COMPOSITE PARAMETER 239 



Table 6.1. Influence of thermal stability on signal level 



(d = 160 km: f = 471 Mc/s: North Sahara Region) 



The dominant propagation mechanisms in January and June are 

 thought to be "diffuse reflection" and "scattering," respectively, and on 

 this basis Misme uses the equations given by Voge to calculate the ex- 

 pected difference in the January and June signal levels. With various 

 assumptions regarding the properties of stable and turbulent layers, a 

 value of 19 (±5) dB is obtained for the ratio of predicted field strengths in 

 January and June, in good agreement with the measured value exceeded 

 for 99 percent of the time. This analysis is facilitated by the fact that the 

 two selected months are characteristic of well-defined climatic situations 

 in the Sahara region. Nevertheless, the results indicate that a parameter 

 which combines the concepts of the equivalent gradient and thermal 

 stability may be of general application. Misme [19] has therefore sug- 

 gested a parameter M of the form: 



M = a ige - iO -\- b [AW]") (6.2) 



where a, h, and n are constants, Qe is the equivalent gradient, and AW is 

 the thermal stability defined above for a 1 km height interval. With 

 a = 0.5 dB/N/km, M provides an estimate of the variation in field 

 strength, E, in decibels, caused by changes in equivalent gradient and 

 stability. It remains to define E in a "standard" atmosphere by selecting 

 a mean value of stability and a suitable value for the term h. 



This composite parameter is thought by Misme to be more representa- 

 tive of the real atmosphere (especially in tropical areas) than A^^ and AA^, 

 and it is certainly of great potential value. However, only fragmentary 

 radio data are available for purposes of comparison in the references 

 quoted, and here again the precise value of the parameter in prediction 

 work can only be determined by a more comprehensive study. 



6.1.5. Potential Refractive Index (or Modulus), K 



This parameter is defined as the value of A'^ = (n— 1)10*^ which an air 

 mass would have if brought adiabatically to a standard pressure, say 

 lOOOmbar, assuming a constant humidity mixing ratio. Examples of the 

 use of K in radio meteorology have been given by Katz [24], and by Jehn 



