126 
PACIFIC SCIENCE, Vol. XXI, January 1967 i 
existing instrument locations indicates two dis- 
tinct travel paths : the high elevation path, 
from Mauna Loa to Desert/Uwekahuna, and 
the lower elevation path, from Desert/Uweka- 
huna to Ahua. Apparent velocity for the high 
elevation path is 0.33 km/sec, which, on the 
basis of the slope distance of 12.6 km, results 
in a ground velocity of 0.35 km/sec. These 
results are consistent with sonic velocities in air 
(Chemical Rubber Publishing Co., 1947:1928) 
of 0.33 km/sec at 0° C, and 0.34 km/sec at 
20° C. Similarly, the lower elevation wave front 
path yields an apparent velocity of 0.37 km/sec, 
a condition observed at ambient temperatures of 
about 70° C. 
Blumenstock (1961) summarized weather 
data collected in Hawaii and concluded that the 
winter mean temperature was 20° at the Na- 
tional Park housing area (see Figure 1) and 
that it decreased 2° for every 1,000 ft of eleva- 
tion. He observed the remarkably "equable tem- 
perature conditions" in Hawaii — that is, the 
small range between winter and summer means 
at any one observation point — but he also 
stressed the great variations in temperature and 
in rainfall caused by very local topographic 
situations. 
Additional, near-surface temperature vari- 
ables which may . bear on our present problem 
are the diurnal temperature and wind-direction 
patterns. In table 3, the 19 and 20 December 
1961 figures represent average traveltimes for 
a large number of events in each of two groups. 
The 19 December events occurred at night; the 
20 December events occurred during the day- 
time. The apparent velocities which occurred in 
the two events are: 
(1) Mauna Loa to Desert (12.6 km slope 
distance): 35.9 sec, or 0.351 km/sec for the 
evening events; 36.4 sec, or 0.347 km/sec for 
the daytime events. 
(2) Desert to Ahua (6.4 km distance) : 
17.6 sec, or 0.364 km/sec for the evening 
events; 17.8 sec, or 0.360 km/sec for the day- 
time events. 
As shown above, the gross velocity increases 
as the sound front moves from the slopes of 
Mauna Loa to the flatter terrain at the summit 
and flank of Kilauea, and the velocities are 
systematically lower in the daytime than at 
night. At one atmosphere pressure, the velocity 
of sound in air increases 0.012 km/sec between 
0° C and 20° C. Therefore, the natural expecta- 
tion would be the reverse of our findings, i.e., 
velocities expectedly would be slightly greater 
during the daytime than during the evening, 
when temperatures are lower. Again, Blumen- j 
stock’s findings (1961:6) can be invoked for ; 
a mechanism which might explain this seeming i 
contradiction: "The usual regime is to have up- 
slope winds by day and downslope winds by 
night.” In our situation downslope winds * 
(nighttime) would augment velocities across 
our recording range; upslope winds (daytime) 
would provide relative decreases in apparent 
velocities. The velocity increase we seek to ex- 
plain by this mechanism is 4 m/sec or about 
8 miles/hr — a modest windspeed vector which 1 
is not unrealistic. 
Thus, some of the diurnal changes in sonic ? 
traveltime shown in Table 3 can be explained 
by assumptions of expectable change due to 
diurnal wind-velocity conditions. However, 
such changes can be only partially responsible 
for the difference between apparent velocity !i 
over the Mauna Loa-to-Desert leg and that over ; 
the Desert-to-Ahua leg. As we have pointed 1 
out, such an assumption would require an un- j 
realistic ambient temperature of 70° C for the 
low-elevation, high-velocity segment. 
ANGLE OF INCIDENCE OVER THE 
RECORDING RANGE 
t 
It has been demonstrated above that the sonic ! 
travel path from Mauna Loa station down to 
Desert and Uwekahuna fits into a reasonable 
model for sound-wave rays moving parallel to ‘ 
the ground across that particular path. By con- I 
trast, the lower elevation segment of the record- 
ing range — that between Desert and Ahua — 
offers evidence of increased velocity which | 
cannot be explained by temperature alone. It 
might be explained by a favorable component ; 
of wind velocity, but that would require wind 
velocities in excess of 50 mph, a condition rarely ! 
observed in Hawaii. The best situation pro- 
ducing such a velocity increase, as well as one,; 
which would also provide for energy focusing, j 
would be encountered if the sonic rays impinged 
upon the low-elevation stations at a steeper angle 
of incidence. 
