Electrical Environment in a Storm Cloud T his article gives an overview of the electrical characteristics of the thundercloud and the predominant mechanisms that are at the origin. The specific cloud that can produce lightning is described and the parameters that control its development and its organization are discussed. According to the variety of the scales of time and space associated with the mechanisms that occur within the thundercloud, it is difficult to simulate them both

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... and numerically. Thus, the advances in the knowledge of the thunderstorm electricity have been sometimes relatively slow and have raised a lot of debates. Furthermore, in-situ observation remains difficult because of the hostility of the thundercloud medium for instrumentation, sensors, aircraft or other carriers of sensors. The responses to the questions in the domain of thundercloud electricity can sometimes remain speculative. However, recent detection techniques and laboratory experiments allow a better knowledge of the cloud electrical environment to be obtained. All aspects about lightning flashes and electrical discharges will be covered by other contributions in this issue. History of the thundercloud electrical description For most researchers, meteorologists and official organizations, lightning and thunderstorms are completely interdependent. Since the time of Benjamin Franklin, during the eighteenth century, it has been understood that the lightning flash is of electrical nature and therefore the thunderstorm that produces it is the seat of electrical processes. The first experiments simply showed that negative charge was present within thunderclouds and especially in their lower part. The difficulty in making in-situ observations has differed the understanding of the nature and causes of thundercloud electrification. Later, at the beginning of the twentieth century, C.T.R. Wilson, a famous scientist known for the Wilson cloud chamber used to follow trajectories of ionizing particles, showed that the thundercloud could hold both signs of charge by performing measurements with new sensors on the ground [1]. The charge structure as a positive dipole (positive charge above negative) of the storm was pointed out. However, all thunderclouds did not correspond with this scheme and reverse structures were sometimes observed from in-situ measurements [2]; [3]. In the second half of the last century, a lot of theories of charging have been proposed, along with some experiments of cloud exploration with new sensors using modern electronics and carried by aircraft or balloons. In parallel, laboratory experiments have simulated cloud microphysics and charge separation at small scale between particles. Resulting from these advances, the question of the effective contribution of the charging processes to the cloud electrification has fed a lot of discussions between researchers in the community of atmospheric electricity [4]; [5] . To return to the electrical cloud structure, a third and smaller charge center was also observed within many storms [6] . Charge was also observed at the periphery of the cloud as screening layers, especially at the cloud top [7]; [8], which was also confirmed theoretically. Finally, more complex charge structures have been observed with repeated experiments of cloud soundings and techniques derived from lightning mapping, or obtained with electrified cloud modeling (see in the following). This paper reviews current knowledge in the electrical characteristics of thunderclouds. The first section describes the thermodynamics and the microphysics of different categories of storms. The second section is devoted to the charging processes that can take part within the thundercloud electrification. The third section describes the main electrostatic structures observed or simulated within thunderclouds. Thundercloud development and organization Thunderclouds are the result of air convection combined with substantial humidity. The convection can initiate when conditional instability is released. In order to describe the conditional instability, the parcel theory is used: when a parcel of air moves in an upward vertical direction, it follows an adiabatic process -no energy is exchanged between the parcel and the surrounding air -which reduces its temperature at a rate of about 10°C every kilometer. If the parcel is found to be less cold than the surrounding air at its new altitude,