90 



SCATTERING AND ABSORPTION OF MICROWAVES 



individual oxygen and water vapor attenuation 

 curves have been plotted in the 0.2- to 10-cm wave- 

 length range. Any change in the water vapor content 

 from the one adopted for this graph (7.5 g per cu m 

 or 6.5 g per kg of air) or the total pressure can be 

 taken into account in computing the combined 

 oxygen and water vapor attenuations, since these 



10. 

 7. 

 5. 



3. 



2. 



l. 

 o. 



s 0. 

 d-0. 



Q 



zO. 



•z. 

 o 



§°- 



io, 



o, 

 0, 





 ,0 

 















7 

 5 



3 

 2 



.1 



,07 



,05 



,03 

 ,02 



0.01 

 0.007 

 0.00 5 



0.00 3 

 0.002 



30 24 15 10 6.0 3.0 1.5 1.0 0.6 0.3 

 FREQUENCY IN IO 3 MC 



Figure 6. Atmospheric one-way attenuation. (1) Oxy- 

 gen and water vapor (total for p = 76 cm Hg, T = 20 C, 

 water vapor. = 7.5 g per cu m). (Van Vleck.) (2) Moder- 

 ate rain (6 mm per hr) of known drop size distribution. 

 (3) Heavy rain (22 mm per hr). (4) Rain of cloudburst 

 proportion (43 mm per hr). 



are proportional to the partial pressures of oxygen 

 and water vapor. For practical purposes the effect 

 of temperature variations can be neglected. 



In Figure 6, curve 1, is plotted the total attenua- 

 tion of oxygen plus water vapor in an atmosphere 



at 76-cm pressure, with the same water vapor content 

 as the curve of Figure 5. Curves 2, 3, and 4 are 

 additional rain attenuation curves computed for a 

 moderate rain of rainfall 6 mm per hr, a heavy rain 

 of 22 mm per hr and an excessive rain of 43 mm per 

 hr, which is of cloudburst proportions. In any rain 

 the result of total attenuation is the sum of the 

 oxygen, water vapor, and liquid drop attenuation. 



It is thus seen that for waves of 3 cm or shorter the 

 rain attenuation may become prohibitive, whereas 

 the gaseous attenuation loses its practical import- 

 ance at waves longer than about 2 cm. In this 

 connection it is to be noted that for millimeter 

 waves the rain attenuation begins to level off at 

 waves of a few millimeters, as Table 10 indicates, 

 and would actually decrease at waves shorter than 

 1 mm. However in this range, the water vapor 

 absorption due to the strong water lines situated at 

 much shorter waves becomes more and more intense, 

 and communication or radar on these bands is almost 

 totally excluded. It is worth noting in this connection 

 that using radiation which is strongly absorbed 

 might, in certain cases, be of great operational 

 interest. In the oxygen band, for example, short- 

 range communication could be achieved without any 

 likely interference by the enemy. 



Electromagnetic theory thus gives a satisfactory 

 picture of the absorption and scattering phenomena 

 of microwaves both by floating or falling water 

 drops, or their equivalent in hail and snow, and by 

 the oxygen and water vapor of the atmosphere. 



Of the approximately 100 reports which were 

 prepared by the Columbia University Wave Propa- 

 gation Group or were presented at the second, third, 

 and fourth conferences on propagation held in 

 February 1944, November 1944, and May 1945, 61 

 have been selected for publication in the Summary 

 Technical Report. Of these, 18 reports, covering 

 standard and nonstandard propagation, are published 

 in this volume; the remainder are published in 

 Volume 2. The reports not included in these two 

 volumes were omitted chiefly because their material 

 was superseded by later documents. 



The reports in the remainder of this volume appear 

 in two sections. Chapters 1 1 through 15 are concerned 

 with standard propagation; Chapters 16 through 27, 

 with nonstandard propagation. 



