SCIENCE Development and field evaluation of a lightning earth-flash counter R.B. Anderson, B.Sc.(Eng.), Ph.D., F.I.E.E., F.S.A.I.E.E., H.R. Van Niekerk, B.Sc.(Eng.), M.Eng., M.S.A.I.E.E., H. Kroninger, B.Sc, Grad.S.A.I.E.E., and D.V. Meal, Assoc.S.A.I.E.E. Indexing terms: Lightning and lightning protection, Reviews of progress Abstract: The lightning-flash counters of the 1950s were difficult to calibrate as far as their range was con- cerned, and their discrimination against inter- and intracloud flashes (now referred to as cloud flashes) was found to be poor. Research in South Africa over the past fifteen years was thus concentrated on improving the response of counters to earth flashes and on their calibration. This resulted in the development of a 10 kHz counter, which, over a long period, has a better than 90% response to earth flashes in a subtropical area where cloud flashes predominate, and, furthermore, has an effective range to earth flashes of almost 20 km, which has been reliably measured. Some 400 counters were installed throughout southern Africa, and the earth-flash density has been determined over a seven-year period. This indicates some significant differences with the previously measured distribution of thunderstorm days. Comparisons have been undertaken with other counters, notably the 500 Hz counter developed in Queensland, Australia, which, together with the 10 kHz counter, have been adopted as standard international counters of CIGRE. 1 Introduction The need to measure the earth-flash density, as opposed to the isokeraunic level based on thunderstorm days, began to emerge in the late 1950s. One of the first so-called 'ground'-flash counters was proposed by Pierce [1] and modified by Golde [2], and was used in several countries in early attempts at the measurement of the earth-flash density. A parallel counter was devised by Horner [3], but it was specifically designed to provide a measure of the level of lightning interference in a given area. In 1963 a task force on lightning-flash counters was inaugurated by CIGRE under Golde. Renewed interest in the development of the counter persisted in various countries from that time onwards. In 1965 Barham [4] developed a transistorised version of the Pierce-Golde counter in Australia, and in 1969 Prentice and Mackerras [5] published their measurement of the recording range of the counter using a network of observers in Queensland. They obtained a value for the effective range of 33 km for that locality. This was the first counter to be proposed by CIGRE (see Reference 6) for these measurements. The work in South Africa at the Council for Scientific and Industrial Research (CSIR) started with the 1962 counter design of Malan [7], which was based on his earlier observations. These observations determined that the ratio of the magnitude of radiation from earth flashes at 5 kHz to that at 100 kHz was in excess of 5:1, whereas for cloud flashes it was in the order of only 1:1. However, it was soon found that, whilst this applied to mean values of the radiation, it was not true for instantaneous values occurring during the return stroke. However, this gave rise to the possibility of using frequency responses for counters in the range of 5-10 kHz to discriminate against that of cloud flashes, which had a lesser output at these fre- quencies than earth flashes. Transistorised counters were thus designed for these frequencies and tested in the field measuring system described in more detail in Section 2. As Paper 2932A (S4/S3/E1), first received 1st June and in revised form 2nd November 1983 The authors are with the Council for Scientific & Industrial Research, National Electrical Engineering Research Institute, PO Box 395, Pretoria, 0001 South Africa was reported by Anderson et al. [8] they indeed showed immediate promise of a high level of discrimination against cloud flashes. 2 Lightning measurement system The effective range of a lightning-flash counter is defined as that radius from the counter within which the number of flashes actually occurring over a long period equals the number registered by the counter over the same period. This radius could be applied to either earth flashes {R g ) or cloud flashes (R c ). Given an assumed uniform distribution of flashes over the recording area of the counter, then R g = [2 [ 1/2 km (1) P(r) is the probability that the counter will count a flash to earth at a range of r km from the counter. This can be obtained by observing, over a sufficiently long period of time, the number of earth flashes occurring at each given range interval and recording whether or not they were registered by the counter. A simple integration of the resulting probability curves (for example the curves shown in Fig. 4) gives the effective range R g , which, in practice, coincides approximately with a 30 to 40% probability of counting. The recording system should thus enable observations of the occurrence of lightning flashes at distances of at least more than twice the expected effective range of the counter, until values of P(r) become insignificantly small. To accomplish this, three recording stations (see Fig. 1) were initially set up in the Pretoria area in a triangle at distances of 20, 35 and 40 km apart and fitted with so called all-sky cameras (Fig. 2) which photographed 360° of azimuth. This gave a 'birds-eye view' of lightning flashes (see Fig. 3), and angular measurements of their direction relative to true north could be made. The stations were operated only at night to enable the camera shutters to remain open until a lightning flash occurred. Following this, the electrical signal of any polarity induced on the aerial of a trigger by the lightning flash was used to acti- vate the camera mechanism to move the film to the next frame. The sensitivity of the trigger was set by trial and 118 IEE PROCEEDINGS, Vol. 131, Pt. A, No. 2, MARCH 1984
Transcript
SCIENCE
Development and field evaluation of alightning earth-flash counter
R.B. Anderson, B.Sc.(Eng.), Ph.D., F.I.E.E., F.S.A.I.E.E., H.R. Van Niekerk,B.Sc.(Eng.), M.Eng., M.S.A.I.E.E., H. Kroninger, B.Sc, Grad.S.A.I.E.E., and
D.V. Meal, Assoc.S.A.I.E.E.
