This updated and expanded new edition of Cooray's classic text provides the reader with a thorough background in almost every aspect of lightning and its impact on electrical and electronic equipment. The contents range from basic discharge processes in air through transient electromagnetic field generation and interaction with overhead lines and underground cables, to lightning protection and testing techniques. New to this edition are discussions of high-speed video recordings of lightning; rocket-and-wire triggered lightning experiments; tower initiated lightning discharges; upper atmospheric electrical discharges; attachment of lightning flashes to grounded structures; energetic radiation from thunderstorms and lightning; global lightning nitrogen oxides production; and lightning and global temperature change. The Lightning Flash, 2nd Edition is a sound introduction to researchers and advanced students working in the field and of value for anyone designing, installing or commissioning equipment which needs to be secured against lightning strikes.
Inspec keywords: computational electromagnetics; electromagnetic fields; discharges (electric); lightning
Other keywords: high-speed video observations; nitrogen oxide production; lightning attachment; lightning flash; atmospheric electrical discharges; energetic radiation emissions; electromagnetic fields; triggered lightning
Subjects: Electric discharges; Physics literature and publications; General electrical engineering topics; Electric and magnetic fields; Atmospheric electricity
Clouds in the Earth's atmosphere are composed of water droplets and ice crystals. Clouds are commonly white in appearance because these liquid and solid particles are large relative to the wavelengths of visible light, and so no selective scattering occurs to colour the cloud. Owing to the abundance of cloud condensation nuclei, clouds appear whenever the air becomes locally supersaturated in water vapour. This supersaturation condition is most often achieved by a lifting process in which air parcels subsaturated with respect to water vapour cool by adiabatic expansion. The lifting process is usually caused by the heating of air near the Earth's surface, which is itself warmed by sunlight. The warmed air parcels become buoyant relative to their surroundings and rise. A second mechanism for lifting depends on the forced ascent of air by horizontal pressure gradient forces. Regardless of the lifting mechanism, the altitude at which the supersaturation condition is achieved in the rising air parcel and cloud begins to form is the lifted condensation level (LCL).
The origin of thunderstorm electrification has long been an unsolved problem in atmospheric physics. Despite a number of simulated laboratory experiments, together with the vast amount of field data collected over the past few decades, our knowledge of how these convective cloud masses get charged still remains sparse at the microphysical level.
The experiments performed by researchers in different countries, notably South Africa, England, Switzerland and the United States, during the last 60 years have greatly advanced our knowledge concerning the mechanism of lightning flashes. However, many pieces of the puzzle pertinent to the mechanism of lightning flashes are still missing and many of the theories put forth as explanation of its mechanism are mainly of qualitative nature. The reason for this slow progress is the impossibility of studying lightning flashes under controlled laboratory conditions. On the other hand, the mechanism of the electric spark, which could be studied under controlled conditions, may guide the researchers in their quest for understanding the mechanism of lightning flashes and creating more advanced theories of the phenomena. After all, it is the observed similarities between the small laboratory sparks and lightning discharges that forced Benjamin Franklin to conclude that lightning flash is a manifestation of electricity. This chapter is devoted to a description of the mechanism of laboratory sparks.
Experimental observations of the optical and electromagnetic fields generated by lightning flashes during the last 50 years have significantly advanced our knowledge concerning the mechanism of the lightning flash. Nevertheless, this knowledge is not as exhaustive as that of long laboratory sparks due to our inability to observe lightning flashes under controlled conditions. Thus, the mathematical description of the mechanism of the lightning flash is relatively poor at present even though the main features of the lightning flashes themselves are well known. The main goal of this chapter is to provide the reader with the important features of the mechanism of the lightning flash. No attempt is made to provide an exhaustive list of the literature since this can be found elsewhere. The chapter is organised as follows. First a basic description of the mechanism of lightning flashes is given to introduce the reader to the terminology used in lightning research. After that, each event associated with the lightning flash is described in detail with particular attention being paid to the electromagnetic fields generated by these events. Nomenclature: In this chapter a positive discharge is defined in such a way that the direction of motion of electrons in such a discharge is opposite to that of the discharge itself; a negative discharge is defined as one in the opposite sense. According to this definition a 'negative return stroke' is a positive discharge and a 'positive return stroke' is a negative discharge. A positive field change is defined to be in the sense of negative charge being lowered to ground or positive charge being raised. According to this definition a lightning flash that transports negative charge to ground gives rise to a positive field change.
