304 RADIO WAVE PROPAGATION EXPERIMENTS 
Correlations with Echo 
Table 1 gives a summary of the results obtained in 
the form of a comparison of precipitation observed in- 
side and just outside the echoes. 
Correlations with Weather Stations 
Correlations with weather stations were notable 
chiefly for the rain we did not detect. Nearly all the 
precipitation observed at the station was light and 
apparently too light to produce echoes effective at such 
distances. Weather reports on the teletype from Ot- 
tawa itself were not correlated with echoes because 
most meteorological echoes within 10 miles were ob- 
scured by permanent echoes and distortion at the 
center of the PPI. The next closest weather station is 
at Canton, N. Y., 57 miles from our set. The rainfall 
for every hour was obtained from a rain gauge at 
Canton, and in addition some of the teletyped weather 
reports were received. Rain was reported from Canton 
on five occasions during our hours of operations. On 
one of these occasions we had an echo directly over 
Canton; on three others we had an echo within three 
miles of Canton; on one occasion we had no echo in the 
vicinity at all. The details are given in the table. It can 
be seen that we detected rain falling at a rate of 0.2 in. 
per hour and failed to detect rain falling at a rate of 
0.03 in. per hour. 
TaBe 2. Rain at Canton, New York (U. S. Weather 
Station, 57 miles from set) during analyzed hours of 
operation. 
Rainfall 
Case in. /hr* Thunder Echo 
1 0.20 Yes Overhead 
2 0.05 Yes 1 mile away 
3 0.3 Yes 2 miles away 
4 Trace No 3 miles away 
5 0.03 Yes No echo 
*All rates taken from gauge reading made at hourly intervals. 
Résumé of Correlations 
Our checks with local observers out to 60 miles 
from the set revealed the following: Inside the echo 
there is sure to be rain, with a 0.3 chance that it is 
moderate or heavy. Just outside (1 to 5 miles) there 
is never more than light rain, and a half chance of 
none at all. Further, the chance of an observer in the | 
vicinity of an echo reporting thunder was 0.4. Our 
checks with weather stations were less relevant, be- 
cause only two echoes passed over weather stations 
during the period studied. But from the numerous 
cases of light rain at these stations that we did not 
detect, we can say that we cannot detect light rain at 
90 miles. By light rain we mean rainfall less than 0.1 
in. per hour, and this can just be detected at 50 miles. 
Further, to judge by one storm that we detected and 
one we missed at Canton, we can detect 0.2 in. per 
hour and cannot detect 0.03 in. per hour at 57 miles. 
Fraction Detected by Radar of 
Total Quantity of Rainfall 
Starting from the proportion of hours of rain that 
give an echo, we have used the distribution with rate 
of rainfall of the hours of rain to give us a value for 
the minimum rate of rainfall that will give us an 
echo. Now using a distribution with rate of rain- 
fall of the quantity of rainfall, we can proceed to de- 
termine the proportion of the total quantity of rain 
that was observed by radar. The proportion is quite 
high: 83 per cent close to the set, 62 per cent at 
50 miles. 
Comparison with Ryde’s Theory 
Computations of the echo strength to be expected 
have been made on the basis of the theory developed 
by J. G. Ryde of the General Electric Company (Brit- 
ish). The experimental results are in satisfactory 
agreement with theory. (See page 269 ff.) 
The Best Frequency for Storm 
Detection 
The sensitivity to rain of the frequencies we have 
been using is such that with the power available we 
can obtain satisfactory performance. At higher fre- 
quencies the sensitivity, according to theory, is con- 
siderably higher, but considerations of absorption 
made by Ryde would keep us from going to much 
higher frequencies. Absorption affects us in two dif- 
ferent ways. In the case of widespread rain, even of 
moderate intensity. there is enough absorption between 
the set and the echo source to reduce our effective range 
considerably. Where there is no widespread rain but 
the rain that we want to see is heavy and concentrated, 
the absorption of high-frequency radiation by heavy 
rain can be so great that hardly any of the radiation 
impinging on the storm makes its way back out to be 
reflected. We could actually fail to detect a storm in 
this way, because the storm was too intense. The fre- 
quency we are using (S-band) is safe against both these 
effects, but increasing it by a factor 3 would lead us well 
into them. 
Ultimate Range—Greater Range 
of a Production Set 
The performance of the prototype set we have used 
has been specified in the previous section by its range 
for aircraft. The performance of the same design of 
set, constructed and installed to the final production 
specification, is known to be better: the range for air- 
craft is approximately twice as great, and some calcula- 
tions show that very roughly the range of a produc- 
tion set for storms will be twice that of our proto- 
type set. ‘ 
The full account of this work is published as: Sum- 
mer Storm Echoes on Radar MEW, Report No. 18 
of the Canadian Army Operational Research Group. 
