ATTENUATION BY ATMOSPHERIC GASES 



273 



where Au^ is the effective Hue width of the absorption bands above the 

 1.35-c'm Hne. The nonresonant term has been increased by a factor of 

 4 over the original Van Vleck formula in order to better satisfy experi- 

 mental results [7]. 



Although Van Vleck gives estimates of the various line widths, more 

 recent experimental determinations were used whenever possible. The 

 line-width values used in this chapter are summarized in table 7.1. 



Table 7.1. Line width factors used to determine atmospheric absorption 



The preceding expressions for gaseous absorption are given as they 

 appear in the literature and do not reflect the pressure and temperature 

 sensitivity of either the numerical intensity factor or the line widths. 

 This sensitivity must be considered for the present application since it is 

 necessary to consider the manner in which the absorption varies with 

 temperature and pressure variations throughout the atmosphere. The 

 dependence of intensity factors upon atmospheric pressure and tempera- 

 ture variations was considered to be that given by the Van Vleck theory. 

 The magnitude and temperature dependence of the line widths is a 

 question not completely resolved. Both theory and experiment indicate 

 the line width to vary as 1/7'^, x <0. Different measurements on the 

 same line of oxygen have given values of x ranging from 0.71 to 0.90 

 with differences in the magnitude of Ai^ of about 2 percent [10, 11]. Ex- 

 periments have also clearly indicated that the line width changes from 

 line to line, with maximum fluctuations of about 15 percent. In the 

 frequency region considered in this chapter (10 to 45 Gc/s) the centroid 

 frequency approximation for oxygen is valid and a mean line width can be 

 used with good accuracy, but in the region of the resonant frequencies of 

 oxygen, the line-to-line line width variations must be taken into account. 

 The expressions used to calculate the absorptions are given in table 7.2. 

 The reference temperatures given are those at which the appropriate 

 experimental determinations were made, and the pressures are to be 

 expressed in millibars. A detailed discussion of the theoretical aspects of 

 the pressure and temperature dependence is given by Artman [12]. 



Experimental measurements on the absorption of microwaves by the 

 atmosphere (performed after our original work) show different values of 

 the loss than those obtained by theoretical prediction methods. There 

 is reasonably good agreement between the predicted and measured loss 

 for oxygen, but the measured loss of water vapor is considerably greater 



