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Electric Power Systems Research 118 (2015) 93–100

Contents lists available at ScienceDirect

Electric Power Systems Research

j o ur na l ho mepage: www.elsev ier .com/ locate /epsr

ay-to-day differences in the characterization of lightning observedy multiple high-speed cameras

.S. Antunes ∗, A.C.V. Saraiva, O. Pinto Jr., J. Alves, L.Z.S. Campos, E.S.A.M. Luz,. Medeiros, T.S. Buzato

LAT, Atmospheric Electricity Group, INPE, National Institute for Space Research, São José dos Campos, SP, Brazil

r t i c l e i n f o

rticle history:eceived 10 March 2014eceived in revised form 28 July 2014ccepted 30 July 2014vailable online 24 August 2014

eywords:ightning characteristicsigh speed camerashunderstorm days

a b s t r a c t

This study aims to analyze how the visible parameters of cloud-to-ground discharges such as the flashmultiplicity and duration, the interstroke interval, the continuing current duration and the number ofground contacts per flash vary between observations conducted on different days. Several authors havealready analyzed these parameters for groups of thunderstorms in some regions. Although some authorsdid not find differences on those characteristics for different regions, there has not been a comprehensiveamount of lightning recordings from the same thunderstorm in order to evaluate such parameters in astorm-to-storm basis. The lightning data for this work was obtained by four high-speed cameras set tooperate at 2500 frames per second. They were located in São José dos Campos, Brazil, and in its neighboringregion as part of the RAMMER project (Portuguese acronym for Automated Multi-Camera Network forMonitoring and Study of Lightning Flashes). Five thunderstorm days were selected for this study, during

which a total of 361 flashes were recorded. This is the first study in which four high-speed camerasobserved the same thunderstorm from different locations. Because of the number of cameras and theirpositions, the coverage area is larger than usual single-camera studies, thereby increasing the number offlashes recorded from the same thunderstorm. These larger samples allow a more representative analysisof the lightning parameters mentioned above.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

The knowledge of characteristics of cloud-to-ground lightnings very important, as it may affect human activities directly. In [1]he authors showed, for example, how important it is to know theatio between single stroke flashes and multiple stroke flashes forhe protection of transmission lines. In addition to causing damagesf the order of billions of dollars [2], lightning is also responsible forosses of human lives. Studies show that about 130 people die eachear due to accidents involving lightning in Brazil [3]. Understand-ng the characteristics of lightning occurring in Brazil is essentialot only for science but also for protection in social and technolog-

cal contexts. Some of the main visible characteristics of negativeloud-to-ground (CG) lightning flashes are the number of returntrokes per flash (multiplicity), the interstroke interval, continu-

ng current and flash duration, as well as the number of groundontacts. Studies of these characteristics have been already made

∗ Corresponding author. Tel.: +55 (12) 3208-6801.E-mail addresses: larissa.silva@inpe.br, antunesla86@gmail.com (L.S. Antunes).

ttp://dx.doi.org/10.1016/j.epsr.2014.07.030378-7796/© 2014 Elsevier B.V. All rights reserved.

by different authors. Table 1 summarizes some of the results ofprevious studies.

High-speed cameras became one of the main tools to study thevisible characteristics of lightning flashes; however, every tech-nique has its limitations. Given the electronic construction of thistype of camera, it cannot record at the same time as it transfersthe video files to the PC, particularly when operated at high framerates. Thus, the time interval between video records depends onthe number of recorded frames and their spatial resolution, whichinfluences the total file size of each individual video. High-endcameras have the ability to store large amount of data in specialstorage hard drives, in which the file transfer is extremely fast.Such devices are extremely expensive and still may not be able torecord all flashes of one single thunderstorm due to visibility issues,like intervening rain shafts. A cost-efficient alternative solution isthe use of multiple intermediate cameras, forming a network toobserve the thunderstorm from different angles. In order to achievethis objective the project RAMMER [11] was created. RAMMER is a

network of high-speed cameras whose main objective is the obser-vation of thunderstorms at various angles, automatically triggered,increasing the total number of recorded flashes per thunderstorm.The observations in this study were made during the summer of

94 L.S. Antunes et al. / Electric Power Systems Research 118 (2015) 93–100

Table 1Summary of the results of previous studies on the characteristics of lightning by various authors.

