188 



STORM DETECTION 



and so very little overlapping between this group and 

 the group of correlations with echo. 



^^•** Correlations with Echo 



Table 1 gives a summary of the results obtained in 

 the form of a comjiarison of precipitation observed in- 

 side and just outside the echoes. 



Table 1. Preoijiitation inside and just outside echoes. 



Observer's position relative to echo 

 Inside Outside 



(1 to 5 miles) 

 Xo. % Xo. % 



11.1.5 Correlations with Weather Stations 



Correlations with weather stations were notable 

 chieily 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., r^7 miles from our set. The rainfall 

 for every hour was olitained from a rain gauge at 

 Canton, and in addition some of the teletyped weather 

 reports were received, liain was reported from Canton 

 on five occasions during our hours of operations. On 

 one of these occasions we had au echo directly over 

 Canton ; on three others we had an echo within three 

 luiles 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 wc 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. 



Table 2. Rain at Canton, New York (U. S. Weather 

 Station, 57 miles from set) during analyzed hours of 

 operation. 



*A11 rates taken from gauge reading made at hourly intervals. 



11.1.0 Resume of Correlations 



Our checks with local observers out to GO miles 

 from the set repealed the following: Inside the echo 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 

 che(I<s with weather stations were less relevant, be- 

 cause only two echoes passed over weatlier stations 

 (luring the period studied. But from the numerous 

 cases of light rain at these stations that wc did not 

 detect, we can say that we cannot detect light rain at 

 110 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 0.3 in. per 

 hour and cannot detect 0.03 in. per hour at 57 miles. 



11 1-7 Fraction Detected by Radar of 

 Total Quantity of Rainfall 



Stalling from the proportion of hours of rain that 

 give an echo, we have used the distribution with rate 

 of rainfall of the hoirrs 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 pi'oceed to de- 

 termine the propoition of the total quantity of rain 

 that was observed by radar. The proportion is quite 

 high : 83 per cent close to the set, G8 per cent at 

 ."JO miles. 



11.1.8 Comparison with Ryde's Theory 



('om])utati(>ns of Iho echo stiength to be expected 

 ha\e l)cen made on the l)asis of the theory developed 

 by .T. 0. Ryde of the General Electric Company (Brit- 

 ish). The experimental results are in satisfactory 

 agreement with theory. (See Section 10.1.) 



11.1.9 rpj^p 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 Kyde would keep us fronr going to much 

 higher frequencies. Absorption affects us in two dif- 

 ferent ways. In the case of widespread rain, even of 

 nidderale intensity, there is enough absorption between 



