90 
PACIFIC SCIENCE, Vol. II, April, 1948 
lower daytime speeds over the land than over 
the ocean (Kaneohe). 
These observations indicate that the diurnal 
speed changes cannot be attributed to the sea- 
land-breeze regime alone. The effect of sea- 
valley and land-mountain winds is apparendy 
strongest in the vicinity of the Waianae Range. 
This is particularly apparent by the comparison 
of Waipahu and Aiea. Part of the explanation 
probably lies in the fact that the Waianae 
Range, in the rain shadow and much drier than 
the Koolau Range, is covered in its lower 
reaches by a more sparse and xerophytic vege¬ 
tation cover than are the moist foothills of the 
Koolau. This could give rise to greater heating 
and cooling effects on the lower layers of air. 
The higher wind speed during the day is 
ordinarily attributed to transfer of momentum 
from higher wind speeds aloft to lower levels 
through increased daytime mixing as a result 
of greater instability. In the case of the island 
stations, a very stable layer is produced at night 
immediately above the ground surface as a result 
of radiative heat loss. The development of a 
nocturnal surface temperature inversion is ap¬ 
parent in many individual Honolulu soundings 3 
and shows up in the mean sounding for January 
presented in Figure 5. Obviously this near¬ 
surface stability will be much more pronounced 
over land than over water, and explains the large 
reduction of nocturnal speeds over the land. 
The depth of the layer of low nocturnal 
speeds also points to stability as the explanation. 
Figure 6 shows the wind speeds at the four pibal 
8 A sounding is a measurement of pressure, tempera¬ 
ture, and relative humidity aloft. It is plotted in the 
form of a graph of temperature vs. pressure as in Fig¬ 
ure 5. Relative humidity at various pressures is usu¬ 
ally entered on the graph. A sounding is taken by 
release of a small radio transmitter (radiosonde) 
carried aloft by a balloon filled with helium. Signals 
indicating the three types of measurements are trans¬ 
mitted by the radio and picked up by a radio receiver 
on the ground. The words "radiosonde observation’' 
are sometimes abbreviated to "raob.” Such observa¬ 
tions are made twice daily by the Weather Bureau at 
Honolulu. 
or rawin 4 scheduled times at Honolulu. It is 
apparent that the diurnal change aloft is smaller 
but in opposite phase to that of surface wind. 
The mean thickness of the ground inversion is 
roughly the same as the depth through which 
the large diurnal wind-speed changes occur. 
FIG. 5 . Soundings above Honolulu. 
Since the strong diurnal change in speed is 
measured at all stations, even at Wheeler Field 
(elevation 850 feet), and is observed at still 
higher localities in central Oahu, it appears that 
the nighttime speed reduction is a near-surface 
phenomenon. It must occur in a layer some¬ 
what less than 500 feet thick, just above the 
general land surface regardless of the topog¬ 
raphy. Insufficient observations are available to 
determine whether this is true for mountain 
crests. 
4 "Pibal” is the abbreviation for pilot balloon. 
"Rawin'' means upper wind observations by means 
of radar. In each case, a balloon filled with helium is 
released, rising at a constant rate. Its position in space 
is tracked by observing it from the ground, through a 
telescope in the case of a pilot balloon, or by a radar 
set in case of a rawin. By plotting its position at 
evenly spaced time intervals, the movement can be 
computed and thus the wind speed and direction at 
each level can be determined. Upper air winds are 
measured four times each day by the Weather Bureau 
at Honolulu, twice by pilot balloon (visual tracking 
of the balloon), and twice by rawin. 
