160 



DIELECTRIC CONSTANT, ABSORPTION AND SCATTERING 



liecuiitly, observations were made available on drop 

 size distributions in clouds of different typcs.^'" The 

 main results of interest to the attenuation problem are 

 that in clouds of different altitudes the diameter of 

 the drops does not seem to exceed 0.02 cm. The liquid 

 water content of the clouds examined by Mazur" varied 

 between about 0.15 and 0.50 g/m^. The results of 

 Diem* are, on the whole, similar. 



Some data on ice clouds are included in Best's 

 memoranda." 



Data on fogs are extremely meager. The diameter of 

 fog droplets appears to be of the same order of mag- 

 nitude as tho.se of liquid water clouds.-*'-^ Humphreys, 

 in his table of i^recipitation values, gives 0.006 g/m' 

 as the liquid water content in fog. 



The data on rains used in this report are those from 

 reference 11. For additional data recently collected 

 see reference 26. 



The most important set of data which is directly 

 usable in this work is contained in Table 6. In the 

 last row of this table p is the precipitation rate or rate 

 of rainfall, expressed in millimeters per hour, and 

 results directly from the total volume of water fall- 

 ing per square meter per second, since p = 36 X 

 10"*!', where V is expressed in cubic millimeters per 

 square meter per second. 



Rains 1 and 2 refer, according to Best,^^ to a rain 

 looking very ordinary, falling over a large area. Type 

 3 is a rain with breaks and sunshine. Type 1 corre- 

 sponds to the beginning of a short rainfall like a 

 thundershower. Type 5 refers to a sudden rain fi-om 

 a small cloud, associated with a calm, sultry atmos- 

 phere. Type 6 was a violent rain like a cloudburst with 



some hail. Types 7, 8, an 9 are for the heaviest period 

 and the period of stopping of a continuous fall which 

 at times took the form of a cloudburst. The preceding 

 characteristics of the rains in Table 6 are quotations 

 from the pajser of Best. 



These data on droj) size distributions are the only 

 data available to the writer. Clearly the rate of rain- 

 fall cannot be correlated from these data to any drop 

 size distribution. A priori, it seems unlikely that a 

 strict correlation between drop size distribution and 

 rate of rainfall should exist. To a rain of given drop 

 size distribution corresponds necessarily a determined 

 rate of I'ainfall, but the reverse is not true, since a 

 given rate of rainfall might be ol)taincd with a large 

 variety of drop size distribution."' In other words, 

 the drop size distribution is the only physical charac- 

 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 availal>le. According 

 to Kei'r and Rado,^" K-band rain absorption experi- 

 ments over a relatively short path (~lkm) have shown 

 that the simultaneous rates of rainfall at three j3oints 

 of such a jiath 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 

 possil)iiity of a quantitative interpretation of the ex- 

 perimental data on attenuation is almost excluded. 



Table 6. Drop size distributions in rains. 