Indexing terms: Lightning and lightning protection, Reviews of progress
Abstract: The lightning-flash counters of the 1950s were difficult to calibrate as far as their range was con-cerned, and their discrimination against inter- and intracloud flashes (now referred to as cloud flashes) wasfound to be poor. Research in South Africa over the past fifteen years was thus concentrated on improving theresponse of counters to earth flashes and on their calibration. This resulted in the development of a 10 kHzcounter, which, over a long period, has a better than 90% response to earth flashes in a subtropical area wherecloud flashes predominate, and, furthermore, has an effective range to earth flashes of almost 20 km, which hasbeen reliably measured. Some 400 counters were installed throughout southern Africa, and the earth-flashdensity has been determined over a seven-year period. This indicates some significant differences with thepreviously measured distribution of thunderstorm days. Comparisons have been undertaken with othercounters, notably the 500 Hz counter developed in Queensland, Australia, which, together with the 10 kHzcounter, have been adopted as standard international counters of CIGRE.
1 Introduction
The need to measure the earth-flash density, as opposed tothe isokeraunic level based on thunderstorm days, beganto emerge in the late 1950s. One of the first so-called'ground'-flash counters was proposed by Pierce [1] andmodified by Golde [2], and was used in several countriesin early attempts at the measurement of the earth-flashdensity. A parallel counter was devised by Horner [3], butit was specifically designed to provide a measure of thelevel of lightning interference in a given area. In 1963 atask force on lightning-flash counters was inaugurated byCIGRE under Golde. Renewed interest in the developmentof the counter persisted in various countries from that timeonwards.
In 1965 Barham [4] developed a transistorised versionof the Pierce-Golde counter in Australia, and in 1969Prentice and Mackerras [5] published their measurementof the recording range of the counter using a network ofobservers in Queensland. They obtained a value for theeffective range of 33 km for that locality. This was the firstcounter to be proposed by CIGRE (see Reference 6) forthese measurements.
The work in South Africa at the Council for Scientificand Industrial Research (CSIR) started with the 1962counter design of Malan [7], which was based on hisearlier observations. These observations determined thatthe ratio of the magnitude of radiation from earth flashesat 5 kHz to that at 100 kHz was in excess of 5:1, whereasfor cloud flashes it was in the order of only 1:1. However,it was soon found that, whilst this applied to mean valuesof the radiation, it was not true for instantaneous valuesoccurring during the return stroke. However, this gave riseto the possibility of using frequency responses for countersin the range of 5-10 kHz to discriminate against that ofcloud flashes, which had a lesser output at these fre-quencies than earth flashes. Transistorised counters werethus designed for these frequencies and tested in the fieldmeasuring system described in more detail in Section 2. As
Paper 2932A (S4/S3/E1), first received 1st June and in revised form 2nd November1983
The authors are with the Council for Scientific & Industrial Research, NationalElectrical Engineering Research Institute, PO Box 395, Pretoria, 0001 South Africa
was reported by Anderson et al. [8] they indeed showedimmediate promise of a high level of discriminationagainst cloud flashes.