Authors Number offlashes

Single strokeflashes (%)

Multiplicity Inter-stroke(ms)

Flashduration

Instrumentation

Kitagawa et al. [4] 193 14.0 6.4 – – Electric field antenna and rotating-film cameraRakov et al. [5] 76 17.0 4.6 60.0 – Electric field and standard TV cameraCooray and Perez [6] 137 18.0 3.4 48.0 – Electric field antennaCooray and Jayaratne [7] 81 21.0 4.5 56.5 – Electric field antennaSaba et al. [8] 233 20.0 3.8 61.0 163.0 High-speed camera and RINDATa LLSb

Saraiva et al. [9] 432 19.5 3.9 61.5 226.0 High-speed camera and RINDATa LLSb

Ballarotti et al. [10] 883 17.0 4.6 59.0 300.0 High-speed camera

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a RINDAT stands for Integrated Network of National Lightning Detection.b LLS stands for lightning localization systems.

012/2013 in the city of São José dos Campos, São Paulo (south-astern Brazil) and its neighboring cities. The RAMMER networkad four sensors strategically placed to cover the area of interest.ive thunderstorm days were chosen due to their large numberf recorded flashes. The objective of this work is to analyze theharacteristics of lightning individually for each thunderstorm day,ince the number of cameras observing the same region allowed thecquisition of a statistically significant database. That will allow thevaluation of possible relevant differences between some lightningharacteristics per day. The results should also improve our under-tanding of the determining factors in the formation of lightningver one given region.

. Instrumentation

The RAMMER network was formed by four observation stationsuring the summer season of 2012/2013. Three stations operatedt fixed locations and were protected by weatherproof housing ashown in Fig. 1. The housing has been designed to protect the equip-ent of climate action, allowing each station to operate outdoors.ne of the stations was operated in a car because the chosen site didot have an adequate fixed location. Each station was mounted in

standard way and contained the same instruments: (a) Phantom9.1 high-speed camera set to operate with a spatial resolution of

200 × 500 pixels at 2500 frames per second; (b) a computer with

TB of hard disk space, responsible for running system control andata storage software; and (c) a GPS antenna to synchronize eachrame recorded with a precision of 1 ns, allowing the correlation

ig. 1. Weatherproof housing for the instruments of each fixed RAMMER station.he station shown in the picture was installed (RAMMER 1, or R1) on the base of aV tower in the northern region of São José dos Campos.

between the lightning data recorded with the information providedby the local lightning location system.

Each fixed stations also had a photodiode sensor that emits atriggering pulse when drastic changes in ambient light occurs. Thispulse activates the camera at the exact moment the return strokelight hits the photodiode, allowing the automatic operation of thesensors. The RAMMER Mobile Station was manually operated anddid not require both housing and automatic triggering system.

The position of each station was carefully chosen so that all cam-eras could observe the central region of the city of São José dosCampos as shown in Fig. 2. Their names were given according to theorder of installation. The first one (RAMMER 1, or R1) was installedon the base of a TV tower in the northern region of São José dosCampos. The second, RAMMER 2 (R2), was installed in a tower ofSimoldes Plasticos, located in Cac apava, a neighbor city of São Josédos Campos. RAMMER 3 (R3) was placed in the tower of AdvancedStudies Institute (IEAv) in São José dos Campos. Finally, the fourthstation, RAMMER Mobile (RM), was adapted in a car and operatedwithin the campus of the University of Vale do Paraíba (UNIVAP).

Among the three stations programmed to work automatically,only R1 did it full time, while R2 and R3 were operated manuallyduring the thunderstorm days chosen for this work. All recordingsoccurred in the summer of 2012/2013, and only those days withthe higher number of flashes recorded, February 18th, 19th and22nd, and March 06th and 08th of 2013, where chosen. There were706 videos recorded during this period. Videos with lightning out-side the field of view were discarded from the analysis, as well asintra-cloud discharges and positive flashes (the polarity was givenby the LLS). After the data reduction, 361 videos containing con-firmed negative CG flashes were used. The LLS used in this workwere BrasilDAT (Brazilian Lightning Detection Network) [12] andRINDAT (Integrated National Lightning Detection Network) [13].Their data allowed us to identify the location and polarity of thereturn strokes.

