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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

Initial steps in the development of a comprehensive lightning climatology of South Africa

Gill, Tracey 08 July 2009 (has links)
The summer rainfall region of South Africa is dominated by convective thunderstorm development from October to March. The result is that lightning is a common event over most of the country during this time. The South African Weather Service (SAWS) installed a stateof- the-art Lightning Detection Network (LDN) in late 2005 in order to accurately monitor lightning across South Africa. Data from this network for 2006 was utilised in order to develop an initial climatology of lightning in South Africa. Analyses were performed of lightning ground flash density, flash median peak current and flash multiplicity on a 0.2° grid across South Africa. The highest ground flash density values were found along the eastern escarpment of the country, extending onto the high interior plateau. There is a general decrease in flash density from east to west, with almost no lightning recorded on the west coast of the country. The regions of highest flash density recorded the highest percentages of negative polarity lightning. The percentage of positive lightning was higher in the winter months, as was the median peak current of lightning of both polarities. The median peak current distribution displayed distinct bands of current values oriented in northwest to southeast bands across the country. The bands of higher median peak current correspond to the regions to the rear of the interior trough axis in areas dominated by stratiform cloud development and were more dominant in the mid summer months. The highest flash multiplicity was recorded in the regions of highest flash density. Along the southern escarpment, on the eastern side of South Africa, flash multiplicity values exceeded 3 flashes per square kilometer. The highest flash multiplicity of negative polarity lightning was recorded in the spring and early summer. Throughout the year, the percentage of single stroke flashes for positive lightning is high. Topography and the position of the surface trough have a very strong influence on the ground flash density and median peak current distributions, but not on the flash multiplicity distribution. The results from the analyses of the three lightning variables were then combined to determine risk indexes of high intensity lightning and of positive polarity lightning. The eastern part of South Africa is at extreme risk from both large amounts of lightning and from positive polarity lightning, whereas the regions in the northwest of the country that are dominated by mining are at extreme risk from mainly positive polarity lightning.
22

On the sixth mechanism of lightning injury

Blumenthal, Ryan 30 April 2015 (has links)
A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the Degree of Doctor of Philosophy / The work presented in this thesis extends and contributes to research in the field of lightning injury mechanisms. Six mechanisms have been described in the literature about lightning injury. This thesis takes an in-depth look at the sixth injury mechanism. The sixth mechanism may be thought of as a ‘pressure-shock wave’ which is directly proportional to the current of the lightning discharge, and which is present immediately surrounding lightning’s luminous channel. A literature review, case studies and two novel experiments helped confirm the sixth mechanism’s existence. The medical data and the lightning data were then aligned. Two main questions were addressed, namely within what range is a human at risk; and what is the risk of lightning’s pressure shock wave. This ‘pressure-shock wave’ may explain some of the more curious lightning injury patterns seen on lightning-strike victims. Knowledge and insight into the sixth mechanism may have direct and indirect applications to those working in the fields of lightning injury and lightning protection. This thesis represents a contribution to the literature in both medicine and engineering.
23

A study of positive cloud-to-ground lightning flashes in mesoscale convective systems

Lu, Chungu 05 August 1988 (has links)
This study is mainly concentrated on examining the positive cloud-to-ground lightning activity associated with Mesoscale Convective Systems. Six MCS events which occurred during the O.K. PRE-STORM program in 1985 are studied. Data indicating the location and polarity of the cloud-to-ground lightning flashes from a lightning location network are analyzed in conjunction with the low-level echo patterns as obtained from radar. Spatial and temporal characteristics of positive cloud-to-ground flashes are identified from the data analysis. For all cases examined, positive cloud-to-ground flashes were found most commonly in the stratiform regions of the MCSs examined, and their frequency tended to peak during the later stages of the storm lifecycle. Two mechanisms responsible for the occurrence of positive cloud-to-ground lightning flashes with the above spatial and temporal characteristics are discussed. Based on the laboratory results, a 1-D charge generation model is developed. The model results show that in-situ charging is unlikely to be the dominant mechanism for charge generation in the stratiform region under normal atmospheric conditions. Sensitivity studies show, however, that in-situ charging processes strongly depend upon the liquid water, graupel and snow contents in the cloud. Under favorable atmospheric conditions, in-situ charging may lead to a significant charge generation. Hence, we cannot completely dismiss in-situ charging mechanism. Analysis of wind fields from dual-Doppler radar in combination with vertical profile of electric fields indicates that charge advection from the convective region to the stratiform region of MCSs may be a potential mechanism responsible for the occurrence of positive cloud-to-ground lightning flashes in the stratiform region. / Graduation date: 1989
24

The warning time for cloud-to-ground lightning in isolated, ordinary thunderstorms over Houston, Texas

