Lightning phenomena

Charge formation in clouds

Numerous theories have been advanced regarding the formation of charge centers, charge separation within a cloud, and the ultimate development of lightning strokes.

1. ·        One theory attributes charge separation to the existence of both positive and negative ions in the air and the existence of a normal electric field directed toward the earth.  Large  drops  of  water  in  the  electric  field  are  polarized,  the  upper  sides  acquiring  a negative charge and the lower sides a positive charge. As the polarized drops of water fall due to gravity, the undersides (positive sides) attract negative ions, while no such action occurs at the upper surfaces. As a result of this action, the drops accumulate negative charge.  Thus, the original charges, which were distributed at random and produced an essentially neutral space charge, become separated. The large drops of  water  carry  the  negative  charges  to  the  lower  portion  of  the  cloud,  causing  the  lower  portion  to  be negatively charged and the upper portion to be positively charged.
2. ·        Another theory is that the interaction of ascending wind currents in the leading head of a cloud breaks up the water droplets causing the resulting droplets to be positively charged and the air to be negatively charged. The positively charged water droplets are unable to fall through the ascending wind currents at the head of the cloud, which causes this portion of the  cloud  to  be  positively  charged  while  the  remaining  larger  portion becomes negatively  charged. 
3. ·        Yet another theory suggests that there are regions of subzero temperature within a cloud and the subsequent formation of ice crystals is an essential factor in the explanation of the charge centers within clouds.

The  important  fact  to  the  designing  engineer  is  that  a  charge  separation  does  occur  in thunderstorm clouds. 

Experiments using balloons equipped with electric gradient  measuring  equipment  have  been  performed  to  investigate  typical  charge  distribution  in thunderclouds,  and  these  experiments  have  shown  that,  in  general,  the  main  body  of  a  thundercloud  is negatively charged and the upper part positively charged. A concentration of positive charge also frequently exists in the base of the cloud. Such charge distribution in a cloud causes an accumulation of charge of the opposite polarity on the earth’s surface and on objects (e.g., trees, buildings, electric power lines, structures, etc.) beneath the cloud.

A typical charged cloud and the resulting electric fields are shown in Figure 1 (Note that the plot in Figure 1 is of the electric gradient as the cloud moves over the ground, not the amount of charge below the cloud.)

Figure: 01

The electrical charge concentrations within a cloud are constrained to the size of the cloud. The cloud size, in relation to the earth, is small. Therefore, the electrical gradient that exists in the cloud is much greater than at the earth. Because of this, an electrical discharge tends to be initiated at the cloud rather than at the ground.