3. Analyses and results

The lightning data were analyzed for each thunderstorm day,and also altogether in order to ascertain whether there are signif-icant differences between days. A total of 361 negative CG flashwere selected for this analysis, among which 26% (93 flashes) didnot have their polarities identified by the LLS. Due to the presenceof high multiplicity and visible branching, characteristics that areprevalent in negative CG flashes [1], these flashes were classified asof negative polarity. For events that were registered by more thanone camera, only the record with the clearest imagery was consid-ered in the analysis. This procedure allowed a great reduction inthe possibility of a single event to be double-counted in the statis-

tical analysis. Fig. 3 shows an example of lightning that occurred onFebruary 18th and was recorded simultaneously by three cameras.In this case only the lightning recorded by R3 was considered in theanalyses.

L.S. Antunes et al. / Electric Power Systems Research 118 (2015) 93–100 95

Fig. 2. Location of the four RAMMER stations.

ed sim

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Fig. 3. Example of lightning flash record

Table 2 shows the amount of daily recorded negative CG flashesor each thunderstorm day and of each RAMMER station. The flashesecorded by more than one camera were counted only once.

The visible characteristics analyzed were flash multiplicity anduration, interstroke time interval, duration of long continuing cur-

ent (CC) (greater than 40 ms) [1,14], duration of short continuingurrent (CC) (between 10 and 40 ms) [15], and number of groundontact points.

able 2umber of recorded lightning flashes per thunderstorm day by the RAMMERetwork.

Date R1 R2 R3 RM Subtotal

02/18 0a 25 24 19 6802/19 11 17 19 27 7402/22 6 14 13 22 5503/06 14 31 34 38 11703/08 18 8 1 20 47

Total 361

a RAMMER 1 was offline due to technical problems.

ultaneously by three different cameras.

3.1. Multiplicity

The total number of return strokes recorded for all days was1563, grouped into 361 flashes, which corresponds to an averageof 4.3 strokes per flash. 20.8% (75) of all flashes were composed byonly one return stroke. These values are similar to the results foundby [8–10] in the same region of the present study.

Table 3 shows the total duration of the observation periods ofeach thunderstorm, the recorded stroke count, the percentage ofsingle stroke flashes, the arithmetic mean of the multiplicity (indi-vidually per thunderstorm), and the total multiplicity of the entiredataset. It is possible to notice a significant variation of some ofthese characteristics between thunderstorm days. It is worth not-ing that the region where these thunderstorms were recorded isbasically the same. On February 22nd we observed the greatestvariation, with 36.4% of single stroke flashes, which helped lower-ing the mean multiplicity to 2.8, a value that is much lower than

those found in literature [4–10]. Between the days with the lowest(02/22) and highest (03/06) multiplicities, a factor of 1.8 was found.

Fig. 4 presents the compiled distribution of the multiplicity forall observed flashes and its shape is similar to what is usually found

96 L.S. Antunes et al. / Electric Power Systems Research 118 (2015) 93–100

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14.8%). These strokes were classified as forked because two or moreground contact points were observed hitting the ground within390 ms time scale (exposure time of the camera). These events willbe addressed in detail in a future work.

Table 4Geometric mean and maximum value of flash durations for each thunderstorm day.

Date Maximum duration (ms) Geometric mean (ms)

02/18 1257.2 235.9

ig. 4. Distribution of flash multiplicity for the entire dataset and for each thunderstre marked on each histogram.

n literature [4–10]. The distributions of flash multiplicities for eachay are also presented separately.

On February 19th, the largest multiplicity was 16 and 36.5% (27)f the flashes presented multiplicities greater than 5.

The flash with the highest number of strokes registered onebruary 22nd had only 10 strokes, only 16% (9) of the flashes hadultiplicities greater than 5. On March 6th, the flash with the high-

st number of strokes of the whole dataset was found, presenting 20trokes. 34% (40) of the flashes in this day had more than 5 strokes.n March 8th, the highest multiplicity was 14, with 19% (9) of theashes presenting more than 5 discharges.

.2. Duration

The duration of the flash was defined as the time intervaletween the moment that the first return stroke touches the groundnd the end of the faintest luminosity of the last subsequent stroker the end of its continuing current (if present). 278 flashes weresed for this analysis because the durations of single stroke flashesnd forked strokes were not included. The geometric mean (GM) ofhe overall durations was 270.2 ms. This value is higher than 163nd 226 ms obtained for the same region in previous studies [4–10].

The longest duration was found on March 6th with 1800.4 ms,

orresponding to a flash with 12 return strokes. This value is one ofhe highest recorded in the literature. Table 4 shows the geometric

ean of the flash durations, as well as the maximum value foundn each thunderstorm day.

able 3bservation period (in minutes, between the first and the last recorded flashes),umber of recorded strokes per day, percentage of single stroke flashes and averageultiplicity for each thunderstorm day.

