Simply put, a charge of any given value on one object will induce an opposite charge on the adjacent surface of any nearby object. Imagine two metal spheres, separated from each other by a small distance. If we charge sphere a negatively, sphere B. will develop a positive charge on the hemisphere adjacent to sphere a. In fact, the charges on both spheres will be concentrated on the spheres' adjacent hemispheres. Negative charge will be concentrated on a's side facing B, and positive charge concentrated on B's side facing a. The degree to which sphere a can be negatively charged without a spark leaping from a to B (equalizing the charges on both spheres) is a function of the distance between the two spheres, the surface areas of the two spheres, and the dielectric constant (the tendency of a material to resist the flow of electricity through it) of the material intervening between them. In our example the material is air and it is the baseline against which all other substances' dielectric constants are scaled, so the dielectric constant of air is equal to 1 as in Table 1 (Ford, 1991). The general formula expressing this function is C = 0.08854(a/S) in which C. is capacitance in picofarads, a is the area of one side of one plate in square centimeters and S. is the distance between plates in centimeters. The rule of thumb from this equation is that capacitance increases as the area of the plates increase, but decreases as the distance between plates increases. With air as the dielectric and the bottom of a thunder cloud representing one plate and the surface of the Earth as another, each many square kilometers in area, the practical capacitance thus formed is staggering. A thunderstorm's base may possess a charge of around 300 million volts and can be hundreds of square kilometers in area.

This positive field will be concentrated on tall objects and sharply pointed ones. The two charges do try to meet. Negative charges trickle into a series of steps from the cloud bottom and the closer these reach the ground, the more intense the positive charge becomes directly below the lowest step as in Figure 1. When the field strength overcomes the air's dielectric strength, a positive spark leaps up from the ground to connect with cloud's lowest step, the two connect and the bottom of the cloud discharges into the ground in a 100 million volt, 30,000 amp, 500 million joule jolt cooking the atmosphere at up to 28,000*C (Moore, 1997).
Appendix

Dielectric Constants for Insulators

Insulator

Dielectric Constants

Plate Glass

Pyrex Glass

Hard Rubber

Polystyrene

Shellac Film

Spar Varnish

Bees wav (purified)

Paraffin wax

Wood (dry)

Air