PARAMETERS DERIVED FROM THE iV-PROFILE 235 



statistically, changes in signal level and functions of a parameter derived 

 from routine surface and upper-air measurements of pressure, tempera- 

 ture, and humidity. 



Following the work of Pickard and Stetson [10], N , and AA'' have been 

 the subject of detailed studies by several workers [1, 2, 11, 12, 13, 14]. 

 Only the more important conclusions are summarized here. Values of the 

 correlation coefficient, r, relating monthly median values of either A^^ or 

 AA'' and field strength, derived from a number of paths in diverse climatic 

 conditions [2], range from 0.4 to 0.95 with a median value of about 0.7. 

 An analysis has also been made of the results obtained by using (a) values 

 of AA'' obtained from the surface readings and at heights other than 1 km, 

 and (b) values of AN between different levels on the profile up to a height 

 of 3 km. A comparison of measured field strength at frequencies near 

 100 Mc/s on 20 paths, 130 to 446 km long, located in various part of the 

 United States, yields the following result: the use of N s gives as good a 

 correlation as any of the AA^ values, due to the high correlation between 

 the surface value and these differences. The values of r relating monthly 

 median values of A''^ and AA'' (the decrease in the first kilometer) have 

 been obtained for the United States [15], France, [16], Germany, [17] and 

 the British Isles [18]. They range from 0.60 to 0.93. Moreover, the data 

 are consistent with the assumption of a reference atmosphere in which A'^ 

 decreases exponentially with height [15]. These results lead at once to 

 a consideration of the value of N s in predicting the geographical variation 

 of monthly median field strength. This question is considered in more 

 detail later in this section. 



It has been shown that, where only past radio or meteorological results 

 are available, one obtains at least as good a prediction of the diurnal and 

 seasonal variations of field strength from long-term meteorological data as 

 from relatively short-term (say 1 year's) radio data. The annual cycle 

 may be represented by a single regression coefficient of 0.18 dB/A^'-unit 

 for either night or day; however, the regression coefficients for the diurnal 

 cycles lie between 0.2 and 1.1 dB/N-umt and vary with distance and 

 season, being greatest for paths between 175 and 200 km long and for the 

 winter months. The possibility of predicting the variation of hourly 

 median values of field strength within any given month by combining the 

 .seasonal and diurnal correlations has also been discussed [14]. It is recog- 

 nized that the development of a prediction procedure based on this ap- 

 proach must account for the complex sensitivity of field strength, E, to 

 changes in A'^s; this requirement in turn leads to considerations of season, 

 climatic region, distance and frequency. The results for the distance 

 dependence of the E-N s regression coefficient are, of course, intimately 

 connected with the propagation mechanism. In particular, in the case 

 of VHF paths about 200 km long there will usually be components in the 

 received field due to diffraction around the earth's .surface and scattering 



