372 
and a maximum (about 200 km) late in summer [28; 
29, pp. 184-187]. In central Europe, the ring is much 
better developed to the east of the source during winter, 
+ SOUND HEARD 
- SOUND NOT HEARD 
200 KN. 400 
Fig. 8.—Observations of sound waves through the strato- 
sphere after explosion of 5000 kg of ammunition, December 
18, 1925. (After Hergesell and Duckert [16].) 
to the west during summer. This is due to the fact that 
the rays have rather large angles of incidence (normally 
between 75° and 90°) so that at the source sin 7 is 
“Ligurian Sea 
Fig. 9.—Sound intensities observed 
THE UPPER ATMOSPHERE 
usually larger than 0.98. If the temperature at the point 
of observation is more than about 10C higher than atthe 
source (continent in summer, ocean in winter), it be- 
comes increasingly probable that this difference makes 
the return of the ray to the ground impossible; the 
rays turn upward again at a level with a temperature 
given by equation (10). Similar effects can be produced 
by prevailing winds (equation (19)). 
Records show relatively small differences in travel 
time of rays to the abnormal zone between day and 
night [19], although details of the records change grad- 
ually (Fig. 116). Wide variations in the amount of 
explosive do not result in different travel times. This 
proves that the return of the rays to the ground is not 
affected by an increase in the ratio of the pressure 
change to the pressure itself and that, even at the 
highest levels reached by the rays, equation (7) holds. 
This conclusion has been confirmed by the agreement 
between the temperatures calculated from equation (7) 
and those found from records made with V-2 experi- 
ments (Fig. 12). Finally, records taken at equidistant 
points in opposite directions from the source did not 
give appreciably different travel times. Therefore, the 
conclusion can be drawn that relatively high tempera- 
tures in the stratosphere account for the sound in the 
outer zones, as suggested by Whipple [30]. 
TT e enn, 
+ » 
5 
c, 
after an explosion near Vergiate, Italy. (After Oddone.) 