Clements, Nathan Chase 15 May 2009 (has links)
Lightning detection over Houston, Texas is possible with the Lightning Detection and Ranging (LDAR-II) network and the National Lightning Detection Network (NLDN). A comparison of the two datasets in conjunction with 37 isolated, ordinary thunderstorms reveals a time separation of 3.1 minutes between the first detected Very High Frequency (VHF) source (i.e. first intracloud discharge) and the first cloud-toground (CG) lightning flash. This CG warning time is increased to 16.1 minutes when using the radar-defined criterion of when the 30-dBZ contour first reaches the -10°C isotherm level. Several attempts were made to establish a similar characteristic that could be used to forewarn the occurrence of the final CG in this storm type. Based on the average radar characteristics during the last CG flash in each thunderstorm case, CG activity comes to an end when the 45-dBZ echo falls below the -10°C isotherm. Detection efficiencies that remain slightly less than perfect for each network may have allowed for some error when analyzing VHF sources and ground flashes for each convective case. Exhibiting this possible error, four cases actually recorded a greater number of CG flashes than intracloud flashes, which is contrary to typical lightning characteristics. Future studies hope to increase the number of thunderstorm cases to analyze as the LDAR network continues to observe more lightning events. Also, similar approaches could be implemented in differing geographic regions of the country to observe if these lightning characteristics vary depending on latitude, longitude, or climate.
25

Low-level convergence and its role in convective intensity and frequency over the Houston lightning and rainfall anomaly

McNear, Veronica Ann 17 September 2007 (has links)
An increase in the amount of lightning and rainfall over the Houston area, compared to the surrounding rural areas, has been well documented in previous studies. The placement of a Shared Mobile Atmospheric Research and Teaching Radar (SMART-R) in the Houston area during the summer season of 2005 presented a unique opportunity to investigate the role of boundary-layer convergence in modulating convective frequency and intensity and, thereby, likely causing the rainfall and lightning anomalies. The role of the urban heat island (UHI) and the sea-breeze, as a source of low-level convergence leading to enhanced convection over Houston, was examined. Hourly average dual-Doppler wind and convergence maps were created on 1 X 1 km grids for an eleven-week period. By using these images along with average lightning, rainfall, and reflectivity for a large Houston-centered domain, it was possible to discern a correlation between low-level convergence and convection. Also, past findings of enhancement in lightning and rainfall over Houston and downwind of Houston were validated. High convergence levels for the Houston area in the mid-morning were followed closely by a peak in convection in the early afternoon. The enhancement of rainfall and lightning over and downwind of downtown was found to be primarily from a large increase in frequency of deep convective events when compared to the surrounding domain. Also, it was found that UHI, rather than sea-breeze, was likely the primary causative mechanism in the development of convection over the Houston area because of the lack of deep convection in areas equally affected by the sea-breeze and the timing of the convection compared to time of peak sea-breeze. An area of weaker enhancement south of Houston, not discussed in previous studies, was found to be present, possibly from the interactions between the bay-breeze off of the Galveston Bay and the seabreeze.
26

Triggered-lightning properties inferred from measured currents and very close magnetic fields

Jhavar, Ashwin B. January 2005 (has links)
Thesis (M.S.)--University of Florida, 2005. / Title from title page of source document. Document formatted into pages; contains 161 pages. Includes vita. Includes bibliographical references.
27

Warm season lightning distributions over the northern Gulf of Mexico coast and their relation to the mesoscale and synoptic scale environments

Stroupe, Jessica Raye. Fuelberg, Henry E. January 2003 (has links)
Thesis (M.S.)--Florida State University, 2003. / Advisor: Dr. Henry E. Fuelberg, Florida State University, College of Arts and Sciences, Dept. of Meteorology. Title and description from dissertation home page (viewed Mar. 1, 2004). Includes bibliographical references.
28

AN ANALYSIS OF THE VOYAGER IMAGES OF JOVIAN LIGHTNING (JUPITER).

WILLIAMS, MARK ANDREW. January 1986 (has links)
In 1979, Voyager I provided the first strong evidence for the existence of lightning on another planet. Two pictures taken while the spacecraft was in Jupiter's shadow reveal about three dozen luminous spots on the night side of the planet. After careful examination of these spots, we conclude that they are lightning flashes occurring somewhere within Jupiter's atmosphere. A search through the additional Voyager I and II images of Jupiter's night hemisphere failed to locate any additional lightning flashes. The lower limit for the planetary lightning rate on Jupiter is found to be 10⁻⁴ km⁻² yr⁻¹. It must be noted that the spacecraft could only detect lightning discharges at least 1000 times higher than typical terrestrial flashes. Furthermore, due to attenuation, any discharges occurring deep within the atmosphere could not have been imaged. Calculations suggest that the actual flash rate could be about 0.1 km⁻² yr⁻¹. Analysis of the lightning images reveals that the flashes group near 50°N latitude. High-resolution photographs of the lightning region made in daylight about 65 hours before the discovery images show long, light ribbon-like clouds. Almost every flash appears to be associated with one of these clouds. Calculations made with a Monte Carlo radiative transfer code that computes the luminosity distribution of the spot on the top of the ammonia cloud that is produced by a point source within the atmosphere indicate that the lightning production region is near the top of the lower cloud deck. The average optical energy radiated by a discharge is calculated to be about 10⁹ J. The total electrical energy is estimated to be about 3 x 10¹² J. Calculations made with a chemical equilibrium model show that lightning synthesis cannot account for the observed abundances of such disequilibrium species as CO, HCN, and C₂H₂.
29

A spectral study of lightning strokes

Orville, Richard Edmonds, 1936- January 1966 (has links)
No description available.
30

The design and testing of a photoelectric photometer for selected lines in the spectrum of lightning

Krider, Edmund Philip, 1940- January 1965 (has links)
No description available.

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