H. G. P. Hunt

Observations of lightning discharges on Brixton Tower

Observations of lightning strikes to Brixton Tower in South Africa are investigated in this paper. The tower is topographically situated in an ideal location to study lightning strikes, and has the benefit of a physical height of 250 m. Observations presented in this paper were made through photographic recordings of lightning events from November 2009 to May 2010. Lightning data from the Southern African Lightning Detection Network (SALDN) is used to match recorded lightning strokes to photographed events. The SALDN provides the associated stroke parameters for assessment of the events in terms of peak current values as well as rise and decay times. Particular focus is given to studying positive polarity events. This includes an investigation into positive strikes to the tower and an event of a downward positive flash that appears to initiate an upward flash from the tower.

Evaluation of the South African lightning detection network using photographed tall tower lightning events from 2009 – 2013

The performance characteristics of the South African Lightning Detection Network (SALDN) are evaluated using photographed lightning attachments to a tall structure over a four year period (2009-2013) after increased numbers (19 to 24) and repositioning of sensors in 2011. The structure is not instrumented and the evaluation only focuses on flash detection efficiency and location accuracy. During the four year period, 66 flashes to the tower were photographed and 185 negative SALDN strokes in 39 flashes were time-correlated with the photographs. It is seen that the flash detection efficiency increased from 50% to 76% after the upgrade with this being considered a low estimate given the majority of events are upward in nature. The median location accuracy is found to be 280 m and maximum location error is decreased after the upgrade.

Comparison between numerical and passive integration of flat plate electric field sensor output

It is required to determine the best configuration for sensors to be used to in an electric field study on lightning occurring in the Johannesburg region. The tests performed were to determine the best sensor configuration to be used and to validate sensor output against a known source. Using a pair of flat plates an impulse was applied to the flat plates resulting in a known electric field. The output of the flat plate electric field sensor, was then compared to the known input. It was shown that despite the resistive load being the simplest to implement, it resulted in problems that require more advanced acquisition tools. Whereas using the passive integration of a capacitor, resulted in a more accurate output, which does not require complex acquisition hardware.

Can LDN data provide corroborating evidence for a Lightning event at a specific geographic location

In many cases of lightning attachment, the exact timing of the event is unknown (property damage, loss of life etc.) making comparison with LDN data difficult. A methodology for matching data from a Lightning Detection Network (LDN) with to a possible location of attachment using only the reported positions of strokes is proposed. A case study of lightning attachment to a tall structure (Brixton Tower, Johannesburg, South Africa) is used to evaluate of the methodology. Photographs of the lightning attachments are time-correlated with data from the South African Lightning Detection Network (SALDN) and these results are compared with the results of the methodology. It is found that the methodology returns 40% false positive results and further evidence is needed to distinguish the time-correlated matches from the false positive results. The median confidence ellipses provided for each stroke are also investigated but it appears that this does not provide the evidence needed to do this. It is concluded that LDN data can provide corroborating evidence of lightning attachment to a location but cannot be used in isolation.

A method for determining LDN confidence ellipse information for ground-strike points

This paper describes a method for determining confidence information for LDN ground-strike points. Ground-strike points can be determined using clustering algorithms and in the process, the strike point location is refined. However, the confidence ellipse information is lost when this is done. By treating stroke detections as multiple observations of the same event, Gaussian merging techniques can be used to develop a confidence ellipse for LDN ground-strike points.

Recognition of lightning-induced trauma to the skeleton: A forensic taphonomic study

Lightning related deaths can be hard to recognize in most circumstances, unless explicitly looked for, and impossible when the remains are fully skeletonized. There is a crucial need for this for forensic authorities in South Africa, where there is a high incidence of lightning deaths and a high rate of recovery of unidentified bodies in the skeletonized state. The effects of a 15 kA, 8/20 μs impulse current applied to a series of pig femurs was investigated as well as a comparison with a skeletal element from a giraffe struck by lightning. A distinct pattern of traumatization was observed and is described in detail. Further analysis and comparison to human results should be carried out and a method for accurately identifying lightning related fatalities should be created.

Preliminary investigation into the interaction between lightning current and resin

An introductory experiment is undertaken which investigates the interaction of the resin of a Pinus Elliottii (Slash Pine) and an impulse response. A resonance is seen in the resulting current waveforms with maximum current amplitudes of around 440A for an impulse charged to 18kV peak voltage. The resin is found to have a high breakdown strength and no heat or damage is observed when the current passes through the test sample. While laboratory generated impulses cannot confirm what is seen in the field when a natural lightning attachment occurs to a tree, it could give a better indication of what could be expected. More tests are needed to confirm the preliminary results seen.

Comparison between South African lightning detection network interpretation of single-channel and branched upward lightning flashes to a tall structure

Lightning flashes photographed attaching to the Brixton tower in Johannesburg, South Africa over the period of 2010-2011 are put into two different categories: branched upward flashes and single-channel flashes. Using time-correlation methods, South African Lightning Detection (SALDN) records are matched to the photographed flashes. The SALDN interpretation of the two categories is then evaluated. It is found that the SALDN detected 25% of the photographed branched upward flashes and 44% of the photographed single-channel flashes. Strokes from both categories have location errors with 2 km. Only the branched upward flashes have detected strokes with location errors greater than 2 km. This is attributed to the smaller peak currents measured by the SALDN for branched upward strokes meaning fewer sensors are able to detect strokes from these events resulting in larger location errors.