280 ATTENUATION OF RADIO WAVES 



in the atmosphere, and expected absorption coefficients due to these 

 trace constituents. The data on molecular absorption coefficients was 

 taken from Ghosh and Edwards [33], that on concentrations from the 

 Glossary of Meteorology [29]. It is readily seen that the attenuation due 

 to these sources is negligible compared to the high absorption due to 

 oxygen and water vapor. 



7.4. Estimates of the Range of Total 

 Gaseous Absorption 



The range in gaseous absorption can be seen by considering the data 

 for the months of February and August at Bismarck, N. Dak., and 

 Washington, D.C., two stations with very different climates. The values 

 of total gaseous absorption (defined as the sum of 71, 72, and 73, where 

 7i = oxygen absorption in decibels per kilometer, 72 = water vapor 

 absorption arising from the 1.35 cm line and 73 = additional absorption 

 arising from absorption lines whose frequencies are considerably higher 

 than that corresponding to the 1.35 line) at each station and elevation up 

 to 75,000 ft are shown in figures 7.9 and 7.10 for each of the four station 

 months for the frequency range of 100 to 50,000 Mc/s. Above 75,000 ft 

 the absorption values for all four station months are identical and are 

 given for each frequency in figure 7.11. The absolute humidity was 

 calculated using the upper air monthly average values of temperature, 

 pressure, and humidity as reported by Ratner [17]. Readings for the 

 relative humidity are not generally given in this report for altitudes 

 greater than about 15 km due to the inability of the radiosonde to meas- 

 ure the small amount of humidity present at these altitudes. It is 

 believed that the climates represented by these station months encompass 

 the range of those of the majority of the continental United States radio 

 propagation paths. 



An interesting property of the annual range of absorption as a function 

 of the frequency may be seen in figures 7.9 and 7.10. For the first 5,000 

 ft above the surface, it is noted that in the frequency range of 10 to 32.5 

 Gc/s, the summer values are greater than the winter values due to in- 

 creased humidity of the summer months. Outside of this frequency 

 range, however, the winter values of absorption are greater due to the 

 increased oxygen density. 



In the frequency range 6 to 45 Gc/s, atmospheric absorption, 7;,, at 

 a frequency v, arises primarily from oxygen absorption, jdi,, and water 

 vapor absorption, juw', i.e., 



7^ = 7rf. + Jwv. (7.4) 



