Mteroscopte Structures of Wind Waves 
latter technique is desirable especially when Miles' calculation has 
not been verified experimentally. 
VII. COMPARISON OF LABORATORY AND OCEANIC RESULTS 
The average wavelengths obtained at various wind velocities 
are replotted in figure 19a. As the wind velocity increases, the wa- 
ves, as described previously, passing the following stages of develop- 
ment : (A) infinitesimal capillary waves, (B) rhombic wave cells, 
(C) long waves accompanied by parasitic capillaries, and (D) brea- 
king long waves. The mean-square surface slope determined at va- 
rious wind velocities are replotted in figure 19b. Taking together 
the results presented in figure 19a, b, we see that capillary waves 
with infinitesimal amplitudes are the sole contributor to mean-square 
slope in stage (A), gravity waves are the sole contributor in stage 
(B), and both contributors in stage (C) and (D) 
Because of the great difference between wind fetches exis- 
ting in the wind-wave tank and the field, the shear velocity rather 
than the wind velocity should provide a basis for comparison of slope 
data. The upwind-downwind components of Cox and Munk's data and 
our laboratory results of the same components are replotted in figu- 
re 20a. This comparison is made possible on the basis (Phillips 
1958b) that high-frequency wind waves, the principal contributor to 
surface slopes, reach equilibrium states at very short fetches. Such 
a concept is further illustrated by Cox's (1958) measurements of 
mean-square slopes, which reach equilibrium states, ceasing to grow 
spatially, at a fetch slightly greater than 3 m. The wind fetch for the 
present experiment is about 6 m, 
It has been shown that oceanic slope data are divided into 
two groups : gravity waves are the sole contributor to sea-surface 
slope at low wind velocities and both gravity and capillary waves con- 
tribute to sea-surface slope at high wind velocities. The portion of 
the oceanic data fitted by a straightline in figure 20a is the second 
group. A straightline is also drawn to fit the laboratory data in figu- 
re 20a. It is interesting to see that the fitted line for the laboratory 
data which contributions from both gravity and capillary waves is 
parallel with the line fitted through the oceanic data which the same 
contributors; see figure 19, 
The same trend of variation between the oceanic and the 
laboratory data further confirms our earlier discussion : the separa- 
tion of oceanic slope data into two groups is indeed due to the fact 
that capillary waves contribute to mean-square sea-surface slope 
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