Mechanism of Lightning Flashes

Abstract

As we saw in Chap. 2, the initiation of an electric discharge requires the electric field in the air to increase beyond a critical electric field, which depends on the air density. At sea level this critical electric field is approximately 3 × 106 V/m. The critical electric field necessary for electrical breakdown decreases with atmospheric density, and at a height of approximately 5 km the value of this field is approximately 1.5 × 106 V/m. It is important to note that these values of the electric fields are applicable in clear air devoid of particles. However, the presence of small particles in air can decrease the background electric field necessary for electrical breakdown due to field enhancement. For example, a spherical particle in a background electric field of strength E gives rise to an electric field that varies as 3E cos θ on its surface (Fig. 7.1). Thus, the maximum electric field on the surface of the sphere is 3E. If the particle has a pointed shape, then the field enhancement will be higher. It is important to recognize that to create an electrical breakdown, it is not sufficient for the electric field to reach the critical value at a point. The electric field should increase above the critical value over a critical region so that the electron avalanche process can be initiated. A thundercloud contains a variety of small particles, such as water droplets, ice crystals, and graupel, and their presence will reduce the background electric field necessary for electrical breakdown to a value on the order of 500 kV/m. However, only rarely are such high electric fields observed inside thunderclouds. Measurements conducted inside thunderclouds consistently show typical electric field values of the order of 100–150 kV/m [1]. These values are significantly below the values necessary for electrical breakdown. The question is how the electric fields necessary for an electrical breakdown are achieved inside thunderclouds and what the significance is of an overall electric field of approximately 100–150 kV/m in the breakdown process.