ATTENUATION BY ATMOSPHERIC GASES 275 



The behavior of water vapor attenuation near the resonant line is very 

 remarkable, as can be seen by inspecting (7.2). Since Av^ is small com- 

 pared to 1/X, it may be neglected in the denominator of (7.2) for non- 

 resonant wavelengths. The attenuation per unit density is thus directly 

 proportional to /^v^. and hence to the total pressure for these frequencies. 

 But at the resonant frequency, the dominant term in the expression is 

 proportional to l/A^s, and thus inversely proportional to the pressure. 

 In the atmosphere, the water vapor density is proportional to the total 

 pressure. Therefore, the attenuation is independent of pressure at the 

 resonant frequency and now depends only on the fraction of water vapor 

 present. For practical purposes, this means that attenuation can occur 

 at high altitudes with the same effectiveness as in the lower, denser layers 

 if the mixing ratio is the same. 



On the other hand, oxygen absorption occurs because of a large number 

 of lines around 60 Gc/s. In the region from 3 to 45 Gc/s the attenuation 

 is proportional to P- and to T-n/^ ^5j ^g ^j^g temperature decreases the 

 attenuation increases gradually. At —40 °C oxygen attenuation is about 

 78 percent higher than at 20 °C due to increased density at low tempera- 

 tures. Table 7.4 shows the pressure and temperature corrections for 

 oxygen attenuation at wavelengths between 0.7 and 10 cm. 



Figure 7.2 shows the attenuation at a pressure of 1 atmosphere and 

 20 °C as a function of wavelength [15]. The solid lines represent values 

 of attenuation measured by Becker and Autler [7]. The dashed line 

 shows values calculated from Van Vleck's theory. The water vapor 

 absorption curve, c, corresponds to a water content of 1 g/m^. 



Since absorption is so sensitive to the absolute humidity level, it is 

 helpful to have information on the climatic variation of absolute humidity 

 throughout the 1 to 99 percent range of values normally used in radio 

 engineering. Estimates of the values of absolute humidity at the surface 

 expected 50 percent of the time for the United States for February and 

 August are given in figures 7.3 and 7.4 respectively [16]. It is evident 

 that for either month the coastal regions display greater values of absolute 

 humidity than do the inland regions. Note that for any location the 

 August values are consistently greater than the February values. Figures 

 7.5 through 7.8 show the values of absolute humidity expected to be 

 exceeded 1 and 99 percent of the time throughout the United States in 

 both summer and winter. 



In addition to oxygen and water vapor, there are a number of other 

 atmospheric gases which have absorption lines in the microwave region 

 from 10 to 50 Gc/s. These gases normally constitute a negligible portion 

 of the general composition of the atmosphere, but could conceivably 

 contribute to attenuation. Table 7.12 shows the resonant freciuencies, 

 maximum absorption coefficients at 300 °K (attenuation coefficient if the 

 fraction of molecules present were equal to unity), expected concentration 



