270 ; RADIO WAVE PROPAGATION EXPERIMENTS 
assumed that ideal conditions prevailed throughout the 
rains under consideration. By this was meant that the 
same sample of rain falling, say, over an area of 1 sqm 
would be found anywhere inside the area covered by 
the rain. Such ideal rains seem to be rather simple 
theoretical models. Considerable fluctuations in the 
rate of rainfall over zelatively short distances (1 km 
or less) have been reported.*° These spatial irregular- 
ities of rains exclude any simple interpretation of the 
experimental data on rain attenuations. The computed 
values of attenuation are based on a few data on drop 
size distributions” in rains. 
In Figure 1,? the individual oxygen and water 
vapor attenuation curves have been plotted in the 0.2- 
to 10-cm wavelength range, using the most acceptable 
data available on the position of line centers and line 
widths. Any change in the water vapor content from 
the one adopted for this graph (7.5 g/m* of air or 6.2 
g per kilogram of air) or the total pressure can be 
taken rapidly into account in computing the combined 
oxygen and water vapor attenuations, since the atten- 
uation values are proportional to the partial pressures 
of oxygen and water vapor. For practical purposes the 
effect of atmospheric temperature variations can be 
neglected. 
20.0 
10.0 
5.0 
ABSORPTION IN DB PER KM 
3.0 60 90 15 30 
FREQUENCY IN 105 MC——+ 
60 90 150 
10 543 215 106 050403 02 
—=———-/ IN GM 
Ficure 1. Oxygen and water vapor absorption versus 
wavelength. (1) Absorption due to water vapor in an 
atmosphere at 76-cm pressure containing 1 per cent 
water molecules, or 7.5 g per cu m. The water resonance 
line is assumed to be at 24,000 mr, and its half-width at 
half maximum (line breadth) i is 3, 000 me. (2) Absorp- 
tion due to oxygen in an atmosphere at 76-cm pressure, 
whose resonance band at 60-108 me is supposed to have 
a line breadth of 600 me. 
15.0 
10.0 
7.0 
5.0 
3.0 
2.0 
50 90 50 3024 I5 10 60 3.0 
<t— FREQUENCY IN 10° MG 
02 03 O7 25 2 3 Elo we doa0 co wo 
A1N CM —e 
15 10 0.6 0.3 
Ficurr 2. Atmospheric attenuation for one-way trans- 
mission. (1) Oxygen and water vapor (total p=76 cm 
Hg, 1=20 C, water vapor=7.5 g per cu m). (Van 
Vleck). (2) Moderate rain (6 mm per hr) of known drop 
size distribution. (3) Heavy rain (22 mm per hr). (4) 
Rain of cloudburst proportion (43 mm.per hr). 
In Figure 2 is plotted the totai (oxygen plus water 
vapor) attenuation (curve 1) in an atmosphere at 
76-cm pressure with the same water vapor content 
as the water curve of Figure 1. Curves 2, 3, and 4 are 
rain attenuation curves computed for a moderate rain 
(rate of rainfall 6mm per hour), a heavy rain (22mm 
per hour), and an excessive rain of cloud burst pro- 
portion (43 mm per hour). The corresponding drop 
size distributions were given by Best.** 
In any rain the resulting total attenuation is the 
sum of the gaseous (oxygen plus water vapor) and 
corpuscle or liquid drop attenuation values. 
It is thus seen that for waves of 3 cm or shorter the 
rain attenuation may become prohibitive, whereas the 
gaseous attenuation loses its practical importance at 
waves longer than about 2 cm. The attenuation of rain 
computed in this report extends from 5 cm toward 
longer waves. In the region A = 1.25—5 em, only 
two attenuation values are available,’? at 1.25 and 3 
em respectively. The dashed portions of the rain atten- 
uation curves are thus extrapolations drawn through 
the two computed points. The shape of these extra- 
polated portions of the curves, in view of the decreas- 
ing trend of the computed dielectric absorption values 
