50 



FACTORS INFLUENCING TRANSMISSION 



20 



Table 1 (Continued) 



22 



24 



26 



28 



30 



32 



34 



36 



38 



40 



86.0 



89.6 



93.2 



96.8 



100.4 



104.0 



89. 6 



93.2 



96.8 



100.4 



A (ft) 



0.0 



32.8 



65.6 



98.4 



131.2 



164.0 



248.1 



328.1 



492.1 



656.2 



820.2 



984.3 



1,148.0 



1,312.0 



1,476.0 



1.640.0 



1,969.0 



2,297.0 



2,625.0 



2,953.0 



3,281.0 



4,921.0 



6,562.0 



h{lt) 



general this correction may safely be neglected un- 

 less the difference between ih and 1,000 is quite 

 large, corresponding to an elevation of the ground 

 level of several thousand feet above sea level.) 



The wet term M^ is obtained fi'om Table 2 as a 

 function of temperature and relative himiidity. 

 Finally, M, = 0.157/;, if h is in meters, or M, = 0.048/i, 

 if h is in feet, may readily be computed by means of a 

 slide rule. M is then obtained bv addition. 



4.1.8 



Atmospheric Stratification 



Ordinarj' weather data give comparatively' little 

 information about the atmospheric stratification 

 near ground level. Radiosonde data are too widely 

 spaced (vertical distances of the order of 100 meters 

 between successive readings) for relial^le determina- 

 tion of the variation of M with height at low levels. 

 Special instruments have thei'efore been developed 

 in recent .years for low-le^'el soundings. Such instru- 

 ments contain temperature and humidity measiu'ing 

 elements that are relatively free of lag; they are 

 attached to airplanes or dirigiljles or they are carried 

 aloft by means of captive balloons or kites. The 

 above tables are for use in connection with such 

 measurements. 



Two main cases must be distinguished. First, the 

 refracti^•e index or M is very nearly a linear function 

 of the height in the lower layers (at heights above 

 about 500 to 800 meters the variation of refractive 

 index with height will deviate from linearity onl}^ in 

 \'ery exceptional instances). This is the standard 

 case where dM/dh is independent of /;, and bj^ using 

 ecjuation (6), k is conveniently obtained from the 

 slope of the M — h curve. It is found that the 

 vertical temperatiu'e gradient has a comparativelj' 

 small influence on k, while fairly small variations 

 of the humidity gradient will affect k appreciably. 



The other case is that of nonstandard refraction. 

 Here M is not a linear function of the height. The 

 most important special case is that of superrefrac- 

 tion, ■ss'hich occurs when, in certain height intervals, 

 M decreases with height instead of following the 

 usual increase with elevation. Such a decrease of M 

 in certain layers of the atmosphere is caused by a 

 steep negative moisture gradient or steep positi\'e 

 temperature gradient, or even more by a combina- 

 tion of both influences. 



With superrefraction, propagation conditions are 

 greatly different from those encountered with 

 standard refraction and the methods to be given 

 later for the determination of the transmitted power 