2 Lightning measurement system
The effective range of a lightning-flash counter is definedas that radius from the counter within which the numberof flashes actually occurring over a long period equals thenumber registered by the counter over the same period.This radius could be applied to either earth flashes {Rg) orcloud flashes (Rc). Given an assumed uniform distributionof flashes over the recording area of the counter, then
Rg = [2 [1/2
km (1)
P(r) is the probability that the counter will count a flash toearth at a range of r km from the counter. This can beobtained by observing, over a sufficiently long period oftime, the number of earth flashes occurring at each givenrange interval and recording whether or not they wereregistered by the counter. A simple integration of theresulting probability curves (for example the curves shownin Fig. 4) gives the effective range Rg, which, in practice,coincides approximately with a 30 to 40% probability ofcounting.
The recording system should thus enable observationsof the occurrence of lightning flashes at distances of atleast more than twice the expected effective range of thecounter, until values of P(r) become insignificantly small.To accomplish this, three recording stations (see Fig. 1)were initially set up in the Pretoria area in a triangle atdistances of 20, 35 and 40 km apart and fitted with socalled all-sky cameras (Fig. 2) which photographed 360° ofazimuth. This gave a 'birds-eye view' of lightning flashes(see Fig. 3), and angular measurements of their directionrelative to true north could be made. The stations wereoperated only at night to enable the camera shutters toremain open until a lightning flash occurred. Followingthis, the electrical signal of any polarity induced on theaerial of a trigger by the lightning flash was used to acti-vate the camera mechanism to move the film to the nextframe. The sensitivity of the trigger was set by trial and
118 IEE PROCEEDINGS, Vol. 131, Pt. A, No. 2, MARCH 1984
error to ensure the recording of flashes over the requiredrange, which was arbitrarily taken as 100 km.
persed around the site in such a way as to avoid inter-ference with each other, and their operations were
Fig. 1 Typical lightning direction finding station
Fig. 2 Automatic 'all-sky' camera
Synchronisation of the stations was achieved by theprovision of frequency-controlled mechanical counters,which were pulsed at one-second intervals and which werestarted simultaneously at all stations, using VHF radioswitching-on tones transmitted at the onset of thunder-storms in the vicinity. The storms were tracked visually bythe operators, and radar assistance was used on occasions.These one-second pulsed counters were situated in thecamera's field of view and were illuminated by small lampseach time the camera shutter was triggered (see Fig. 3).
The various lightning-flash counters under test were dis-
Fig. 3 Lightning flash recorded with 'all-sky' camera
transferred to mechanical counters also placed in thecamera field of view and likewise illuminated with eachoperation of the trigger. The resulting 30 m film strips,which could accommodate some 1200 frames, made it pos-sible to determine the identity of each flash, its azimuthand whether or not the particular counters being moni-tored had operated.
In order to obtain distances, flashes which occurredsimultaneously according to the timing of the respectivecounters were triangulated. An accuracy of range to within1 km was usually possible as judged from the frequentoccurrence of triangulation results from at least threestation records. In fact, two station records were thereafteraccepted with confidence when the angle of intersectionwas reasonably acute.
3 Effective range of lightning-flash counters
The various counters, which were monitored at eachstation and reported upon in this paper, were as follows:
(a) the transistorised version of the 500 Hz Pierce-Goldecounter, having a six-wire horizontal aerial first adoptedby CIGRE [6] and later replaced by an equivalent verticalaerial version [9]. This was designated the CIGRE 500 Hzcounter and is fully described in Reference 10
(b) the 10 kHz vertical aerial RSA 10 counter [8], laterdesignated the CIGRE 10 kHz counter, also fullydescribed in Reference 10
(c) the RSA 5 counter, which was the original 500 Hzcounter referred to in (a), but fitted with a vertical aerialwithout, however, changing the sensitivity level.
Recordings of the probability function P(r) with respect tothe range r were carried out over a period of 12 to 13years. The results for the individual counters mentionedabove are shown in Table 1 and depicted in Fig. 4.
