Passage V

Lightning is a visible electrical emission that often occurs in a cloud that has positive charges at the top and negative charges at the base (bottom). Two theories attempt to explain the mechanisms by which thunderstorm clouds become electrically charged and produce lightning.

Gravitational Theory

As shown in Stage I of Figure 1, in a mature thunder-storm cloud (a cloud beginning to precipitate), most precipitation particles, which may include raindrops, snow, or ice crystals, are initially neutral (no charge). As the larger particles (water droplets or ice crystals) in the upper regions of such a cloud fall due to gravity, they pass and rub against the smaller particles (see Stage II). The friction between them generates static electricity and places a negative charge on the larger particles as they settle to the cloud base. The smaller particles, which become positively charged through this process, rise to the top of the cloud (see Stage III). This separation of the positive and negative charges within the cloud is necessary for lightning formation.

Convective Theory

In this theory, the wind circulation within thunder-storm clouds is believed to cause the charge separation. Near the ground, some air molecules are positively charged. As updrafts (vertically rising currents of air) from the surface initiate the development of thunderstorm clouds, they also carry these positive charges aloft into the cloud (see Stage I of Figure 2). Initially, the entire cloud has a positive charge (Stage II). Negatively charged particles in the atmosphere are attracted toward the positively charged cloud and form a layer of negative charge around the edges of the cloud (Stage III). Strong downdrafts form as precipitation begins to fall and the thunderstorm reaches mature state (Stage IV). These downdrafts strip its the layer of negative charge from the cloud edge and carry it downward toward the base. The net effect of the convective circulation is to carry positively charged particles from the atmosphere to the cloud top and deposit negatively charged particles from the cloud's edge at the base of the cloud. This process continues through the remainder of the cloud's life cycle.