DIFFRACTION (GENERAL SURVEY) 



59 



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 extremel.y complex 

 structure; the scattered intensity A\-ill 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. 