Date Observationperiod (min)

Recordedstroke count

% ofsingle-strokeflashes

Averagemultiplicity

02/18 140 289 17.6 4.202/19 141 350 17.5 4.702/22 133 157 36.4 2.803/06 209 599 12.8 5.103/08 126 168 32.0 3.6

All 749 1563 20.8 4.3

ay separately. The identification of day, sample size (N) and arithmetic means (AM)

Fig. 5 shows the general distribution of all flash durations and thedistributions per thunderstorm day. Again, February 22nd had themost different distribution of the whole set. The geometric mean offlash duration of the day was 193.1 ms, while all days had an overallgeometric mean of 270.2 ms.

3.3. Interstroke interval

The 361 analyzed flashes produced 1160 interstroke intervals.The very short intervals (of the order of 0.4 ms) were associatedwith forked strokes, i.e., strokes that hit the ground at virtually thesame time [16]. Table 5 shows the number of forked strokes perday which, although small, had a relatively large range (from 7.3% to

02/19 1448.0 358.502/22 852.0 193.103/06 1800.4 280.103/08 1082.4 248.7

All days 1800.4 270.2

Table 5Number of forked strokes, sample size (N) and geometric mean (GM) of the inter-stroke intervals for each thunderstorm day.

Day Forked strokes(%)

Interstrokeintervals(number)

Interstrokeintervals GM(ms)

02/18 7.3 213 56.202/19 14.8 271 69.602/22 7.3 91 61.603/06 9.4 468 52.003/08 8.5 117 47.8

Total 9.7 1160 56.7

L.S. Antunes et al. / Electric Power Systems Research 118 (2015) 93–100 97

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(168) of flashes with multiple ground contact points among thosewith multiple strokes.

Fig. 9 shows the distribution of the number of contact pointsfor the entire dataset and the distribution of contact points for

Table 6Statistics of short continuing current duration per thunderstorm day.

Date Number ofstrokes withshort CC

% of flasheswith short CC

GM of short CCdurations (ms)

02/18 25 10.3 17.302/19 46 29.7 18.902/22 36 21.8 16.303/06 44 15.4 16.003/08 18 12.8 15.4

Total 172 18.0 16.5

Table 7Statistics of long continuing current duration per thunderstorm day.

Date Number ofstrokes withlong CC

% of flasheswith long CC

GM of long CCdurations (ms)

02/18 15 11.8 79.402/19 30 27.0 121.0

ig. 5. Distribution of flash duration of all 278 flashes and for each day separatelyay, sample size (N) and geometric means (GM) are marked on each histogram.

The longest intervals were associated with long continuing cur-ent. Intervals longer than 1 s were discarded since they wererobably related to a different flash captured in the same record.one of these longer intervals corresponded to sequences of strokes

ollowing the same channel to the ground, which strongly sug-ests that these events are not the same flash. The longest intervaleasured was 772 ms on March 6th, and it occurred between the

econd and third return strokes. They followed the different patho ground and the second stroke presented a long continuing cur-ent of 162 ms. The overall GM was 56.7 ms, a value similar to thoseound in other studies (around 60 ms [6–10]).

Table 5 shows the sample size and the GM of the interstrokentervals for each day, and the number of forked strokes. The highestM was obtained on February 19th, which also had the highestumber of strokes with long continuing current (30 or 8.6%). Theay with the lowest geometric mean of interstroke intervals wasarch 8th (47.8 ms).Fig. 6 shows the distribution of interstroke intervals for all

ashes and the distribution of interstroke intervals for each day; noignificant differences were found among the thunderstorm days.

.4. Continuing current

The continuing current (CC) can be defined as a constant varia-ion of the electric field, reflected as a continuing persistence of thehannel brightness after the return stroke process and can be clas-ified into two major groups depending on their duration [4,14,15].he CC is considered long when its duration exceeds 40 ms [4,14]nd short when its duration is between 10 and 40 ms [15].

In this study, CC was identified on video records of high-speedameras. However, these measurements could be underestimatedecause of the distance between the return stroke and the camera,r when the channel is obscured, for example, due to the presencef the rain shafts. This effect will be addressed in a future work.