The measurements during the years 1971-74 were madeon the original version of the CIGRE counter having ahorizontal aerial, while those in 1978/79 were made on thevertical aerial equivalent referred to in (a) above. In themeantime, it was well established [11] that the two typesof counters installed in the same location recorded valueswhich did not differ by more than 10%, and were therefore
IEE PROCEEDINGS, Vol. 131, Pt. A, No. 2, MARCH 1984 119
Table 1 : Effective range R0 of counters for earth flashes
0 0.2 0A 0.6 0.8 1.0 1.2 U 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0distonce of floshcounter effective range
Fig. 4 Probability of counting as a function of the ratio of the distanceto an earth flash to the counter effective range
— • - RSA 10 (10 kHz)_ - • - - CIGRE 500 Hz
O - - RSA 5
regarded as equivalent within the accuracy of the measure-ment required.
The value of Rg for the 500 Hz counters of 36.8 km forearth flashes was close to the value reported by Prenticeand Mackerras [5], namely 33 km for Brisbane, S.E.Queensland, Australia. This was approximately at thesame latitude, but virtually at sea level on the eastern sea-board, compared with inland at an altitude of 1600 m forPretoria, Transvaal, South Africa.
On the other hand, the effective range of the 10 kHzcounter as established in the Pretoria area has been par-tially corroborated by measurements made in the TampaBay area, Florida, as reported by Darveniza and Uman[12]. In this report a value of 21.7 ± 0.3 km was deter-mined using proprietary lightning location equipmentpatented by Krider and Noggle [13]. This value is withinthe range of measurements reported in Table 1 over aneight-year period.
In this case the location was on the western gulf sea-board at approximately sea level, and where the meanlightning-flash density appeared to have been between 11and 14 flashes/km2, which is of the same order as that ofthe maximum rate found in South Africa.
Measurements of Rg for the counter in more temperateareas have, as yet, not been reported.
The RSA 5 counter is included because it was later dis-covered to be more sensitive to cloud flashes than the
other two CIGRE counters referred to above, as describedin the following Section.
4 Discrimination against cloud flashes
Assuming the general relation, namely that the earth-flashdensity N is given by
(2)UK9
where K = total registration of a counter
Yg = correction factor (< 1) to account for a
and
proportion of cloud flashes recorded by thecounter
R = effective range of counter to earth flashes
and assuming a uniform distribution of earth and cloudflashes, Prentice [6] showed that the following relationshipholds, namely:
Thus, in order to calculate the value of Yg it would benecessary to know not only the ratio of the equivalentranges Rc/Rg, but also of the densities of cloud and earthflashes, Nc/Ng.
Considerable difficulty attends the measurement ofthese quantities in practice. For example, whilst the mea-surement of Rc was undertaken for all counters, it wasobserved that good accuracy could not be achievedbecause cloud flashes often extended over large distances,certainly in excess of 10 km and more. Generally, cloudflashes operated the counters at close range, i.e. when lessthan 10 km, so that the distance to flashes could only bearbitrarily judged. Similarly, the measurement of Nc/Ng
(the so-called cloud/ground ratio) would have had to beundertaken within a given radius in which all flashes ofboth kinds had to be observed. This would have entailed avery sensitive trigger system capable of ensuring everyflash being photographed within approximately 10 km,and this would have resulted in an enormous increase inthe number of records requiring processing which couldnot be physically accomplished with limited staff.
Thus it was decided rather to measure the value of Yg
directly by observing that
Y= EF9 EF + CF
(4)
where EF = number of identified earth flashes whichoperated the counter observed over along period
and
CF = number of identified cloud flashes whichoperated the counter observed over the sameperiod
The 10 kHz counter was particularly suited to this mea-surement because of its relatively confined range. Theresults accumulated over nine years are given in Table 2.
Thus in 4910 observations only 345 cloud flashes wereobserved to operate the counter, giving an overallweighted correction factor Yg = 0.93.
Greater accuracy of the observation was attained by thefact that cloud flashes which operated the counteroccurred within a shorter range of the counter than theearth flashes, and were thus not readily missed. Earth
120 IEE PROCEEDINGS, Vol. 131, Pt. A, No. 2, MARCH 1984
flashes, on the other hand, which occurred too far away tobe photographed but nevertheless operated the counter,were excluded, and their addition to the records wouldhave increased the value of Yg towards unity.
It was thus established with some confidence that the 10kHz counter could be relied upon to count earth flasheswithin an accuracy of 10% if cloud flashes were ignored. Itcould, of course, be relied upon for even better accuracy ifa correction Yg was applied to the result, whereupon, for
g
the conditions in SouthAfrica, with Rg = 19.9 km and
K1340
earth flashes/km' (5)
This also illustrates the point that the counter could beused for accurate measurement of an earth-flash density ofless than one per km2.
