Chapter 6 
STORM DETECTION 
STORM DETECTION BY RADAR* 
siti sumMer (194+) a study was made of the 
meteorological echoes observed on a Canadian 
microwave S-band early warning radar set at Ottawa. 
These were correlated with check observations on the 
weather made by a large number of local observers dis- 
tributed over the area covered by the set. It was found 
that observers inside the source area of the echo always 
reported rain; just outside the echo (1 to 5 miles) 
there was a half chance of light rain. Atmospheric 
electrical disturbance was present in less than half 
the cases checked. Echoes became less frequent at in- 
creasing distances from the set but in some cases were 
seen at 160 miles. 
The display system that we have used is a plan posi- 
tion indicator [PPI] tube. This tube provides a map 
of a circular area centered on the location of the set 
and extending out at choice to 40 or 80 or 160 miles. 
The last-mentioned setting was the one used most of 
the time. 
Procedure 
During the hours of operation a 16-mm motion- 
picture camera was kept running, taking pictures of 
the PPI display and a clock alongside, exposing one 
frame of the film for the duration of each revolution 
of the array. Thus about four photographs were ob- 
tained per minute. At the same time we watched the 
progress of moving echoes across the screen and made 
telephone calls to any observers that were available, in 
the neighborhood of any echo. From the observer the 
existing state of the weather was determined ; his re- 
marks and the exact time were carefully noted. 
We checked the echoes and their movement as re- 
corded on the film against the information obtained 
about the weather from the observers. We also made 
charts of the echoes, based on the film, at 30-minute 
intervals. 
Weather Information 
Facilities of which we availed ourselves for obtain- 
ing information were, among others: 
1. Ottawa meteorological stations. These stations 
provided us with as many as three forecasts a day and 
with weather information generally. 
2. Distant meteorological stations. Apart from the 
Ottawa stations, the nearest weather station, 57 miles 
away, is at Canton, N. Y. Its reports are included in 
® By Col. J. T. Wilson, Director of Operational Research, 
NDHQ, Canada. 
303 
the teletype sequences that come to us. 
3. Unofficial observers, consulted by telephone. Since 
the official weather stations did not provide the close 
network that we required, we compiled a list of per- 
sons whose location could be closely marked on our 
map and whom we could consult about existing 
weather conditions to the extent that an untrained 
ybserver would be competent. 
Correlations 
Our aim was to correlate observed echoes with 
weather conditions. At first clouds were thought to be 
possible sources of echoes, and it was thought that 
fronts might produce some sort of echo quite inde- 
pendently of cloud or precipitation along the front. 
The earlier part of our work showed that on after- 
noons with heavy cumulus clouds but with no pre- 
cipitation there were no weather echoes. On days with 
scattered showers, however, echoes were observed. 
Also, the passage of a front did not seem to produce 
any peculiar sort of echo or any echo that could not 
be attributed to precipitation along the front. 
In analyzing our correlations, we have found it 
convenient to consider them in two groups. First, there 
are correlations with echo, that is, correlations when 
the observer was in the vicinity of an echo, although 
not necessarily right inside the echo. All correlations 
involving telephone calls to local observers were of 
this type, for we didn’t make such a telephone call 
unless there was an echo in the vicinity. Second, there 
are correlations with weather, or correlations with 
weather stations, when we first select an occasion when 
precipitation is reported (by an official station) and 
then go looking in our records for an echo to match. 
Nearly all the precipitation recorded at the weather 
station during our hours of operation was light, and 
too light, as it proved, for us to detect it at the distance 
we were away. Thus there is only a very small number 
of echoes associated with correlation with weather, 
and so very little overlapping between this group and 
the group of correlations with echo. 
Taste 1. Precipitation inside and just outside echoes. 
Observer’s position relative to echo 
Inside Outside 
(1 to 5 miles) 
No. % No. % 
Cases of no rain 0 0 19 48 
Very light rain 6 13 7 17 
Light rain 16 33 13 33 
Moderate rain 13 27 1 2 
Heavy rain 13 27 0 0 
Total 48 100 40 100 