Tables 6 and 7 show the incidence of short and long CC,espectively, for each thunderstorm day. Figs. 7 and 8 show the

istribution of short and long CC durations, respectively, for thehole dataset. The percentage of flashes with long CC was 21% (76),hile 18% (65) of the flashes presented short CC. Therefore, 39% of

he flashes showed some kind of continuing current. The largest CC

ingle and forked stroke events were not included in the analysis. Identification of

found was 757.1 ms on February 19th, on the fifth and last returnstroke of a lightning flash that had four different contact points tothe ground.

3.5. Number of ground contacts

Table 8 shows how many flashes produced a certain number ofground contact points, on each day. The average for all flashes was1.8 and the average for multiple flashes (excluding single strokeflashes) was 2.0. Nearly half (46.5%) of the flashes observed pre-sented more than one ground contact point. Excluding the singlestroke flashes from the total, the percentage increases to 58.7%

02/22 22 29.0 120.703/06 24 15.4 129.403/08 16 15.4 183.0

Total 107 21.0 123.1

98 L.S. Antunes et al. / Electric Power Systems Research 118 (2015) 93–100

Fig. 6. Distribution of interstroke intervals for the combined dataset for each thunderstorm day. Identification of day, sample size (N) and geometric means (GM) are markedon each figure.

Fig. 7. Distribution of short continuing current durations for the whole dataset and distg

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eometric means (GM) are marked on each histogram.

able 8umber of flashes with multiple ground contacts, percentage end arithmetic mean.

Date Number of flasheswith multipleground contacts

Flashes with % ofmultiple groundcontacts

Arithmetic mean ofnumber of groundcontacts

02/18 37 54.4 2.002/19 35 47.3 2.102/22 22 40.0 2.003/06 56 47.9 1.903/08 18 38.3 2.0

Total 168 46.5 2.0

ribution of short CC durations from the 5 thunderstorm days, sample size (N) and

each thunderstorm day. The number of multiple stroke flashes withmore than three contact points was 25 (or 8.7%) this value is higherthan a previous statistics obtained by [9] in the same region (2.6%).

4. Discussions

The first parameter that was analyzed was flash multiplicity. In[8] and [9], the arithmetic mean of multiplicity found for the sameregion was 3.9. In this work, the arithmetic mean for all flashes

was 4.3. Although these numbers are very similar, the presentedanalysis showed considerable variation per day. On March 6th, forexample, the average multiplicity was 5.1 with 34.2% (40) of theflashes presenting multiplicities greater than 5, while on February

L.S. Antunes et al. / Electric Power Systems Research 118 (2015) 93–100 99

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ig. 8. Distribution of long continuing current durations for the whole dataset aneometric means (GM) are marked on each histogram.

2nd the average was 2.8 and only 16.4% (9) presented multiplici-ies greater than 5.

It is interesting to notice that March 6th also presented the low-st percentage of single stroke flashes (12.8%), while February 22ndad the highest percentage of single stroke flashes (36.4%). Thisalue is almost twice the mean value found for single flashes (20.8%)or all thunderstorm days. By correlating the last two columns ofable 3, one can verify that average multiplicity is inversely propor-ional with the occurrence of single stroke flashes. Fig. 10 shows alot of this relationship and the linear fit presented an R-value of

.94.

The geometric mean for all flash durations was 270.2 ms, higherhan 163 and 229 ms, values previously found for the same regiony [8,9], respectively. However, Ballarotti et al. [10] found a closer

Fig. 9. Distribution of ground contact points for the entire dataset and for each day

ribution of long CC durations from the 5 thunderstorm days sample size (N) and

value to the one found in this work (300 ms).The daily geometricmean also presented variations. The lowest geometric mean of flashdurations occurred on February 22nd (193.1 ms), and almost twicethat value was observed on March 8th (358.4 ms). The flash whichshowed the longest duration occurred on February 18th, lastingabout 1800 ms. As stated before, this is one of the highest valuesrecorded in literature to date. On February 22nd, the maximumduration found for a flash was 852 ms, which is much lower thanthe values found in the other days.

Intriguingly, February 22nd had the lowest number of sub-

sequent strokes (91) and the geometric mean of the interstrokeintervals was 61.6 ms. In contrast, March 6th had the highest num-ber of subsequent strokes (461) and the geometric mean of theinterstroke intervals was 52.0 ms.

, sample size (N) and geometric means (GM) are marked on each histogram.

100 L.S. Antunes et al. / Electric Power Syste

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ig. 10. Relationship between flash multiplicity and percentage of single strokeashes. The continuous line is the linear fit applied to the data.

