ATTENUATION IN CLOUDS 



7.7. Attenuation in Clouds 



291 



Cloud droplets are regarded here as those water or ice particles having 

 radii smaller than lOO/i or 0.01 cm. For wavelengths of incident radiation 

 well in excess of 0.5 cm, the attenuation becomes independent of the 

 drop size distribution. The generally accepted equations for attenuation 

 by clouds usually show the moisture component of the equations in the 

 form of the liquid water content (g/m^). Observations indicate that the 

 liquid water concentration in clouds generally ranges from 1 to 2.5 g/m^ 

 [18], although Weickmann and aufm Kampe [19] have reported isolated 

 instances of cumulus congestus clouds with a reading of 4.0 g/m^ in the 

 upper levels. In ice clouds, it will rarely exceed 0.5 and is often less than 

 0.1 g/m^. The attentuation of cloud drops may be written as: 



K = KiM, 



where K = attenuation in dB/km, 



Ki = attenuation coefficient in dB/km/g/m', and 



M = liquid-water content in g/m^. 



Values of Ki by ice and water clouds are given for various wavelengths 

 and temperatures by Gunn and East in table 7.6. 



Table 7.6. One-way attenuation coefficient, Ki, in clouds in dB/km/gm/m^ 



Several important facts are demonstrated by table 7.6. The decrease 

 in attenuation with increasing wavelength is clearly shown. The values 

 change by about an order of magnitude for a change of X from 1 to 3 cm. 

 The data presented here also show that attenuation increases with de- 

 creasing temperature. Ice clouds give attenuations about two orders of 

 magnitude smaller than water clouds of the same water content. The 

 attenuation of microwaves by ice clouds can be neglected for all practical 

 purposes [20]. The comprehensive works of Gunn and East [15] and 

 Battan [20] on attenuation offer excellent sources of detailed information 

 on this subject. 



