376 : PROPAGATION THROUGH THE STANDARD ATMOSPHERE 
the Fresnel-Kirchhoff formula is often applicable 
to short-wave propagation problems. It is treated 
in Chapter 8. 
Diffraction by Targets 
This problem can be dealt with theoretically by 
methods similar to those used in computing diffrac- 
tion by terrain features, the main difference being 
that the angle of scattering is nearly 180 degrees 
sh {= 
Figure 19. Reflecting pattern of an airplane. 
instead of approximately 0 degrees. 
In the case of target diffraction, theory is less 
useful than in many other problems of wave propa- 
gation. This is due to the fact that radar targets 
such as airplanes or ships*have an extremely complex 
structure; the scattered intensity will therefore 
often change by many decibels as a result of only a 
small tilt of the target. Figure 19 shows a typical 
reradiating or reflecting pattern for an airplane. 
Numerous measurements of the average radar cross 
section of planes and ships have been made. 
A phenomenon of great importance in the micro- 
wave region is the scattering of radiation by water 
drops in the atmosphere. Small droplets such as are 
found in fogs and most types of clouds do not give 
reflection visible on the scopes. Only drops large 
enough to produce actual precipitation give appre- 
ciable radar echoes. However, this does not neces- 
sarily mean that rain is falling at the locality indi- 
cated by the scope; frequently vertical updrafts of 
air will maintain drops afloat that in still air would 
fall to the ground; moreover, drops falling from a 
comparatively high cloud can evaporate before 
reaching the ground. Especially in tropical regions, 
the last-named phenomenon is more common than 
is ordinarily thought. 
