Radar System Engineering

60 TIIE RADAR EQUATION [SEC.2.15
wtivelengths, in contrast to the behavior of the water-vapor curve, a
residual absorption persists up to wavelengths of the order of 30 cm.
This effect is small enough to have escaped experimental detection,
butitis clearly predicted by the theory.’ Except near thecenter of the
absorption maximum, the attenuation due to oxygen should vary about
as the s~uare of the pressure; hence the effect rapidly diminishes at
high altitudes. It is perhaps unnecessary to remark that attenuations
Wavelengthin cm
F,c. 216.-Attenuation caused by water vapor (Curve a) and oxygen (Curve b).
Curves appliesto an atmospherecontaining10goi water vapor per m3. Curve b applies
to an atmospherewhichisone fifth oxygen, at a total pressureof 76cm Hg.
due to independent causes, such as water vapor and oxygen, are directly
additive.
Water drops in the atmosphere can affect the passage of microwave
radiation in two ways. In the first place liquid water is a very imperfect
dielectric at microwave frequencies, and absorbs energy from an oscillating
electric field just as any 10SSYdielectric WOuld. . For extremely small
drops, such as those in fog or clouds, this is the only important effect,
and in this limiting case the attenuation at a given wavelength is simply
proportional to the aggregate liquid water content of the atmosphere,
1J H. Van Vleck, op. cit.; seealso Vol. 13.