FUNDAMENTAL PROBLEMS AND LIMITATIONS 



3 



1.2.3 



Standard Atmosphere 



Propagation of radio waves in the troposphere is 

 materially influenced by the distributions of tem- 

 perature, pressure, and water vapor. The condition 

 most nearly approximated in the Temperate Zone has 

 been accepted as the so-called standard at7nosphere, 

 and propagation under this condition has been stud- 

 ied and calculations made thereon. 



In the standard atmosphere specified by the 

 National Advisory Committee on Aeronautics 

 [NACA] the temperature is assumed to decrease with 

 altitude at the rate of 6.5 C per kilometer, starting 

 from 15 C at sea level, with a sea level dry air 

 pressure of 1013.2 millibars, which is equivalent to 

 760 mm Hg pressure (see Table 1). 



the total pressure and moisture vapor pressure, re- 

 spectively, in millibars, at height h above sea level. 

 In the moist standard atmosphere, 7i decreases lin- 

 early with height h at the approximate rate of 

 0.039 X 10-« units per meter. 



There are several reasons why this book concerns 

 propagation in the moist standard atmosphere. 



1. The atmosphere in certain places (particularly 

 the temperate zones) and over considerable periods 

 of time is substantially standard in character. 



2. Calculations based on the standard atmosphere 

 serve as a standard against which propagation in 

 nonstandard atmospheres may be compared. 



3. A great deal of propagation information now 

 available in the field is based on propagation cal- 

 culated for standard conditions. 



Table 1. Properties of the atmosphere. 



To simulate the actual atmosphere of the temper- 

 ate zones it is necessary further to specify a water 

 vapor pressure. The value chosen is 10 millibars at 

 sea level, decreasing with altitude at the rate of 

 1 miUibar per 1,000 ft up to 10,000 ft. With this 

 addition the conditions for a moist standard atmos- 

 phere are specified in Table 1. This value of water 

 vapor pressure yields a value of relative humidity 

 approximating 60 per cent. 



Listed also in Table 1 is the index of refraction n. 

 The gradient of this quantity, dn/dh, controls the 

 curvature of the rays for a wave moving in the ap- 

 proximately horizontal direction; n is given by the 

 formula 



(n - 1)106 = _ I p - e -1- ——— 1, (1) 



1.2.4 



Propagation in the Moist 

 Standard Atmosphere 



where T is the absolute temperature, p and e are 



The radiation energy emitted by a transmitter is 

 a wave spreading out in three dimensions, which 

 may be represented by a series of concentric spherical 

 wave fronts or by a sj'^stem of lines called raj-'s. The 

 velocitj^ at any point on the wave front is given by 



c 3 X 10^ , , .o^ 



V = — = meters per second. (2) 



n n 



Since n decreases with height, the upper portions of 

 the wave front move with higher velocities than the 

 lower portions, and the wave paths as represented 

 by the rays are curved slightly downward, as shown 

 in Figure 2. 



The radius of curvature of the rays p is given by 



