158 



DIELECTRIC. CONSTANT. ABSORPTION AND SCATTERING 



size of the drops but oiil}' on tiie total mass of liquid 

 water per unit volume contained in the air. It is in- 

 teresting to find, for a given wavelength, tlie largest 

 diameter for which the approximation (51) can still 

 be used in practice. If it is practical to use (51), (as 

 in reference 1"2), for 



c«p 



2< 



10 



(52) 



then in order that (51) shall represent the attenuation 

 factor witliin 10 per cent, tlie diameter of the spheres 

 must (for given A), l)e equal to or le.ss than Dc with 



"-m 



cm. 



(53) 



In Tal)le -i appear the values of Cj, c,, and D^ in tlie 

 wavelength range 1 to 100 cm. The values of c^ are 

 not included, since this coefficient turns out to be 

 practically constant, in this range, increasing from the 

 value of 1.234 for A = 1 cm to 1.339 for A = 100 cm. 

 It is evident that for values of p which are not too 

 small, equation (48) or (50) has to be used. When 

 p is sufficiently close to unity these series cease to give 



any good values of the absorption cross section Qt or 

 the attenuation factor a. In the K and X bands, Eyde 

 and Hyde'- have, therefore, computed tlie attenuation 

 factors exactly. These computations were included 

 (without being elieclied) in Tables 4 and 5, where Qt 

 and a have been computed for a series of drops ranging 



X, CAXl- 



Table 4. Absorption cross section Qt (cm-) of water drops with diameter D (cm). 



0.05 



0.10 



0.15 



0.20 



0.25 



D, cm 

 0.30 



0.35 



0.40 



0.45 



0..50 



0.55 



Table 5. Attenuation a/N (db/km) in fictitious rains with a concentration of one drop per cubic centimeter of D cm 

 diameter. 



X, cm 



0.05 



0.10 



0.15 



0.20 



D, cm 

 0.25 0.30 



0.35 



0.40 



0.45 



0.50 



0.55 