Having established a reliable calibration for the 10 kHzcounter and knowing at least the values of Rg for the othertwo counters, it was thus possible to calculate the corre-sponding values of their correction factors Yg as illustratedin Table 3 for the values of Ng measured.
Table 3: Calculated mean correction factors ¥„ for all counters
It is thus clear that the 10 kHz counter results indicatean improved discrimination against cloud flashes com-pared with the other two counters, and that the RSA 5was, in fact, the most sensitive to cloud flashes. If Rc forthis counter was known, it could be used to measurecloud-flash density. In fact, Rc was established approx-imately from 630 observations as 7.9 km. Accordingly,using the value of earth-flash density calculated from the10 kHz counter as a base, the resultant cloud-flash densityfor Pretoria was calculated as indicated in Table 4.
As can be seen, there is a consistency with regard to theabove results which engenders some confidence that thevalues are correct. The mean ratio of 2.5 obtained mightappear low for a subtropical area, but it is clearly defined
by the consistent annual measurement of the earth-flashrange and the correction factor Yg for the RSA 5 counter.However, the threshold level used for triggering of thecamera equipment and the optical sensitivity of the filmboth play a significant part; hence the term 'apparentNc/Ng ratio'. The ratio obtained is, however, greater thanthe value of 2.24 obtained in a three-year investigation inSouthern Rhodesia (now Zimbabwe) from field changeobservations [14].
It follows from eqn. 2 that the ratio of the registrationof two different counters operating in the same area isgiven by the following relation:
(6)
Thus, using the values shown for Rg and Yg given in Table3, the CIGRE 500 Hz counter should register approx-imately four times the number of counts of the 10 kHzcounter under the conditions measured; there was ampleconfirmation of this (see Reference 11 and Table 3).
It was observed, however, that all parameters varied,sometimes within wide limits from storm to storm, ormonth to month, and even from year to year, as illustratedin the Tables presented, and it was thus necessary toobserve data over a sufficiently long period to ensure thatstatistically significant results were obtained. Measure-ments made over short periods would often indicate quitedivergent results, which obviously could not be relied uponnor accepted.
5 Earth-flash density for southern Africa
Approximately 400 counters of the CIGRE 10 kHz type(then the RSA 10) were installed throughout the Republicof South Africa and South-West Africa (Namibia). Thecounters were spaced more closely together in regions ofhigh isokeraunic level than in the more arid areas on thewest of the subcontinent. This also roughly coincided withthe distribution of population in corresponding townswhere counters could be read and monitored. The bulk ofthem were purchased and operated by public organis-ations, including the Department of Posts and Telegraphs,the Electricity Supply Commission, the South AfricanBroadcasting Corporation, the Department of WaterAffairs, National Parks, the Weather Bureau, Municipal-ities and some individuals. Representatives of these bodiesserved on a task force which met biannually to monitorthe operation of the project and consider the results of theprocessing of the data carried out by the National Electri-cal Engineering Research Institute (NEERI) of the CSIR.
Counters were read mostly daily and recorded on
IEE PROCEEDINGS, Vol. 131, Pt. A, No. 2, MARCH 1984 121
prepaid postcards (see Fig. 5), which, when received, wereentered into a computer data bank for analysis. The
in Fig. 8 and reported by Eriksson (see Anderson andEriksson [17]).
Fig. 5 Prepaid postcard recording form for lightning-flash-counter dailyreadings
program devised by one of the authors [15] sorted andsmoothed the data to portray weighted mean conditions. Amap showing lines of equal flash density (isokerauniclevels) could be plotted at monthly intervals, or cumulativedata could be acquired every six months or annually asrequired.
The counters which weighed less than 1 kg and wererelatively small (see Fig. 6) were returned by post for reca-libration to the standards laid down for CIGRE 10 kHz
counters [10] and for fitting new batteries. At the lastreview 98.4% of all counters tested were still within 5%,and 93.4% were within 2% of the specified calibrationlimits. Furthermore, of 380 counters and in nearly 2200counter-years, only 41 have been damaged, six directly bylightning, but only one counter could not be repaired. Fiveprinted-circuit boards, five surge suppressors, six capac-itors, 16 transistors, eight diodes and 12 relays werereplaced in total.
