92 TECHNICAL SURVEY 
oxygen and water vapor attenuations, since these 
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 
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Ficure 6. Atmospheric one-way attenuation. (1) Oxy- 
gen and water vapor (total for p = 76 cm Hg, T = 20C, 
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). 
drops, or their equivalent in hail and snow, and by 
the oxygen and water vapor of the atmosphere. 
