294. RADIO WAVE PROPAGATION EXPERIMENTS 
Since, presumably, y is the only quantity in this 
equation which is a function of p, and since y = 0 for 
p = 0, the plot of F against p extrapolated to zero 
humidity will yield a value of Qs. Consequently, y is 
determined as a function of p. 
Examples of the y versus p curves so obtained are 
shown in Figure 18 for the wavelengths 0.96, 1.16, 
1.28, and 1.69 cm. Data were also taken at the wave- 
% OB PER NAUTICAL MILE 
ING PERCU M 
Figure 18. Attenuation in water vapor. 
lengths 1.06, 1.31, 1.37, and 1.49 cm, These lines are 
all concave upward with the exception of those at 
1.28, 1.31, and 1.37 cm. There is some evidence that 
the line at 1.31 cm is.concave downward, while within 
experimental error the 1.28- and 1.37-cm lines are 
straight. 
The curvature is surprising, since it was believed 
that y would be proportional to p. The reason for the 
curvature is not understood, and it is possible that it 
arises from some systematic experimental error. How- 
ever, it is difficult to conceive of a systematic error 
which disappears at resonance. 
Because of this curvature, it is not possible to draw 
a single attenuation curve showing absorption as a 
function of wavelength for all humidities. Figure 19 
shows the variation with wavelength of the attenuation 
coefficient y/p in decibels per nautical mile per gram 
of water vapor per cubic meter for humidities of 10 g 
per cubic meter and of 50 g per cubic meter. It is to 
be noted that the peak of this curve,.at 1.32 cm, is 
very close to the standard K-band wavelength. 
These experimental results are in agreement with 
other results for the water vapor attenuation at K 
band. Furthermore, for all practical radar purposes, 
0.05, 
AL | 
BN 
0,04) 
0.01 
AIN CM 
Ficure 19. Attenuation coefficient, water vapor, 45 C. 
and within the range of the measurements, they are 
in agreement with Van Vleck’s theory of the absorp- 
tion of this water vapor line. 
Discussion 
Comments were made on the great accuracy of the 
experiment, pointing out that the quantity being 
measured was extremely small and that other experi- 
ments, particularly one made in Florida by measure- 
ment of the sky temperature, were leading to substan- 
tial agreement with the present findings. The best 
available data on the performance of K-band radars 
supported the experimental result obtained and would 
be of great help in choosing wavelengths for radar and 
other apparatus in the future. 
The good agreement between theory and the experi- 
mental result achieved was stressed. Whereas in infra- 
red absorption measurements the results had disagreed 
with theory by as much as 10 db or more, the discrep- 
ancy in these results amounted to a few per cent only. 
It was noted that from the purely physical standpoint 
this water vapor line and the 0.5-em oxygen absorption 
line were the most carefully investigated lines in the 
spectrum, aside from some lines in the visible region. 
The explanation was given that the microwave meas- 
urements were much more instructive than the optical 
ones from the standpoint of the collision broadening 
theory because the width of the microwave absorption 
line was comparable to the frequency of the radiation. 
This results in a shape factor or line form which can 
be studied in detail. The reported dependence on den- 