The lightning ground-flash-density pattern determinedfrom the results for seven-years accumulated data is givenin Fig. 7, which shows also thunderstorm day levels fordirect comparison. As can be seen, whilst there is a faircorrelation between the two at lower values, the areawhere the flash density is maximum does not coincide withthe area shown for the maximum thunderstorm day levelswhich were determined from observations over 12 years bythe Weather Bureau [16]. This accounts for the ratherpoor correlation between the two quantities illustrated
Fig. 7 Lightning-flash density compared with isokeraunic level (thun-derstorm days) for southern Africa
a Lightning ground-flash density map of southern Africab Isokeraunic levels in South Africa (thunderstorm days)
20
10
5
2 -
1 -
0.5 :
0.2
0.1
based on 120 observations '•1» ]J• • "** j[
.r'/•:•'
Ng =0.023 Tp3 *./X. ' '
1 ' •
* i i i
/
1 2 A 6 10 20 40 60 100 200thunderstorm days (TQ)
Fig. 8 Correlation between lightning-flash density and isokeraunic level
• 1976/1977x 1977/1978
122 IEE PROCEEDINGS, Vol. 131, Pt. A, No. 2, MARCH 1984
Measurements are to be continued for at least an11-year solar cycle, for past records for Pretoria show [11]that there is a probability of having at least one year in tenwhen the lightning density exceeds a normal year by afactor of 1.7. However, many more years of observationwould possibly be required to determine extreme values.
6 Global application of counters
One of the factors mitigating against the use of countersworld wide has been the uncertainty as to whether light-ning characteristics are sufficiently uniform on average, sothat the distribution of field changes and their frequencycharacteristics and polarity do not differ materially at dif-ferent latitudes or altitudes of the locations. Evidence onbalance as presented by Anderson and Eriksson [17] sug-gests that lightning is similar the world over, and some ofthe results reported here support this, at least with regardto their influence upon the effective range of counter toearth flashes. However, cloud-flash activity is known tovary (see Prentice and Mackerras [18]), and cognisancemust be taken of this, especially with counters having rela-tively poor discriminating characteristics.
One of the authors developed a method [19] wherebythe approximate effective range of a counter could bechecked in any situation. Referring to Fig. 9, two identical
volume enclosed underneathboth probability curves
""distance""
Fig. 9 Presentation of the probability of simultaneous operation of twocounters at A and B, with overlapping operation areas
counters situated at distance d apart and having a prob-ability function, for example, as depicted in Fig. 4, wouldhave an overlapping operational area shown enclosedbetween the two respective probability curves.
Consequently, lightning flashes occurring in the neigh-bourhood have a certain probability of being countedsimultaneously by the two counters or a probability ofbeing counted by one of the counters only, according tothe distance of the flash from the respective counters. Thefirst probability can be calculated in terms of the ratio ofsimultaneous counts of both counters to the total counts ofeach counter (averaged if different), namely KJK, for earthflashes only, and plotted against the ratio d/Rg.
For the situations where there are no cloud flashes, i.e.only earth flashes, the characteristic function is shown bythe curve for Yg = 1.0 in Fig. 10. For situations wherecloud flashes occur and are partly counted, a unique curvecould be drawn, provided that both RJRg and NJNg areknown, as mentioned in Section 4.
However, it was demonstrated by calculation that thedifferences in values of d/Rg that would be obtained forvalues of Yg varying between 0.9 and unity would be lessthan 10% if the counters were placed a distance apart, d,approximately equal to the expected value of the effectiverange Rg (i.e. d/Rg= 1).
Thus, if the number of simultaneous counts for thecounter can then be obtained by relaying the operation ofone counter to the other by whatever convenient means(such as land-line or radio data transmission) over a rea-
sonably long period, Fig. 10 can be used to estimate theratio d/Rg and thus check the value of Ra. This is particu-
3£~0.8 l
oo- 0 . 6oin
3 0.4oit
§0.3"5I 0.2
2 0
Yo=0.5
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0ratio of separation to range (d/Rq)
Fig. 10 Ratio of simultaneous counts to total counts as a function of theratio of station separation distance to the effective earth-flash range
larly convenient if the CIGRE 10 kHz counter is used,whereby the expected value of Rg is 20 km, and theexpected value of Yg lies between 0.9 and unity for moretemperate parts of the globe. For areas lying within thetropics, Yg for this counter can be assumed to lie between0.8 and 0.9. The limits for the 500 Hz counter would belarger according to the evidence presented in this paper.
