Page 14 - Spring 2006

Fig. 6. In the daytime, the temperature normally decreases with altitude. This causes sound to be refracted upward, which reduces the sound level for an observ- er on the ground. Although the effects of wind often overwhelm those of tempera- ture with respect to refraction, when winds are light, the daytime temperature decrease can cause unexpectedly low sound levels from distant sources.
ment... In consequence, however, of the rotatory motion of the balloon... the valve-line had become entangled, and [Mr. Coxwell] had to leave the car and mount into the ring to readjust it. I then looked at the barometer, and found its read- ing to be 9 3/4 in., still decreasing fast, implying a height exceeding 29,000 feet.
“Shortly after I laid my arm upon the table, possessed of its full vigour, but on being desirous of using it I found it powerless...trying to move the other arm, I found it power- less, also...then I fell backwards, my back resting against the side of the car and my head on its edge...I suddenly became unconscious as on going to sleep...Mr. Coxwell told me that while in the ring he felt it piercingly cold, that hoarfrost was all round the neck of the balloon, and that on attempting to leave the ring he found his hands frozen. He had, therefore, to place his arms on the ring, and drop down...when he felt insensibility coming over him too, he became anxious to
mental characteristic of the atmosphere.
In 1749, Alexander Wilson, Professor of Practical
Astronomy at the University of Glasgow, suggested a direct measurement: “Mr. Wilson proposed...to explore the tem- perature of the atmosphere in the higher regions, by raising a number of paper kites, one above another, upon the same line with thermometers appended...Upon launching these kites...the uppermost one ascended to an amazing height, disappearing at times among the white summer clouds...To obtain the information they wanted, they contrived that ther- mometers, properly secured, and having bushy tossels of paper tied to them, should be let fall at stated periods from some of the higher kites; that was accomplished by the grad- ual singeing of a match-line.” Success depended on recover- ing and reading the thermometers quickly. Considering the difficulty of interpreting the thermometer readings after hav- ing fallen back to Earth, it is not surprising that Wilson did not publish the results of these measurements.
A century after Wilson’s kite experiments there was a dramatic increase in manned balloon flights for scientific observation. Large fabric balloons filled with “carbureted hydrogen” (methane) lifted scientists and their instruments to tens of thousands of feet altitude. Although the instru- mentation was crude by today’s standards, the investigators built sun shields to reduce direct solar heating of their ther- mometers and they forced air past the thermometers to has- ten their response to changes in temperature. Temperature measurements from these flights convinced Reynolds that temperature-induced refraction might be possible.
Without oxygen, these aeronauts measured temperature, pressure, and humidity to remarkable altitudes. The British Association for the Advancement of Science funded a series of balloon flights, the first few with John Welsh as the science observer. Then, James Glaisher flew a longer series of more ambitious flights. In fact, “ambitious” hardly does them jus- tice. With Mr. Coxwell as pilot, Glaisher ascended from Wolverhampton on September 5, 1862: “Discharging sand, we... attained the altitude of five miles, and the temperature had passed below zero... we ascended still higher... and I also found a difficulty in seeing clearly... I could not see the column of mercury in the wet-bulb thermometer, nor the hands of the watch, nor the fine divisions on any instru-
Fig. 7. Before the development of radio telemetry, much of the information about temperature, humidity, and wind speed in the atmosphere came from scientific bal- loon flights. Some of these expeditions were harrowing. James Glaisher’s flight on September 5, 1862 probably exceeded 30 000 feet and nearly killed Glaisher and Coxwell, the pilot. However, data from these 19th century balloon flights came at the right time to support the concept of refraction of sound. In a curious reversal, once atmospheric measurements established refraction as an important effect, sound was later used to probe the upper atmosphere determining both temperature and wind speed at altitudes well in excess of even unmanned balloon flights. (From Glaisher’s Travels in the Air.)
12 Acoustics Today, April 2006