162 



D, cm 



0.05 

 0.10 

 0.15 

 0.20 

 0.25 

 0.30 

 0.35 

 0.40 

 0.45 

 0.50 



Liquid water 



2.46 



28.5 

 71.8 

 31 

 3.13 

 2.70 



0.130 



DIELECTRIC CONSTANT. ABSORPTION AND SCATTERING 



Table 7. Xumber of raindrops per cubic meter in rains of different precipitation rates 



3.6 



476 



512 



27 



22 



0.439 



4.0 



70.^ 

 30.8 

 11.4 

 31.2 



0.21 ; 



Distribution 

 D E 



G 



H 



6.0 



p, mm/lir 

 15.2 



18.7 



22.6 



34.3 



61.4 

 25.6 

 14 

 15.6 

 4.0 



0.242 



0.521 



0.673 



0.930 



1.25 



43.1 



1.55 



ill space. For reasons stated al)o\c, tlieovetieal results 

 are significant only with regard to orders of magnitude. 



10 1 * Attenuation by Idealized 



Precipitation Forms 



The data included in the preceding section show, 

 first of all, that in clouds and fogs the attenuation 

 can be given rigorously. Indeed, Table 3 indicates 

 that the critical diameter even for waves of i-cm 

 wavelength is over 0.06 cm. Since we have seen that 

 in clouds and fogs the drop diameters never exceed 

 0.02 cm, it appears that formula (51) is applicable, 

 and the attenuation of all waves of wavelength A. > 

 1 cm is independent of the size of the drops. Further- 

 more, taking m = 1 g per cubic meter in formula 

 (.51) one probably obtains an upper limit for the 

 attenuation of these waves.* In Figure !) the atten- 

 uation is pl(.)tted down to A ^ 0.2 cm. The dielectric 

 constant of water has l3een computed in this range by 

 using the Debye formula for wavelengths A > 1 cm. 

 Clearly the attenuation in fogs and clouds even in 

 the region A — ' 1 cm is not of great importance ex- 

 cept for long ranges and radar observations. The 

 attenuation becomes negligible for waves with A > 

 10 cm. 



Table 3 also shows that tbc attenuation becomes 

 practically independent of the drop size distribution 

 for wavelengths equal to or larger than about 20 cm. 

 In the 5- to 20-cm range the three-term formula 



(48) or (50) in connection with (54:) will represent 

 fairly well the attenuation in different rains, with 

 increased accuracy at longer wavelengths. Below A 

 = 5 cm this formula is inapplicable, liut there Ryde 



to 



5 



z 



O 



0.5 



0.2 



- 0.1 



f 



0.05 



0.02 



0.01 



0.004 

 0.2 



0.5 



1.0 

 A IN 



10 



CM 



'Attention may be called to the absence of data on the 

 liquid water distributions in lieav}' sea fogs. 



Figure 9. Attenuation factor in liquid clouds and fogs. 

 / = 18C. 



