CONCLUSIONS 333 



Table 8.3. Percentage occurrence of surface ducts during the years 1952 to 1956 



Table 8.4. 



Percentage occurrence of surface subrefractive layers during the years 

 1952 to 1956 



8.2.7. Conclusions 



The conclusions of the present study could be considerably modified 

 by the analysis of many more examples, although it is evident that hori- 

 zontal variation of n near the earth's surface produces the most marked 

 deviations from the ray paths obtained by assuming horizontal stratifica- 

 tion of n. The effect of horizontal changes occurring more than a kilom- 

 eter above the surface appear from our present examples, to have little 

 effect. Further, the effects of horizontal changes appear to be most 

 pronounced in the presence of surface ducts and at small elevation angles. 

 The tentative conclusion is reached that the effect of horizontal n change 

 is normally small, since ducting will occur less than 15 percent of the time. 



8.3. Comparison of Observed Atmospheric Radio Re- 

 fraction Effects With Values Predicted Through 

 the Use of Surface Weather Observations 



8.3.1. Introduction 



The atmospheric radio refraction effects considered in this section are 

 of two general types: errors in measuring distance by means of timing 

 the transit of radio signals between two points, known as radio range 

 errors, and errors in estimating the elevation angle of a target by means 

 of measuring the angle of arrival of radio signals from the target, known 



