. Compendium of meteorology. Meteorology. Fig. 10.âYearly period of the distance from the source at which the first ring of audibilitj- begins (circles and thin curve) and of the distance of the maximum number of reported obser- vations (dots and thick curve) based on data collected by Wegener [28, 29]. in the speed and direction of the wind, or (2) an increase in temperature above the value near the ground at ele- vations varying, in central Europe, between about 30 km in late winter and 40 km in late summer. Either (1) (a) ' 2 SEC.' ALIENLUNNE 98,43 km UFFELN 128,75 km. l^/|J^I^^A^(W^â¢'^'V&
. Compendium of meteorology. Meteorology. Fig. 10.âYearly period of the distance from the source at which the first ring of audibilitj- begins (circles and thin curve) and of the distance of the maximum number of reported obser- vations (dots and thick curve) based on data collected by Wegener [28, 29]. in the speed and direction of the wind, or (2) an increase in temperature above the value near the ground at ele- vations varying, in central Europe, between about 30 km in late winter and 40 km in late summer. Either (1) (a) ' 2 SEC.' ALIENLUNNE 98,43 km UFFELN 128,75 km. l^/|J^I^^A^(W^â¢'^'V>-V^ i 3 S 79 iiSlii i It eS 0Z168H2 ^ghgm^^s Fig. 11a.âDirect sound waves, recorded by Kiihl's undograph, showing dispersion. (After Schulze [27].) or (2) or both operating jointly may produce this effect. Since the travel times for the rays arriving in the second abnormal zone were always close to twice the travel time at half the distance in the first zone (Fig. 12) it was concluded that the second ring is produced by rays reflected at the surface of the earth [11, 29]. Similarly, the following zone (travel-time curve c in Fig. 12) is probably due to twice-reflected waves. Still later phases may be due to waves which left the ground under too small an angle of incidence to be turned back in the warm layer near 55 km, passed upward into the colder layer above, and finally were turned back (Fig. 14) in the layers at an elevation about 100 km [5], where the temperature increases beyond that near 55 km. Calculations of the temperature in the part of the stratosphere in which the sound waves are turned down to produce the first abnormal zone can be made as follows [10, 11]. First, equation (7) is used to calculate the sound velocity in the troposphere, and possibly in the lower part of the stratosphere. Equation (17) gives the angles of incidence corresponding to the travel-time curve [b in Fig. 12); usually they change little with distance. Equation (14) gives the horizont
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