7 Conclusions
The paper sets out to show by long-term measurementsthat the 10 kHz counter now adopted by CIGRE has char-acteristics which favour its use for measuring earth-flashdensity with reasonable accuracy. Furthermore, whilst theindications are that the parameters measured in the Preto-ria area are applicable in other areas of the globe, methodsare described whereby the effective range may be checkedin any situation. A counter suitable for measuring theapparent cloud-flash density has also been developed.
8 Acknowledgments
The authors acknowledge the support of the Council forScientific and Industrial Research and, in particular, theencouragement of the Director of the National ElectricalEngineering Research Institute for the successful conclu-sion of this project. Thanks are due also to Mrs. M.A.Smith for analysing the field data over many years.
9 References
1 PIERCE, E.T.: The influence of individual variations in the fieldchanges due to lightning discharges upon the design and performanceof lightning flash counters', Arch. Met. Geophys. & Bioklim Ser. A.,1956,(1), pp. 78-86
2 GOLDE, R.H.: 'Lightning flash counter'. Electrical Research Associ-ation of Great Britain Report, July 1957
3 HORNER, F.: 'The design and use of instruments for counting locallightning flashes', Proc. /££ , 1960, 107, Part B, pp. 321-330
4 BARHAM, R.A.: 'A transistorised lightning flash counter', Electron.Eng., 1966, 38, p. 164
5 PRENTICE, S.A., and MACKERRAS, D.: 'Recording range of alightning flash counter', Proc. / ££ , 1969, 116, pp. 294-302
IEE PROCEEDINGS, Vol. 131, Pt. A, No. 2, MARCH 1984 123
7 MALAN, D.J.: 'Lightning counter for flashes to ground'. Proc. Inter-national Conference on Gas Discharges, Butterworth, 1962
8 ANDERSON, R.B., VAN NIEKERK, H.R., and GERTENBACH,J.J.: 'Improved lightning earth-flash counters', Electron. Lett., 1973, 9,pp. 394-395
9 BARHAM, R.A., and MACKERRAS, D.: 'Vertical-aerialCIGRE-type lightning-flash counter', ibid., 1972, 8, pp. 480-482
10 ANDERSON, R.B., VAN NIEKERK, H.R., PRENTICE, S.A., andMACKERRAS, D.: 'Improved lightning flash counters', Electra,1979,(66), pp. 85-98
11 ANDERSON, R.B., MEAL, D.V., and SMITH, M.A.: 'Eleventhprogress report on the development and testing of lightning flashcounters in the Republic of South Africa during 1978/79'. CSIRSpecial Report ELEK 168, May 1979
12 DEVENIZA, M, and UMAN, M.A.: 'Research into lightning protec-tion of distribution systems II—Results from Florida field work 1978
and 1979'. Paper 83 SM454-6 presented at No. 855 to the IEEESummer Power Meeting, July 1983
13 KRIDER, E.P., and NOGGLE, R.C.: 'A detection system for light-ning'. US Patent Office 4115732, September 19, 1978
14 ANDERSON, R.B.: 'The lightning discharge'. Ph.D Thesis, Uni-versity Cape Town, CSIR Special Report ELEK 12, Part 1, 1971, pp.48-49
15 KRONINGER, H.: 'Newsletter—national lightning recording schemeNo. 3'. CSIR Special Report ELEK 181, September 1979
16 South African Weather Bureau, Department of Transport: 'Climateof South Africa'. Pt8 General Survey W.B.28, 1965, p. 298, Fig. 154
17 ANDERSON, R.B., and ERIKSSON, A.J.: 'Lightning parameters forengineering application', Electra, 1980, (69), pp. 65-102
18 PRENTICE, S.A., and MACKERRAS, D.: The ratio of cloud-to-cloud-ground lightning flashes in thunderstorms', J. Appl. Meteoroi,1977, 16, pp. 545-549
19 VAN NIEKERK, H.R.: 'Calibration of lightning flash counters'.CSIR Special Report ELEK 49, July 1974
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