half right. Seasonally reversing winds in the stratosphere do produce a seasonal variation in the zones of audibility but this is coupled with an increase in temperature at high alti- tude. Wind would produce zones of audibility only in the direction toward which the winds were blowing. The observed zones of audibility often extended in full or almost full rings around the source so wind could not be the sole agent of refraction.
Although it was the least “reasonable” of the mechanisms for high-altitude refraction, it seemed that a high-altitude temperature increase was the only mechanism compatible with the observations. What could produce such a tempera- ture increase?
In 1923, F. A. Lindemann and G. M. B. Dobson pub- lished their predictions of the temperature and density of the upper atmosphere based on observation of visible meteor tracks: “...existing observations enable us to say with consid- erable certainty that the density at heights above 65 km is
very much higher than is commonly supposed, and that the temperature must increase from its value of something like 220° [K] at heights between 12 and 50 km., to something like 300° [K] at these heights...”
Edward Gowan followed with a paper in 1928 suggesting that ozone was the cause of this upper atmosphere tempera- ture increase: “Lindemann and Dobson have calculated the density of the atmosphere from observations of meteors. From the abnormally high values obtained they conclude that the temperature above 60 km is of the order of 300° K...The author’s [Gowan’s] suggested explanation of the occurrence of a high temperature at such heights was the strong absorption of solar energy in the ozone8 that has been observed. Whipple refers to the zones of audibility that occur at some distance around big explosions. Assuming a reason- ably sharp transition of temperature from 220° to 280° K at a height of about 60 km., he makes a rough estimate of the minimum radius of the outer zone audibility. This agrees well with observations that have been made.”
A decade later, Gutenburg gave one of the strongest endorsements of the role of acoustics in upper atmosphere research: “The best information regarding the temperature between 30 and 60 km is derived from observations that give the velocity of sound waves at these altitudes. [However] at higher levels the absorption of sound increases so fast with elevation that the usefulness of sound waves for temperature determina- tions decreases rapidly above the level of 60 to 70 km.”
Gutenburg continues: “Many attempts have been made to explain the high temperatures at heights between about 40 and 60 km...there is little doubt that absorption of the solar radiation by ozone and water vapor play the major roles in
Fig. 11. Once refraction was understood, the propagation of sound became an impor- tant method for probing the upper atmosphere. This graph shows the variation in temperature with altitude inferred from measurements of sound in Northern Germany (adapted from Gutenberg, 1946). The winter profile is shown in blue; the summer profile in red. At the time of Gutenberg’s work, the variation in temperature above 60 km was conjecture. Later work verified the decrease from 60 to 80 km and a subsequent increase in temperature above 80 km. The temperature profiles from the ground to 10 km were determined from balloon measurements.
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