STORM DETECTION 305 
FIRM ECHOES: 
GROUND 
OlFFUSE ECHOES: 
SNOW 
Figure 1. Typical S-band PPI display of snow echoes. 
S-BAND RADAR ECHOES FROM SNOW? 
Since June 1944, the Canadian Army Operational 
Research Group has been studying the nature and ap- 
plication of S-band radar echoes from storms. During 
the past winter we studied echoes from snow, obtained 
on occasions when snow was present and rain definitely 
was not. 
Heavy snow has been detected on five occasions, 
with maximum ranges varying from 30 to 65 miles. 
One moderate snowfall which kept all aircraft 
grounded was not detected at all, even at the minimum 
range of 10 miles. 
Roughly, 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 case 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 
bBy J. S. Marshall, Canadian Army Operational Research 
Group. 
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- 
erate snow can be 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- 
nadian Meteorological Service, and the Royal Canadian 
Air ‘Force. 
