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OVERVIEW AND RECOMMENDATIONS 4 ). If a dart leader forges a different path to ground than the previous stroke, then the lightning will actually strike the ground in more than one place and will have a forked appearance. Chapters 2 and 3 discuss the physics of lightning in greater detail. Among the more important advances that have been made in recent years has been the discovery that both in- cloud and CG discharges produce very fast-rising currents, i.e., rise times of tens to hundreds of nanoseconds and rates of change of current (dI/dt) on the order of 1011 A/sec. Most of the standard waveforms that are used to test the performance of lightning protectors and the integrity of lightning protection systems currently have current rise times and dI/dts that are substantially slower than the above values; therefore, these standards may not be an adequate simulation of the true lightning threat to aircraft and other structures (see Chapter 5). A variety of nonequilibrium trace gases are produced within high-temperature lightning channels and by the shock wave that can affect tropospheric and stratospheric chemistry (see Chapter 6). Recent spacecraft observations have shown that lightning may be present in the atmospheres of Jupiter, Venus, and Saturn; the upcoming Galileo probe will carry a lightning detector to Jupiter. In the future a study of lightning in atmospheres that are radically different from the Earth's may lead to a better understanding of the formation and characteristics of lightning on Earth. Cloud Electrification Although the vast majority of terrestrial clouds form and dissipate without ever producing precipitation or lightning, they can be weakly electrified. In some clouds, the electrification intensifies as convective activity increases, and strong electrification usually begins when there is rapid vertical and horizontal growth of the cloud and the development of precipitation. Most lightning on Earth is produced by cumulonimbus clouds that are strongly convective (i.e., they contain a vigorous system of updrafts and downdrafts) and that contain both supercooled water and ice. A small fraction of warm clouds are also reported to produce lightning. The updrafts and downdrafts and the interactions between cloud and precipitation particles act in some still undetermined manner to separate positive and negative charges within the cloud. These processes usually transfer an excess of positive charge to the upper portion of the cloud and leave the lower portion with a net negative charge. Recent research has shown that the negative charge is usually concentrated at altitudes where the atmospheric temperature is between â 10Â°C and â 20Â°C (i.e., 6 to 8 km above sea level in summer thunderstorms and 1 to 3 km in winter storms) and that this altitude remains constant as the storm develops. This finding sets important criteria that must be met by any proposed thunderstorm charging mechanisms. The positive charge that is above the negative may be spread through deeper layers and does not exhibit as clear a relationship with temperature as does the negative charge. Positive charges are found at levels between â 25Â°C and â 60Â°C depending on the size of the storm, and this temperature range usually lies between 8 and 16 km above sea level. Cloud electrification processes can be viewed as acting over two spatial scales: a microscale separation that ultimately leads to charged ice and water particles and then a larger-scale separation that produces large volumes of net positive and negative charge and eventually lightning. The microscale separation includes the creation of ion pairs, ion attachment, and charge that may be separated by collisions between individual cloud and precipitation particles. The larger cloud-scale separation may be due to precipitation or large-scale convection or some combination of the two. Numerous mechanisms have been proposed for the electrification of clouds and thunderstorms, and several of these might be acting simultaneously. Feedback can occur through changes in the ion concentrations and electric field, and thus it is diffi
OVERVIEW AND RECOMMENDATIONS 5 cult to identify or evaluate the primary causes of electrification in a cloudy environment. Currently, there is a great need for more measurements to determine the locations, magnitudes, and movements of space charges within and near the cloud boundary. There is also a need to determine the charge-size relationship that is present on both cloud and precipitation particles, how these charges evolve as a function of time, and how these distributions are affected by lightning. Laboratory experiments have provided valuable information about the physics of selected microscale processes and are expected to continue to provide important data on the relative magnitude of various processes. Theory and numerical models also have played an important role in simulating and evaluating possible charging mechanisms on both the microscale and the cloud scale. During the early nonprecipitating cloud stage, charging can occur by diffusion, drift, and selective capture of ions. Later, during the rain stage, there can be additional electrification due to drop breakup and other mechanisms based on electrostatic induction. Drift, selective ion capture, breakup, and induction are probably responsible for the charges and fields that are found in stratiform clouds; however, it is difficult to explain with just these mechanisms the stronger electrification that is found in convective clouds more than a few kilometers deep. For clouds in the hail stage, thermoelectric and interface charging mechanisms can provide strong electrification on the microscale. In thunderclouds, the charges that are generated on a microscale can be subsequently separated on a larger cloud scale by convection and/or gravitational settling. Particles near the boundary of the cloud will become electrified by ion attachment, and the convection of these charges may play an important role in the electrification. Convection also plays a role in the formation and growth of cloud particles by forcing the condensation of water vapor until the particles are large enough to coalesce. Interactions between cloud particles, particularly when there are rebounding collisions, may also produce charge separation. If the larger particles tend to carry charge of predominantly one sign, they will fall faster and farther with respect to the convected air and leave the oppositely charged, smaller particles at higher altitudes. As the populations of charged particles increase, the mechanisms that discharge these particles become more effective. Two kinds of discharging are possible: (1) discharge by ionic conduction, point discharge, or lightning and (2) discharge by collision and/or coalescence with cloud particles of opposite polarity. The attachment of ions to cloud particles will be a function of the particle charge and the electric field of the cloud, and strong fields may also produce corona discharges from large water drops and the corners of ice crystals. Corona ions and lightning will increase the local electrical conductivity, and this, in turn, may prevent or reduce any further buildup of space charge in this region of the cloud. Collisional discharge will take place at all stages of cloud particle growth. These mechanisms are enhanced if the interacting particles are highly charged and of opposite polarity; therefore, if a charging mechanism is to be effective, it must separate charge at a rate that is sufficiently high to overcome the discharging processes. It is worth noting that the electric forces on charged elements of precipitation can be several times larger than gravity; therefore, the terminal velocities and frequency of collisions of these particles will be a function of the electric field. More detailed discussions of the various processes involved in cloud electrification are given in Chapters 8, 9, and 10. Recently, there have been attempts to analyze the patterns of the Maxwell current density that thunderclouds produce at the ground in order to define better the characteristics of the cloud as an electrical generator. The Maxwell, or total current density, contains components due to ohmic and non-ohmic (corona) ion conduction, convection, precipitation, displacement, and the charge-separating currents within the cloud. Under some conditions, there is evidence that the total current density may be