86 



SCATTERING AND ABSORPTION OF MICROWAVES 



in the theoretical predictions of scattering or absorp- 

 tion by rain are due to the relatively limited knowl- 

 edge of drop size distributions in rains of different 

 rates of fall. There is no evidence that a rain with a 

 known rate of fall has a unique drop size distribution 

 though the latest studies on this problem seem to 

 indicate that a certain most probable drop size 

 distribution can be attached to a rain of given rate 

 of fall. 446 Results of this study are included in Table 

 9. On the basis of these results the absorption cross 



Table 9. Drop size distribution. 



section of raindrops of different size has been com- 

 puted for use in Table 10. This table gives the decibel 

 attenuation per kilometer in rains of different rates 

 of fall and for radiation of wavelengths between 

 0.3 and 10 cm. In Table 11, similar to Table 10, 

 another set of results is contained for rains of 

 measured drop size distributions. This table is 

 extended to include radiations of wavelengths up to 

 100 cm. It seems equally interesting to give a graphi- 

 cal representation of those results. Figure 2 corres- 

 ponds to Table 10 and Figure 3 to Table 11. All 

 these data refer to raindrops at 18 C. 



0.1 0.2 CO 0.5 0.7 1 2 3 



MN CM 



Figure 2. Graphical presentation of data given in Table 

 10. 



Since the scattering coefficients a n and b n depend 

 on the temperature, because of its effect on the 

 dielectric properties of water, it seems important to 

 evaluate the attenuation of rains whose drops are 

 at temperatures different from those included in the 

 preceding tables. Table 12 contains the necessary 

 data relative to the changes of attenuation with 

 temperature and is to be used primarily in connec- 

 tion with Table 10. 



It will suffice to mention here that, for waves 

 larger than about 3 cm, the attenuation produced 

 by hail of the same water precipitation rate as a rain 

 will be but a few per cent of the rain attenuation. 

 At shorter waves, in the millimeter region, hail 

 attenuation may become larger than that of rain. 

 Similarly the attenuation of snow should be con- 

 siderably less than rain; however, Canadian reports 

 indicate approximately the same value for the same 

 water content. 



As mentioned above, the whole theory of attenua- 

 tion is based on equation (14). The formulas giving 



Table 10. 



Attenuation in decibels per kilometer for different rates of precipitation of rain. Temperature 18 C, X in cm. 277 



