. Compendium of meteorology. Meteorology. 374 THE UPPER ATMOSPHERE from selected observed values of the travel-time curve considering the calculated angles of incidence at the ground. This results in a travel-time curve for rays SOUND VELOCITY (M/SEC.) 320 340 --^°ilE«._Eo: ,_, ^^, I. -20 0 20 TEMPERATURE (°C) Fig. 13.—Temperature and sound velocity in the atmosphere from various sources. at the level where the temperature observations end. Equation (18) then permits the calculation of the high- est point H* above the level of reference reached by a (Fig. 10) and of the travel times (Fig. 12)
. Compendium of meteorology. Meteorology. 374 THE UPPER ATMOSPHERE from selected observed values of the travel-time curve considering the calculated angles of incidence at the ground. This results in a travel-time curve for rays SOUND VELOCITY (M/SEC.) 320 340 --^°ilE«._Eo: ,_, ^^, I. -20 0 20 TEMPERATURE (°C) Fig. 13.—Temperature and sound velocity in the atmosphere from various sources. at the level where the temperature observations end. Equation (18) then permits the calculation of the high- est point H* above the level of reference reached by a (Fig. 10) and of the travel times (Fig. 12) is affected by the yearly period of the wind, but it is caused mainly by the annual period of the temperature (and thus by the similar period of the ozone content) at elevations between about 25 and 60 km. Annual changes in the direction of the wind near and above the tropopause shift the whole zone in the direction of the prevailing wind, but cannot explain the annual expansion and contraction of the zones which has been observed. Observations of the amplitudes of sound waves through the stratosphere are very scarce. The maximum near the inner boundary of the abnormal zone results from the concentration of energy connected with the cusp of the travel-time curve at the minimum distance (Fig. 14) reached by the rays [11], where the very large values of di/dA produce a focal point according to equation (32). The intensity of the sound waves there may be so large that ^^'indowpanes are broken [14]. The records of the Helgoland explosion [4] showed good agreement between the observed sound-intensity and the energy calculated from equation (32). How- ever, Cox [4] has pointed out that the decrease in short waves relative to the long waves with increasing dis- tance (to be expected from the increase in absorption, see Fig. 3) is not confirmed by these records. The differences in recorded periods are relatively small and may be a consequence of local effects. In very large explos
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