TEMPERATURE EFFECTS ON NONSTANDARD RANGES 



59 



result was established to within an accuracy of ±2 db. 



For about 80 per cent of the time (June 1, 1944 to 

 Septeml)er 30, 1944) the normal interference pattern 

 Was replaced by an entirely different form of signal 

 fading. The period of the fading was about 5 minutes, 

 the field strength at maximum was usually between 

 10 and 13 db above the free space signal, and the 

 peaks of signal were sharp and the minima broad. The 

 latter characteristic is entirely different from interfer- 

 ence fading between two rays which must lead to 

 broad peaks and narrow minima; it is more akin to 

 receiver noise or the form of the signal echo received 

 from "window" on radar sets. Thus it would seem 

 plausible to suppose that the signal was the result of 

 a large number of contributions with uncorrelated 

 phases. 



The type of fading described in the previous para- 

 graph is especially interesting in view of the very high 

 signal maxima. It was shown that the occurrence of 

 these variations was not associated with the reflection 

 at the surface of the sea. It would appear therefore 

 that atmospheric conditions can exercise a very im- 

 portant effect on the propagation of radio waves over 

 a completely optical path. 



5 ^ TEMPERATURE EFFECTS ON 



NONSTANDARD RANGES' 



Experimental work carried out in the Irish Sea has 

 shown the following three characteristics, all of which 

 are in disagreement with existing theory. 



1. It is well known that the present theory requires 

 the contribution of the temperature gradient to be in 

 general small compared with the contribution of the 

 water vapor gradient. In fact if we write 



dfi _ de dT 



dh dh dh 



where /* = refractive index, 



e = partial pressure of water vapor, 

 T = temperature in degrees absolute, 

 h = height coordinate, 

 a, h are positive constants, 



then except in rare cases the present theory requires 

 the (a) (de/dh) term to be large compared with the 



(h) (dT/dh) term. Thus, since the radio propagation 

 conditions depend on d/x/dh, it is to be expected that 

 a better correlation will exist between the radio signal 

 and the measured value of (a) {de./dli) than between 

 the radio signal strength and the measured value of 

 ( — b ) (dT/dh). The reverse has, however, been found 

 to be the case ; the correlation between the radio signal 

 strength and the measured value of (a) (de/dh) is 

 poor, while the correlation with the measured value of 

 i — b) (dT/dh) is good. 



2. A situation similar to that mentioned in (1) has 

 been encountered in attempting to forecast 12 hours 

 ahead for operational radar sets. Table 1 summarizes 

 the results obtained. 



Table 1 



Forecasting 

 system 



No. of cases 

 examined 



% of correct 

 forecasts 



(de/dh) term 600 



Continuity method 600 



{dT/dh) term 150 



56 

 57 



75 



"iBy F. Hoyle, Ultra Short Wave Panel, Ministry of Sup- 

 ply, England. 



The continuity method of forecasting consists in pre- 

 dicting that tomorrow's conditions will be the same as 

 tho^e observed today. Forecasting from the (dT/dh) 

 term was empirical, it being assumed that a tempera- 

 ture inversion of 1.5 C per 100 ft would give very good 

 propagation conditions, an inversion of O.TS C per 

 100 ft would give good conditions, and a temperature 

 lapse would correspond to standard or natural condi- 

 tions. 



3. The value of tV/(//i calculated from equation (1) 

 using the measured values of de/dh and dT/dh. are too 

 small to explain the large signals frequently observed 

 on a wavelength of about 80 mc. 



These experimental results enable tlie following 

 conclusions to be drawn so far as the meteorological 

 conditions around the British Isles are concerned. 



1. The radio sigJial strengths computed from the 

 observed M curve do not agree with observation. The 

 method of observing the temperature and water vapor 

 gradients to obtain an M curve is therefore unsatisfac- 

 tory. 



2. A reasonably satisfactory forecasting system can 

 be obtained on the basis of temperature gradient alone. 

 This system is empirical and does not depend on com- 

 plicated mathematical computations. Accordingly the 