The geometric mean of long CC for all flashes was 123.1 ms andongest CC was found on February 19th, 753.7 ms, associated with

flash with multiplicity 6 and 4 ground contact points.Considering all the thunderstorm days, 8.7% (25) of flashes pro-

uced more than three distinct contact points to the ground. Thisalue is higher than 2.6%, which was previously found for the sameegion [9]. Considering only the 58 flashes of February 19th, thisercentage increases to 16.4% (9).

In this preliminary study, no analysis on the thunderstorm char-cteristics was performed. A future work will address if there is anyelation between days with more or less lightning flashes and thehunderstorm type.

. Conclusions

In order to verify if there are variations on some visibleharacteristics of negative cloud-to-ground lightning flashes perhunderstorm day, an observation campaign was conducted duringhe summer of 2012/2013 in São José dos Campos. There were fourigh-speed cameras observing the same region, so that it was pos-ible to record a statistically significant number of flashes in eachay. The number of flashes recorded on each day varied apprecia-ly. While on March 6th 117 flashes were recorded, on February 8thhis number was 47. The measurements of lightning characteristicser thunderstorm day are presented for the first time.

When the analysis was done considering the whole dataset, thealues found are similar to those found in other studies that exam-ned the same parameters [1,5–9]. When the analysis was madeor each thunderstorm day, some parameters found were distinct,ike the multiplicity that varied between 2.8 and 5.1, or the amountf single stroke flashes. However, the two parameters seem to belosely related, as shown in Table 3 and Fig. 10.

The presented results showed that February 22nd is a verynteresting case study. The lower flash durations and average mul-iplicity indicate that either the thunderstorm that produced themr the synoptic conditions of that day did not allow the occurrencef long lasting flashes and consequently higher multiplicities. The

igher average of interstroke intervals for this day still needs to bevaluated, but it is equal to previous works that observed flashes inhe same region. This day will be studied in more detail in a futureork.

[

ms Research 118 (2015) 93–100

In the next stage of this work, the database will be dividedaccording to the type of storm in order to check if there are still sig-nificant differences in the characteristics of the lightning flashes.For this purpose, the camera data shall be used alongside auxil-iary tools (like weather radar data and satellite images) to properlydescribe the synoptic scenario of these days and even analyze thethunderclouds with more details.

This work is based on the oral presentation at the 2013 Interna-tional Symposium on Lightning Protection (XII SIPDA) [17].

Acknowledgments

The authors would like to thank Vanguarda TV channel,Simoldes Plásticos and Instituto de Estudos Avanc ados (IEAv)for their support during the 2012/2013 observation campaign.A.C.V. Saraiva would like to thank FAPESP under grant number2010/01742-2 for the support during the RAMMER campaign.O. Pinto Jr. also thanks FAPESP for grant number 08/56711-4. L.Z.S. Campos is grateful to FAPESP for scholarship number2013/18785-4.

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[3] I. Cardoso, O. Pinto Jr., I. Pinto, R. Holle, Lightning casualty demographics inBrazil and their implications for safety rules, Atmos. Res. (2014).

[4] N. Kitagawa, M. Brook, E. Workman, Continuing currents in cloud-to-groundlightning discharges, J. Geophys. Res. 67 (2) (1962) 637–647.

[5] V.A. Rakov, M.A. Uman, R. Thottappillil, Review of lightning properties fromelectric field and TV observations, J. Geophys. Res.: Atmos. (1984–2012) 99(D5) (1994) 10745–10750.

[6] V. Cooray, H. Pérez, Some features of lightning flashes observed in Sweden, J.Geophys. Res.: Atmos. (1984–2012) 99 (D5) (1994) 10683–10688.

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14] M. Brook, N. Kitagawa, E. Workman, Quantitative study of strokes and contin-uing currents in lightning discharges to ground, J. Geophys. Res. 67 (2) (1962)649–659.

15] T. Shindo, M. Uman, Continuing current in negative cloud-to-groundlightning, J. Geophys. Res.: Atmos. (1984–2012) 94 (D4) (1989)5189–5198.

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17] L.S. Antunes, A.C.V. Saraiva, O. Pinto Jr., J. Alves, L.Z.S. Campos, E.S.A.M. Luz,C. Medeiros, Characterization of lightning observed by multiple high-speedcameras, in: Paper presented at the International Symposium on LightningProtection (XII SIPDA), 2013.