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OVERVIEW AND RECOMMENDATIONS 14 In recent years, cooperative field programs, such as the Thunderstorm Research International Program (TRIP), have improved our knowledge about the overall electrical structure of thunderstorms. These programs have also provided a framework wherein a number of different investigators using different techniques can study the same thunderstorms at the same location at the same time. For instance, in situ and remote electrical measurements have been made in New Mexico in conjunction with Doppler radar studies of the cloud precipitation and dynamics and in- cloud sampling of the larger cloud particles. Laboratory experiments also continue to provide information on the effects of electricity on cloud microphysics and charge separation mechanisms that are critical to the interpretation of data collected by such field programs. Finally, numerical models of the electrical development and structure of thunderstorms have provided an important framework in which to interpret the cloud measurements and laboratory experiments. The techniques that are used to determine the electrical structure of the fair-weather atmosphere are diverse. Usually, vertical profiles of one or more atmospheric-electrical variablesâtypically electric field, conductivity, and current densityâare measured over a relatively short time span. Sensors are carried aloft on aircraft, balloons, or rockets, and the data are presented both as profiles and as numerically integrated results. The vertical profiles represent almost instantaneously measured parameters rather than time averages. Aircraft or constant-pressure balloons, however, do have the capability of measuring temporal variations at a given level. Profiles have been measured over land because of the convenience and to study specific terrain effects and over water in attempts to eliminate distortions caused by land. Profiles have led to the detection of convection currents in the planetary boundary layer that are comparable in magnitude with the total current, the electrode effect over water under stable conditions, the response of columnar resistance to pollution, and the diurnal variation in ionospheric potential. In addition to the standard electrical parameters, determinations of ion and aerosol contents and compositions, mobility, and chemistry are all critical to an understanding of conductivity. Again, these quantities are usually displayed as vertical profiles and, to a certain extent, are characteristic of the type of platform on which the instrumentation is carried aloft (e.g., some airplane measurements provide horizontal profiles but are limited in their vertical extent). Vertical measurements are probably a good first approximation to the global electrical structure; however, horizontal variations should also be measured as a function of time to complement the profile data. Tethered balloons can provide the time variations of selected electrical properties at a few locations, and this has been attempted on an experimental basis. For understanding the global circuit, it would be valuable to have a number of vertical profiles taken at the same time and, in addition, to repeat certain profiles to obtain the time variations. Knowledge of the upper-atmosphere current systems is important for understanding the interactions among the ionosphere, magnetosphere, and the solar wind. Some of these current systems were studied during the International Magnetospheric Study, 1976-1980, and during the NASA Dynamics Explorer satellite program. The goals of these programs have been to investigate the coupling of the solar-wind energy through the magnetosphere and into the ionosphere; but little effort was made to couple these current systems into the global electrical circuit. Additional measurements of magnetosphere-ionosphere currents are planned for the International Solar-Terrestrial Physics program. Societal Impact Severe weather phenomena that disrupt our lives include tornadoes, hail, high winds, hurricanes, floods, snowstorms, and lightning. Among them, lightning ranks as
OVERVIEW AND RECOMMENDATIONS 15 the number one killer, followed closely by tornadoes. Lightning is much less dramatic than a tornado passing through an area or a severe snowstorm that paralyzes a city, but lightning can strike quickly and kill with little or no warning. Lightning is a leading cause of outages in electrical power systems and was the initial cause of the massive power blackout in New York City on July 13, 1977. The possible effects of lightning on advanced aircraft, nuclear power stations, and sophisticated military systems are problems of increasing concern. The detailed physics of how lightning strikes a structure, a power line, or an aircraft and its effects are still not known. The approaching leader is not influenced by the object that is about to be struck until it is perhaps a few tens of meters away. At that time, an upward-moving streamer leaves the object and similar discharges may also leave other objects nearby. When the upward-moving streamer attaches to the downward-moving leader, the return stroke begins. When the details of this attachment are better understood we should be able to predict with higher probability what will and what will not be struck under various conditions and thereby provide better lightning protection. For example, the positioning of overhead ground wires above power transmission lines and the protection of complex structures could be optimized (see Chapter 5). The current rise time is an important parameter for lightning protection because if the current interacts with an inductive load, the voltage on that load is proportional to the rate of increase of the current. Most of the standard surge waveforms that are used to verify the performance of protectors on power and telecommunications circuits specify that open-circuit voltage should have a rise time of 0.5, 1.2, or 10 microseconds and that the short-circuit current should have a rise time of 8 or 10 microseconds. These values are substantially slower than recently measured lightning current rise times, which are in the range of tens to hundreds of nanoseconds; therefore, it is probable that the degree of protection that is provided by devices tested to present-day standards will not be adequate for protection against direct lightning surges. The unusually destructive nature of lightning that lowers positive charge to ground is only partially documented and is poorly understood (see Chapter 3). Because of the large and long continuing currents, positive lightning may ignite a disproportionately large number of fires, especially in grasslands and forests. The apparent pattern is for positive lightning to strike preferentially outside areas of rainfall, and this further enhances the likelihood of its starting a fire. Positive lightning may be correlated with storm severity and tornado occurrence, and its detection could enhance our present severe-storm detection and warning systems. Newly developed lightning-detection equipment now makes it possible to make real-time decisions on the preparations for repairs of utility systems, early warning and detection of lightning-caused forest fires, and a variety of other warning functions in situations that allow protective action to be taken, such as launches at the NASA Kenedy Space Center and outdoor recreational activities. Among the main users of lightning location data at present are the Bureau of Land Management (BLM) in the western United States and Alaska and the Electric Power Research Institute in the eastern United States. The BLM and the Forest Services of most Canadian provinces utilize the time and location of lightning storms to determine when and where to look for forest fires. Early detection of these fires provides considerable savings both in the natural resources and in the cost of fighting the fires. In the eastern United States the lightning data are being used to accumulate statistics on lightning occurrence and for real-time applications by electric power utilities. For warnings of lightning-intensive storms, these data are also disseminated in real time to many National Weather Service offices and to a growing number of television stations. Although cloud electrification processes are ultimately responsible for producing lightning, these processes can also electrify an aircraft flying in a cloud. It is quite