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7 CHAPTER 1 Background Lightning Properties, Behavior, thought of as a spherical capacitor, with the earth as the lower and Terminology conducting surface and the atmosphere as a slightly conduc- tive medium topped by a highly electrical region in the upper A nearby lightning strike is a dramatic event that immedi- atmosphere, where unfiltered solar radiation effectively ion- ately invokes fear and awe. As an obvious hazard for airport izes atmospheric molecules and atoms into a highly conductive operations, it demands respect. Properly grounded buildings region called the ionosphere. The ionosphere (sometimes also and well-designed electronics with surge protectors usually termed the electrosphere) is positively charged, while the provide adequate protection to structures and electronic sys- earth's surface has a net negative charge. This charge imbal- tems. Fueling operations, which are at risk from sparks or ance creates an atmospheric electric field (roughly 100 V/m other electrical discharges, are normally suspended during near the earth's surface) and a corresponding air-earth elec- lightning activity. The greatest lightning danger is to airport trical current directed downward from the ionosphere to the ramp workers, who need to be moved indoors until the light- ground, where the direction of the current is defined as the ning ends, which essentially shuts down ramp operations. direction that a hypothetical positive charge would flow. Lightning is a complex process that, even after decades of Without a mechanism to recharge the ionosphere, the air- intense investigation, is still quite mysterious. The electric earth current would quickly discharge this global capacitor. fields and currents that help drive lightning are global in scale, While historically there have been suggestions that charged while many of the charge separation processes that lead to a particles from the solar wind might help maintain the positive lightning strike involve microscopic interactions between small charge in the ionosphere, most atmospheric scientists now particles of ice and water in the core of intense thunder- accept that the global population of thunderstorms transfer storms. For every generality about lightning behavior, there electrical charges back to the ionosphere in a thunderstorm seem to be exceptions. driven global circuit (see Figure 1). At any one time there may In this review of lightning properties and behavior we will be as many as 2,000 thunderstorms occurring around the start with a discussion of the earth's electric field and then globe, generating a total of perhaps 40 lightning flashes every move on to the clouds and thunderstorms that create light- second. Our knowledge of atmospheric electricity is still ex- ning. This discussion involves a wide range of often unfa- panding. Recent discoveries of a variety of electrical discharges miliar words and specialized terminology. For reference, a extending upward from the tops of active thunderstorms have glossary of lightning terms, extracted from the American been termed jets, sprites, and elves. Meteorological Society's Glossary of Meteorology, is pro- The presence of the atmospheric electric field may con- vided in Appendix B (1). This discussion also makes extensive tribute to the earliest phases of cloud electrification. Even use of a number of standard reference books and Internet though relatively weak, the field can induce a degree of charge references (211). separation in water drops and ice particles, helping them cap- ture ions and other charged particles that are components of the fair weather current and giving them a net charge. The Earth's Electric Field The relatively low level electrification of small, shallow and Cloud Electrification clouds is not, in itself, a hazard. The development of lightning The earth's atmosphere is an integral part of a natural elec- requires additional charge separation in strong convective trical system in which the earth and its atmosphere can be clouds. Airplanes flying through seemingly benign stratiform
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8 Electrosphere 65 ALTITUDE (km) Rc Fair We urrent ather Storm C Current 15 Earth Figure 1. A simple conceptual model of the main global circuit. Thunderstorm "generators" drive current to the highly conductive electrosphere and back to the ground through the fair weather current (2). clouds may, however, trigger an electrical discharge. Depend- In addition to the charge separation within the cloud, the ing on their history, these clouds may have moderate electrical lower area of strong negative charges induces a compensating fields as a result of earlier convective activity or from electri- area of positive charge to form immediately below the cloud fication associated with the melting of precipitation. In-flight on the earth's surface. Eventually, when the charges build up lightning strikes are relatively frequent (averaging about one to a high enough level to cause an electrical breakdown in the strike for every 3000 hr of flight), but they seldom do much air separating the charge centers, the built-up charges can dis- damage since aircraft are generally well shielded against light- charge in a lightning stroke. This can either happen between ning by their metal airframes (12). the cloud and the ground, or between the positive and nega- tive charge centers within the cloud. The majority of natural Thunderstorm Electrification and Lightning While small and mid-sized convective clouds may become electrified, they seldom produce natural lightning. Lightning requires a tremendous amount of charge separation before a discharge, and this generally happens only in the large con- vective storms we call thunderstorms. While there are still many unknown factors in the initiation of a lightning strike, years of studies have made it clear that the process involves collisions between super-cooled water and ice (including graupel and small hail) in the presence of strong updrafts and downdrafts. Most often, cloud tops have to cool to at least -20 °C before lightning begins, with the critical charge separation processes occurring in the portion of the clouds with temperatures between -5 °C and -20 °C (24 °F to -5 °F). Particle collisions, combined with size sorting and strong updrafts and downdrafts, separate the positive and negative charges. The descending particles tend to collect negative charges, and the ascending particles are predominately posi- tively charged. The idealized result of these interactions is a simple cloud dipole, with positive charges grouped at the top Figure 2. An idealized small and negative charges grouped in the middle and lower areas thunderstorm with charges separated of the cloud, in the -5 °C to -20 °C zone (see Figure 2). into a simple electrical dipole (5).
