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PHYSICS OF LIGHTNING 31 have also been used to infer properties of the thundercloud charge distribution that is affected by lightning (see Chapter 8, this volume). Figure 2.1 Cloud-to-ground lightning over Tucson, Arizona. Within the category of cloud-to-ground lightning, there are flashes that effectively lower negative charge to ground and those that lower positive charge. Most ground flashes are negative, but there is recent evidence that positive discharges are often unusually deleterious, and, therefore, this type is discussed separately in Chapter 3 (this volume). Before describing some of the recent advances in lightning research in more detail, the processes that occur during a typical negative lightning flash to ground will be reviewed briefly. There are still many open questions about basic lightning phenomena, such as whether or how the characteristics of individual flashes depend on the type of thunderstorm, the season, and the type of terrain that is struck. CLOUD-TO-GROUND LIGHTNING Simplified sketches of the luminous processes that occur during a typical cloud-to-ground discharge are given in Figures 2.2, 2.3, and 2.4. For more detailed discussions of these phenomena, the reader is referred to Schonland (1964), Uman (1969), and Salanave (1980). Figure 2.2a shows an assumed cloud charge-distribution just before the lightning begins. Concentrations of negative charge are shown at altitudes where the ambient air temperature is â 10 to â 20Â°C, typically 6 to 8 km above mean sea level. The positive charge is more diffuse than the negative and most of it is at higher altitudes (see Chapter 8, this volume). Cloud-to-ground lightning almost always starts within the cloud with a process that is called the preliminary breakdown. The location of the preliminary breakdown is not well understood, but it may begin in the high-field region between the positive and negative charge regions, as shown in Figure 2.2b. After several tens of milliseconds, the preliminary breakdown initiates an intermittent, highly branched discharge that propagates horizontally and downward and that is called the stepped-leader. The stepped-leader is sketched in Figures 2.2c and 2.2d, and this process effectively lowers negative charge toward ground. The individual steps in the stepped-leader have lengths of 30 to 90 m and occur at intervals of 20 to 100 Âµsec. The direction of the branches in a photograph indicates the direction of stepped-leader propagation; for example, in Figure 2.1 each stepped-leader propagated downward. When the tip of any branch of the stepped-leader gets close to the ground, the electric field just above the surface becomes very large, and this causes one or more upward discharges to begin at the ground and initiate the attachment process (see Figure 2.3). The upward propagating discharges rise until one or more attach to the leader channel at a junction point that is usually a few tens of meters above the surface. When contact occurs, the first return stroke begins. The return stroke is basically an intense, positive wave of ionization that starts at or just above the ground and propagates up the leader channel at about one third the speed of light (Figure 2.2e). The peak currents in return strokes range from several to hundreds of kiloamperes, with a typical value being about 40 kA. These currents carry the ground potential upward and effectively neutralize most of the leader channel and a portion of the cloud charge. The peak power dissipated by the return stroke