Sea Temperatures- — 1EIPPER and ANDERSON 
231 
Figure 23 shows the average temperature- 
depth curves for two specific points. The 
curves labeled "H” are taken from H cruise 
data. These cruises consisted of 386 bathy- 
thermograph observations from March, 1944, 
to February, 1945. All data were collected 
at 21° 18'' N and 157° 53' W, approximately 
0.75 mile south southwest of Honolulu in 
100 feet of water. These data were averaged 
together by months with the results as shown 
in the figure. The curves labeled "Q” are taken 
from Q cruise data. These cruises consisted 
of 427 ba thy thermograms from February, 
1944, to February, 1945. The station was 
located at 21° 18' N and 157° 58' W about 
1.75 miles due south of the entrance to Pearl 
Harbor in about 600 feet of water. 
These two stations are located approx- 
imately 5 miles apart, and the data were taken 
during the same year. It is interesting to note 
that during the months of March and April 
the average curves were identical, but during 
the other months the H cruise data showed 
appreciably lower temperature at all depths. 
Typical bathy thermo grams 
Figure 24 contains copies of actual bathy- 
thermograms selected as typical of a northern 
and a southern portion of the area under 
study for the months of March, June, Sep- 
tember, and December. These single records 
show several prominent features which are 
not apparent in curves drawn through average 
temperatures at selected depths. One of these 
features is the deep layer of nearly isothermal 
water which is present on several of the in- 
dividual curves. Another is the strong then 
mocline. Each of these characteristics is typ- 
ical of nearly all individual bathythermo- 
grams in this area but the process of aver- 
aging and the decreasing frequency of tem- 
perature readings with depth causes them to 
disappear from the average temperature-depth 
curves. Thus it is desirable to consider typ- 
ical single observations as well as average 
curves. 
Depth of the virtually isothermal layer 
The average depth of the virtually isother- 
mal layer for summer and winter periods is 
shown in Figure 25. The charts were pre- 
pared by reading from the bathythermograms 
the layer depth to the nearest foot and aver- 
aging the results by 1 -degree quadrangles. 
These data were then plotted, and isolines of 
depth were drawn. The virtually isothermal 
layer is, on the average, deeper than the 180- 
foot maximum depth for the 'shallow 5 ' bathy- 
thermograph; hence, the "deep” bathyther- 
mographs are the only ones that reach the 
therrnocline. As a result, only 40 per cent of 
the total number of bathythermograms avail- 
able for this study were used in establishing 
the layer depth. Two conclusions may be 
drawn from Figure 25: the layer depth varies 
considerably with position; and during the 
winter the depth is greater than during the 
summer. 
An attempt was made to draw layer-depth 
charts for each month of the year. Although 
many observations were available, the vari- 
ability within any given month and any given 
quadrangle was so great that it was not pos- 
sible to draw isolines that exhibited good 
continuity. 
Diurnal and annual variation of temperature 
In August, 1941, the U.S.S. "Pyro” took 
bathythermograph observations every hour in 
the area 21° to 34° N and 138° to 157° W. 
These data were analyzed by H. U. Sverdrup 
and others to determine the diurnal variation 
of temperature. Figure 26 shows the results 
of this analysis. 
The wind-stirred layer at the time of these 
observations was approximately 120 feet thick 
with 75° F. as the average temperature of the 
layer. Between the surface and 120 feet the 
temperatures were read at 30- foot intervals. 
The temperature differences between 120 feet 
and each other level were determined and 
plotted. The figure shows that the upper 
layer is nearly isothermal during the early 