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9 lightning strikes (about 75% to 80%) occur within the storm Cloud discharges and CG flashes both radiate energy over cloud itself. a wide spectrum of frequencies, predominately the radio fre- quency (RF) bands. During the "stepped" process that creates new channels, there are strong emissions in the very high fre- Anatomy of a Lightning Strike, Part I quency (VHF) range. High current discharges along previously Even in this simple model of a thunderstorm, lightning established channels ("return strokes") generate powerful strikes are quite complex. Figure 3 shows the development of emissions in the low frequency (LF) and very low frequency a typical negative cloud-to-ground lightning strike. Both neg- (VLF) ranges. Medium frequency (MF) emissions are centered ative and positive flashes can occur, but negative flashes are in the AM radio band and are responsible for the static we more common. Negative flashes bring negative charge to hear on AM radio during lightning storms. Figure 6 illustrates the ground, while positive flashes bring positive charge to the the relative energy spectrum of CG and IC flashes in the VLF, ground. In negative flashes, the descending current from the LF, MF, and VHF frequency bands. cloud moves downward in a series of short jumps, called a Cloud and ground flashes produce significantly different "stepped leader." The individual steps in this process branch RF emissions over different time scales, which can be used to out in different directions, looking for the path of least resist- distinguish between these two classes of lightning. With their ance toward the ground. As a leader gets close to the ground, high current and predominately vertically oriented return a corresponding streamer of positive charge moves up from strokes that generate magnetic fields, CG flashes produce the surface to meet the descending negative current. When strong signals that can easily be associated with a single posi- these two currents connect they provide a highly conductive tion near the point they strike the earth's surface. The strong channel for charge transfer between the cloud and the ground. LF and VLF pulses generally follow the curvature of the earth The initial descending negative charge is followed by an even and can be detected for ranges of 300600 km (185375 mi). stronger "return stroke" of positive charge from the ground, IC strokes, on the other hand, are identified by their VHF which seems to move up the channel and into the cloud. The emissions, which are a line of sight transmission that can actual charge transfer is, however, done by free electrons so the normally only be detected out to ranges of 200 to 300 km return stroke is really just a progressive draining of negative (125 to 185 mi). charge downward, with the upper limit of the drained path In summarizing years of lightning research, the National moving upward as electrons flow to the ground. Multiple Severe Storm Laboratory has concluded that taller, more com- strokes of dart leaders and return strokes can follow, produc- plex storms produce more lightning and more CG flashes than ing flickering strobe-like flashes of light (see Figure 4). do smaller, isolated storms. The first flashes produced by a The entire multiple discharge sequence of a lightning strike storm are usually IC flashes, and if detected, they can signal is normally called a flash and is typically made up of two to four the initiation of a thunderstorm. The ratio of IC flashes to CG separate strokes. In some cases, as many as 15 or more strokes flashes is quite variable, but cloud flashes predominate, often have been observed. The subsequent strokes generally follow by a factor of five or more. the established conducting channel, but the final strike point on the earth's surface can jump around from strike to strike, Lightning Climatology with separations of up to several hundred meters or more. These cloud-to-ground flashes are normally called CG Figure 7 shows two views of a lightning climatology for lightning, or simply ground lightning. the continental United States (CONUS), Mexico, and southern Canada. The lightning data were extracted from a global database based on observations from two National Anatomy of a Lightning Strike, Part II Aeronautics and Space Administration (NASA) instru- Electrified thunderstorms are seldom as simple as the ide- ments in low-earth-orbit, the Optical Transient Detector alized dipole shown in Figure 2. There are complex areas of (OTD) and the Lightning Imaging Sensor (LIS). The OTD charge throughout the cloud, resulting in complex electrical data set was collected between May 1995 and April 2000, fields. Figure 5 illustrates a more normal situation and gives while the LIS data set was collected between January 1998 examples of a number of different types of lightning flashes, and December 2005--essentially a 10-year data archive. The including discharges between clouds (intercloud) and within satellite data are based on optical detection of lightning a single cloud (intracloud). Both of these classes of lightning flashes, both during the day and at night, and represent the can be grouped together under the single term IC lightning, "total" lightning distribution, including both IC flashes and or cloud lightning. Unlike CG lightning flashes, IC strokes are CG flashes as seen from space. not followed by return strokes, and they do not carry as much The summaries have been processed to display the number current as is typical for a CG flash. of flashes per square kilometer per year. The upper panel
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10 Figure 3. Anatomy of a lightning strike (5).