ATTENUATION BY FOG 



303 



At wavelengths of 1 and 3 cm with 7 = 0.126 (7 = 2a/X; a = radius 

 of drop) Kerker, Langleben, and Gunn [27] found that particles attained 

 total-attenuation cross sections corresponding to all-melted particles when 

 less than 10 percent of the ice particles were melted. When the melted 

 mass reached about 10 to 20 percent, the attenuation was about twice that 

 of a completely melted particle. These calculations show that the attenu- 

 ation in the melting of ice immediately under the °C [28] isotherm can 

 be substantially larger than in the snow region just above, and under some 

 circumstances, greater than in the rain below the melting level. Further 

 melting cannot lead to much further enhancement, apparently, and may 

 lead to a lessening of the reflectivity of the particle by bringing it to 

 sphericity or by breaking up of the particle. This effect, combined with 

 the fact that hail has greater terminal velocities than rain, gives rise to 

 the so-called "bright band" near the 0° isotherm. 



7.12. Attenuation by Fog 



The characteristic feature of a fog is the reduction in visibility. Visi- 

 bility is defined as the greatest distance in a given direction at which it 

 is just possible to see and identify with the unaided eye (a) in the day- 

 time, a prominent dark object against the sky at the horizon, and (b) at 

 night, a known, preferably unfocused moderately intense light source [29]. 



Although the visibility depends upon both drop size and number of 

 drops and not entirely upon the liquid-water content, yet, in practice, 

 the visibility is an approximation of the liquid-water content, and there- 

 fore, may be used to estimate radio-wave attenuation [28]. On the basis 

 of Ryde's work, Saxton and Hopkins [30] give the figures in table 7.11 

 for the attenuation in a fog or clouds at °C temperature. The attenua- 

 tion varies with the temperature because the dielectric constant of water 

 varies with temperature; therefore, at 15 and 25 °C the figures in table 

 7.11 should be multiplied by 0.6 and 0.4 respectively. It is immediately 

 noted that cloud or fog attenuation is an order of magnitude greater at 

 3.2 cm than at 10 cm. Nearly another order of magnitude increase 

 occurs between 3.2 cm and 1.25 cm. 



Table 7.11. Attenuation caused by clouds or fog [SO] 

 Temperature=0 °C 



