196 



TROPOSPHERIC PROPAGATION AND RADIO METEOROLOGY 



metrical conditions, the radio horizon appears ex- 

 tended as compared to the geometrical horizon, be- 

 cause of the curvature of the rays. In Figure 11 the 

 rays have been straightened out, but a line that was 

 straight in Figure 10 appears curved in Figure 11. 



The value of % for A; is a good average for the at- 

 mosphere in the middle latitudes. For particular 





Figure 11. Rays in a homogeneous atmosphere. Equiv- 

 alent radius ka. 



atmospheric conditions the value of k may be con- 

 siderably different. The moisture content of the at- 

 mosphere is small at the low temperatures of the 

 arctic regions and increases considerably with the 

 higher temperatures of the tropics. However, the 



value of k depends more particularly on the manner 

 in which the moisture content varies with height 

 above the surface of the earth, and to a lesser extent 

 on the distribution of temperature with height. 

 Figure 12 has therefore been constructed to show the 

 dependence of k on the gradient of relative humidity, 

 measured in per cent per 100 m, for a series of surface 

 temperatures varying between T = — 30 C and 

 To = +40 C.| It has been found convenient to plot 

 1/k rather than k itself. The lines drawn correspond 

 to the assumption of saturation humidity at the 

 ground; if the humidity at the ground is less than 

 100 per cent the correction read from the auxiliary 

 table is added to the value of 1/k obtained from the 

 graph. The standard temperature gradient of —0.65 

 C per 100 m is assumed for all the curves. 



The curves of Figure 12 indicate that as the tem- 

 perature increases, smaller and smaller values of rela- 

 tive humidity gradients are required to produce 

 changes in k of considerable magnitude. This should 

 be of greater importance in the tropics where the 

 moisture content is relatively high. 



Changing k from its standard value of % has an 

 important influence on the strength of the field at any 



Figure 12. Graph: 1/k versus RH gradient and temperature for 100 per cent RH at ground. 



