246 TRANSHORIZON PARAMETERS 



France) and Reading (England). The variations in N s obtained from the 

 Crawley radiosonde station (close to the midpoint of the path) are also 

 shown. Here N s has a range of ± 10 A^ units, and the signal level a range 

 of no more than ±4.5 dB. In fact, from March to September the total 

 variation in observed monthly median field strength is only ±1.5 dB. 

 The measurements of field strength in these experiments were estimated 

 to be subject to possible errors of about ±3 dB. Consequently, one 

 would not be justified in assessing the value of A^^s as a radio-meteorological 

 parameter on the basis of the correlation coefficient (about 0.25) calculated 

 from the two curves in figure 6.11. During the period studied in this 

 work (0900-2300 UT daily, January-November 1959) the highest signals 

 were observed during anticyclonic winter weather, with elevated inversion 

 layers at a height of about 0.6 km. Extended stable layers at this height 

 were rarely seen on the sonde ascents during the summer months. This 

 result may partly explain the fact that the seasonal variation in Ns 

 (highest values in the summer months) is not accompanied on the Lille- 

 Reading path by a corresponding variation in monthly median field 

 strength. 



6.1.10. Conclusions 



A review of available data shows that no radio-meteorological param- 

 eter has yet been proved to be superior to A^s or AA^ for general application 

 in the prediction of field strength distributions. The value of these 

 parameters has been established by studies of many paths in diverse 

 climatic conditions. However, it should be noted that much of the 

 radio data has been obtained at frequencies near 100 Mc/s, and there is 

 a clear need for further analysis of the several parameters in conjunction 

 with field strength measurements at higher frequencies. 



Pending more detailed results, it does not seem likely that the equiva- 

 lent gradient, Qe, affords any significant advantage over AA'', especially 

 in view of the many calculations required in its derivation. Some data 

 from selected areas (Congo, Sahara, west Africa) suggest that A''^ and AA'' 

 may have limited value in these regions; however, the present lack of 

 adequate radio data precludes any definite conclusions. These examples, 

 and allied work on vertical motion and thermal stability, emphasize the 

 importance of a parameter (such as that suggested by Misme) which takes 

 account of the influence of elevated layers. This approach would prob- 

 ably prove fruitful not only in equatorial areas but also in temperate 

 regions where the annual range of N ^ and AN is small. 



Analysis of some results obtained at a frequency of 100 Mc/s has shown 

 that it has proved feasible to provide at least as good a prediction of the 

 annual cycle of field strength variations from long-term meteorological 

 data as from relatively short-term radio data. 