Most of what is known about the structure and time evolution of lightning was determined by high-speed photography. The first measurements were obtained using a two-lens streak camera, named Boys camera after its inventor. In a streak camera, a relative movement between the lens and the film is used to record the phases of a lightning discharge. Subsequent improvements of Boys camera allowed measurements of several lightning parameters.
An understanding of the physical properties and deleterious effects of lightning is critical to the adequate protection of power and communication lines, aircraft, spacecraft, and other objects and systems. Many aspects of lightning are not yet well understood and are in need of research that often requires the termination of lightning channel on an instrumented object or in the immediate vicinity of various sensors. The probability for a natural lightning to strike a given point on the Earth's surface or an object of interest is very low, even in areas of relatively high lightning activity. Simulation of the lightning channel in a high-voltage laboratory has very limited application, since it does not allow the reproduction of many lightning features important for lightning protection and it does not allow the testing of large distributed systems such as overhead power lines. One promising tool for studying both the direct and the induced effects of lightning is an artificially initiated (or triggered) lightning discharge from a thunderstorm cloud to a designated point on ground. In most respects, the triggered lightning is a controllable analog of natural lightning. The most effective technique for artificial lightning initiation is the so-called rocket-and-wire technique. This technique involves the launching of a small rocket extending a thin wire (either grounded or ungrounded) into the gap between the ground and a charged cloud overhead.
Tall objects (higher than 100 m or so) located on flat terrain and objects of moderate height (some tens of meters) located on mountain tops experience primarily upward lightning discharges initiated by upward-propagating leaders. Upward (object-initiated) lightning discharges always involve an initial stage that may or may not be followed by downward-leader/upward-return-stroke sequences. The initial-stage current often exhibits superimposed pulses whose peaks range from tens of amperes to several kilo amperes (occasionally a few tens of kilo-amperes).
Electromagnetic fields from lightning can couple to electrical systems and produce transient overvoltages, which can cause power and telecommunication outages and destruction of electronics. Knowledge of electromagnetic fields at high altitudes produced by return strokes in cloud-to-ground (CG) lightning are required in the study of transient luminous events in the mesosphere. Electric and magnetic field pulses from various electrical breakdown events in the lightning are used in detecting and locating lightning flashes. Therefore, calculation of the electric and magnetic fields from different lightning processes has several practical applications. In this chapter, expressions for electric and magnetic fields are derived for charge and current configurations applicable to lightning. In general lightning currents and charges vary with time. First, simple expressions for non-time varying cases are presented. Then, electric and magnetic field expressions from time-varying lightning sources are presented.
A lightning flash is initiated by electrical breakdown of air in a cloud. This process, commonly known as the preliminary breakdown, signifies the initiation of a stepped leader. Such a stepped leader propagates towards the Earth in a succession of nearly discontinuous surges or steps. The stepped leader leaves a charged, conducting channel in its wake. When the leader reaches the ground, the current flowing in the channel increases abruptly, marking the beginning of the return stroke. After the first return stroke, several subsequent return strokes may occur, each of which is preceded by a fast, continuously moving dart leaders propagating from cloud to the Earth down the channel made by the stepped leader. This chapter is concerned with the mathematical modelling of first and subsequent return strokes.
