190 



STORM DETECTION 



grouudeil was not iletvctrd at all, even at the niinimuin 

 range of 10 miles. 



Eonglily, rain and snow of the same intensity, ex- 

 pressed in inches of liquid water per hour, produce 

 about the same echo and are detectable to the same 

 range. Further, there seems to be no useful difference 

 in pattern between echoes from the two sources. Fig- 

 ure 1 shows a typical PPI picture of snow echoes made 

 during the course of the study. 



Theoretically, this equality is not directly signifi- 

 cant : in the ease of snow there is a much greater bulk 

 of lighter material, falling more slowly and reflecting 

 less well. 



Operationally, there are two reasons why radar 

 storm detection is less useful in winter (in Canada). 

 A given intensity of precipitation in the form of snow, 

 say 0.1 in. of water per hour, is much more hazardous 

 to flying and to ground activities than the same inten- 

 sity in the form of rain. Further, great intensities of 

 precipitation such as lead to long-range echoes in 



summer are almost nonexistent in winter in this 

 region ; therefore, detection at great ranges is not 

 achieved. Thus S-band radar in summer can detect 

 important storm areas to a radius of about 100 miles; 

 in winter it detects hardly any weather beyond 50 

 miles and misses some important snow even at 10 to 20 

 miles. 



For the greatest total contribution of radar to fly- 

 ing it is a good thing that echoes from snow are weak. 

 This is important, for while the cumulo-nimbus ac- 

 tivity detected in summer must always be avoided by 

 aircraft because of violent air currents, flying in mod- 

 crate snow can he safe with good blind-flying control. 

 It is fortunate, therefore, that echoes from snow are 

 probably not strong enough to interfere with any radar 

 elements of this control. 



This work has been done with the cooperation of 

 the National Research Council of Canada, the Ca- 

 2iadian Meteorological Service, and the Eoyal Canadian 

 Air Force. 



