Air, a mixture of gases, is largely composed of nitrogen and oxygen. It is generally considered as insulator, and would be an excellent one if all the oxygen and nitrogen molecules were in the neutral state. However, the air is actually composed of varying quantities of neutral molecules and positive and negative ions. As the number of ions in the air is increased, the air becomes a progressively better conductor.
In general, gradually more ions are found the higher we ascend until, at about a height of fourty to fifty miles, a region called the ionosphere is reached. Here, there are sufficient numbers of ions to reflect radio waves. The ionosphere, although conductive, can be considered as a whole as being uncharged. This is due to the number of positive ions being equal to the number of negative ions plus electrons that are distributed in layers varying in height and in degree of ionization. In contrast, the earth has a surplus of electrons and is actually about 300,000 to 400,000 volts negative with respect to the ionosphere. This potential difference together with the total conductive qualities of the atmosphere are sufficient to cause the earth to continually lose electrons to the ionosphere. The entire earth’s surface and the ionosphere may be considered to be oppositely-charged plates of a vast capacitor with the air between them acting as a rather inferior insulator, for it leaks continuously. In addition to the presence of ions, which make the atmosphere slightly conductive, various meteorological processes called precipitation or hydrologic cycle, contribute to the leakage rate of this earth capacitor. T. H. Moray reported that his radiant energy receiver produced more power when it was raining. Falling rain, tends to bring down the less-mobile large ions toward the earth while electrons are carried upwards on rising moisture-laden air. This steady loss of electrons from the earth is called ionic current, and, infinitesimal as it is, it has been measured and amounts to about 9 microamps for every square mile of the earth’s surface. This current flows from the earth via the most convenient conductive path or those offering the least resistance. Most of the electrons are discharged at natural and man made points that project into the atmosphere. Static discharge can also occur when electrically charged particles (raindrops, snow, dust, etc.) strike the antenna, inducing a current impulse in the associated circuitry and thereby producing broadband noise.
“Origin of Lightning,” Lockheed Service Digest.