The return stroke speed is one of the main parameters in the modelling of return strokes. The available experimental observations show that the return stroke speed decreases with height, both in the first and the subsequent return strokes. The experimental data also seem to indicate that there is no relationship between the return stroke current and the return stroke speed. This observation is somewhat against the theoretical intuition where a larger electric field at the return stroke front associated with a larger return stroke current is expected to expedite the neutralisation process giving rise to a larger return stroke speed. What parameters of the lightning channel that control the return stroke speed is one of the most important questions in lightning research. A current pulse propagating along a perfect conductor located above the ground will move at a speed equal to the speed of light in air. Since the speed of propagation of the return stroke front is significantly less than the speed of light one may hypothesise that the process of electrical breakdown that converts the partially ionised leader channel to a highly conducting return stroke channel decides the speed at which the return stroke is propagating. In this chapter, we will consider some of the models that have attempted to take this point into consideration and predict the return stroke speed as a function of other current parameters.
The knowledge concerning the characteristics of electromagnetic fields generated by lightning flashes is of importance in evaluating the interaction of these electromagnetic fields with electrical networks and in the remote sensing of lightning current parameters from the measured fields. However, electromagnetic fields generated by lightning flashes change their character as they propagate over the ground surface due to selective attenuation of the high frequency signals by finitely conducting ground (i.e. propagation effects). Thus, depending on the distance of propagation and the conductivity of ground, the peak and the rise time of the lightning-generated electromagnetic fields and their time derivatives measured at a given distance from the lightning channel may deviate more or less from the values that would be present over perfectly conducting ground.
In this chapter, the authors present the theory describing the interaction of lightning electromagnetic fields with overhead lines, with particular reference to electrical power networks. The first part of the chapter, presents different approaches and formulations that can be used to describe the coupling between an external electromagnetic field and a transmission line. Then, the selected field-to-transmission line coupling model was extended to include the effects of a lossy earth serving as a return conductor and to deal with the case of multiconductor lines. The time-domain representation of coupling equations, useful for analysing nonlinearities, was also be dealt with. The experimental test and validation of coupling models using data from natural and artificially triggered lightning, EMP simulators, or reduced scale models were presented in the first part of the chapter. In the second part of the chapter, the illustrated mathematical models were applied to compute lightning-induced overvoltages on overhead power distribution lines. This chapter particularly discussed the influence on the amplitude and waveshape of lightning-induced voltages of: the finite ground conductivity; the presence of shielding wires; the downward leader phase of the lightning discharge that precedes the return stroke phase and; the corona effect.
Thunderstorms are natural weather phenomena and there are no devices and methods capable of preventing lightning discharges. Direct and nearby cloud-to-ground discharges can be hazardous to structures, persons, installations and other things in or on them, so that the application of lightning protection measures must be considered. The decision for the need for protection and the selection of protection measures should be determined in terms of risk, which means that these measures should be adequate to reduce the risk to a tolerable level.
A grounded structure can interact with a lightning flash in two different ways. It can interact either with a downward or upward lightning flash. The initiation of a downward lightning flash takes place in the cloud, whereas in the case of upward lightning flash, the point of initiation is usually at the tip of a tall structure. In other words, upward lightning flashes are created by the grounded structure itself. In this chapter, a brief description of various models used to study the lightning attachment is given together with some of their predictions.
In order to understand why lightning has again become a major item of concern for many applications of electricity a short look into the history of electromagnetic compatibility (EMC) is necessary.
In this segment models for in-the-field strike and telephone mediated strike have been developed, and proposals regarding pathways have been made. An estimation of the magnitude and the timecourse of the insult have been given.
