280 RADIO WAVE PROPAGATION EXPERIMENTS 
Tasiz 6. Drop size distributions in rains. 
Number of drops/m?/sec in nine different types of rain 
D, cm 1 2 3 4 5 6 7 8 9 
0.05 1,000 1,600 129 60 100 514 679 i 
0.10 200 120 100 280 50 1,300 423 524 233 
0.15 140 60 73 160 50 500 359 347 113 
0.20 140 200 100 20 150 200 138 295 46 
0.25 sie 29 20 0 0 156 205 7 
0.30 57 200 0 138 81 0 
0.35 0 0 0 28 32 
0.40 50 0 0 20 39 
0.45 : 200 101 rae 0 
0.50 sie Bae 25 
Total No 
of drops 1,480 1,980 488 540 500 2,300 1,840 2,180 500 
Total 
volume 
mm!/m?/sec 1,005 1,112 1,656 681.2 5,258 11,970 9,535 6,298 4,236 
p mm/hbr 3.6 4.0 6.0 2.46 18.9 43.1 34.3 22.6 15.2 
teristic of a rain as far as attenuation and back scat- 
tering (echo) of radiowaves are concerned. 
In any one location, even the drop size distribution 
of a rain is but an instantaneous characteristic of that 
rain. No data are available concerning the fluctuations 
in time of drop size distribution. 
The space distribution of raindrops is another prob- 
lem on which too few data are available. According 
to Kerr and Rado,’® K-band rain absorption experi- 
ments over a relatively short path (~4km) have shown 
that the simultaneous rates of rainfall at three points 
ot such a path were almost invariably appreciably 
different. The rates were measured at the location of 
the transmitter, the receiver, and at a point in be- 
tween. Needless to say, under such circumstances the 
possibility of a quantitative interpretation of the ex- 
perimental data on attenuation is almost excluded. 
It may be mentioned here that the earlier attenuation 
experiments on 1-cm waves by Robertson and his col- 
laborators?’ as well as those of Mueller® on K/2 band 
were made over a shorter path (about 400 meters) 
and the rate of rainfall was measured only at one 
place, roughly in the middle of the path. Since the 
path length of the Oxford workers”® was 2 km, there 
was ample room for vossible fluctuations in the rate 
of precipitation. The K-band radar transmission 
studies by the Bell Telephone Laboratory workers 
were made over longer paths,”® and here, too, a situa- 
tion somewhat similar to those reported by the Radia- 
tion Laboratory workers might have existed, as the 
authors duly noticed it. 
The meteorological irregularities which thus seem 
to be inherent in precipitation data eliminate the pos- 
sibility of a quantitative theory of attenuation and 
back scattering of radiowaves by rains or other pre- 
cipitation forms. Although the data contained in 
Table 7 are used extensively in this report, the re- 
sults thus obtained should be regarded as semiquan- 
titative indications rather than rigorous theoretical 
predictions. 
Given the number of raindrops of known dimen- 
sions falling over a certain area in a given time and 
given also the terminal velocity of the drops, the 
spatial concentration of raindrops can be derived at 
once. In Figure 7 the terminal velocity curve is drawn 
as a function of drop diameter. These velocities were 
measured at Porton and are quoted in Best’s paper.** 
From Table 6 we may obtain data for Table 7, 
giving raindrop concentration NV; of drops with diam- 
eter k—=D cm. These concentrations, as are the data in- 
TasLe 7. Number of raindrops per cubic meter in rains of different precipitation rates. 
Distribution 
A B Cc D E F G H I 
p, im/hr 
D, em 2.46 3.6 4.0 6.0 15.2 18.7 22.6 34.3 43.1 
0.05 28.5 476 752 61.4 "3.33 Bees 323 245 47.6 
0.10 71.8 512 30.8 25.6 59.7 12.8 134 108 333 
0.15 31 27 11.4 14 21.5 9.52 66 68.4 95.2 
0.20 3.13 22 31.2 15.6 7.2 23.4 46.1 21.6 3h2 
0.25 2.76 S65 on0 4.0 0.96 0 28.3 21.5 0 
0.30 990 500) sete 7.2 0 25.3 10.2 17.6 0 
0.35 500 Seis p00 090 3.83 0 3.35 0 0 
0.40 4.48 5.75 2.3 0 0 
0.45 0 ooo Sais 11.3 22.5 
0.50 060 000 2.71 5 as A 
a | ce es 
Liquid water 
g/m 0.130 0.439 0.217 0.242 0.521 0.673 0.930 1.25 1.55 
a 