Our understanding of upper atmospheric discharges has advanced significantly since the early 1990s, and a large amount of dedicated observational, experimental, theoretical, and modeling efforts have been made to study various aspects of TLEs. Many papers, including a few recent extensive review papers, have been published in this field [Pasko, 2007, 2008; Neubert et al., 2008; Roussel-Dupre et al., 2008; Mishin and Milikh, 2008; Ebert and Sentman, 2008; Siingh et al., 2008; Pasko, 2010; Ebert et al., 2010; Pasko et al., 2011; Stenbaek-Nielsen et al., 2013; Pasko et al., 2013]. A book dedicated to TLEs was also published in 2006[Fϋllekrug et al., 2006], and discussions on different aspects of TLEs appear in several other books [Rakov and Uman, 2003; Leblanc et al., 2008; Cooray, 2012] and in journal special issues or sections [Ebert and Sentman, 2008; Sentman, 2010; Gordillo-Vάzquez and Luque, 2013]. In this chapter, we attempt to give an overview of the TLE research and our knowledge of them. The general phenomenology of different types of TLEs is described in section 17.2. Section 17.3 discusses similarity laws for gas discharges at different pressures or gas densities, and presents an example illustrating possible distinctions between the discharges at different pressures. In section 17.4, we present example studies of modeling sprites (arguably the best documented TLEs) and their fine structures in order to show how TLE modeling is carried out and how it helps improve our understanding of TLEs.
Electrical discharges in gases can be roughly divided into two categories: those whose behavior is governed by low-energy electrons, with energies less than a few tens of eV, and those whose behavior is governed by high-energy electrons, with energies often reaching several tens of MeV. The first category, which we shall refer to as low-energy discharges, is also called conventional discharges, which includes a wide range of phenomena such as corona discharges, including streamer and glow discharges, Trichel pulses, Townsend discharges, and spark breakdown [Loeb, 1965]. In this chapter, we first review the observations of high-energy atmospheric physics processes within our atmosphere, including x-ray emissions from lightning and gamma-ray emissions, such as gamma-ray glows and TGFs, from thunderclouds. We then introduce and explain the mechanisms involved in high-energy discharges and compare these mechanisms to their low-energy counterparts. We also discuss recent models that have been developed to explain TGFs, including “dark lightning.”
The intense heating of air molecules by a lightning discharge and subsequent rapid cooling of the hot lightning channel results in the production of nitrogen oxides (Chameides, 1986). The lightning nitrogen oxides, or “LNOx”for brevity (where NOx = NO + NO2), indirectly influences our climate since these molecules are important in controlling the concentration of ozone (O3) and hydroxyl radicals (OH) in the atmosphere (Huntrieser et al., 1998; see also Crutzen 1970, 1973, 1979; Chameides and Walker, 1973; Hidalgo and Crutzen, 1977). Analyses of Tropospheric Emission Spectrometer (TES) data show that tropical upper tropospheric ozone has the largest radiative impact (Aghedo et al., 2011). In addition, the distribution of ozone forcing can have a substantial influence on regional rainfall patterns, even more so than its global mean annual average forcing would suggest (Shindell etal., 2012). Since LNOx controls ozone and is the most important source of NOx in the upper troposphere (particularly in the tropics), lightning is important to climate (see the review by Schumann and Huntrieser, 2007). Furthermore, a substantial amount of LNOx is transported to higher latitudes via the stratosphere, extending its influence even farther (Grewe et al., 2002, 2004).
Lightning is one of nature's most beautiful and awesome sights. Yet it can also be extremely dangerous, presenting a major natural hazard in many different environments, from power utility companies to civil aviation, to golfers, and more. Thousands of people are killed every year by lightning bolts, while tens of thousands are injured (Cooray et al., 2007). Lightning impacts both our daily commercial and recreational activities. In the United States alone, damages due to lightning strikes amount to tens of millions of dollars annually (Curran et al., 2000). In recent years, with great interest in renewable energy, wind turbines have become extremely vulnerable to lightning damage (Glushakow, 2007). Furthermore, most commercial airliners are struck about once a year by lightning; however, due to the protective metal skin, generally little damage is incurred. Tens of thousands of fires are also ignited by lightning every year, generally in temperate or high latitudes (e.g., Canada, Siberia, etc.) (Stocks et al., 2002). In such cases, tens of fires can be ignited locally on the same day as a storm passes through, causing major problems for fire crews and fire management. Hence, knowledge of how lightning activity may change as the Earth's temperature changes is of critical importance and interest.