The Atmospheric Capacitor

The earth is a fairly good conductor of electricity. So is the upper part of the atmosphere known as the ionosphere. The air between these two conducting regions, in the troposphere (where most weather occurs) and in the stratosphere, is a poor conductor of electric current. When a poor conductor is sandwiched in between two layers having better conductivity, the result is a capacitor.

A capacitor has the ability to store an electrostatic charge. This produces a potential difference (voltage) between the conductors of the capacitor. The amount of charge that a capacitor can hold is proportional to the surface area of the two conducting surfaces or regions. The earth-ionosphere capacitor is shaped like one huge sphere inside the other. It is nearly 13,000 km (8000 mi) in diameter (just a little larger than the diameter of the earth), resulting in a huge surface area between two spherical "plates" whose spacing is small compared with their diameters. There is a high voltage between the surface and the ionosphere (Fig. 4-6), and this gives rise to a constant electric field in the troposphere and stratosphere.

Ionized layers of upper atmosphere

Capacitor World From The Ionosphere

Fig. 4-6. A large electrostatic potential difference (voltage) exists between the earth and the ionized layers of the upper atmosphere.

High voltage i

Fig. 4-6. A large electrostatic potential difference (voltage) exists between the earth and the ionized layers of the upper atmosphere.

The troposphere and stratosphere of the earth compose a good electrical insulator, but it is not perfect. Vertical air currents, and regions of high moisture content, produce channels of higher conductivity than that of the surrounding air. Cumulonimbus clouds, which sometimes reach to the top of the troposphere, present paths of better ground-to-ionosphere conductivity than stable, dry air. The average effective resistance between the ground and the ionosphere is about 200 ohms, roughly the same resistance as a 75-watt bulb in a household electrical circuit. In and near thunderstorms, the resistance is considerably less than this. Cumulonimbus clouds thus present an attractive environment for electrical discharges to occur. Some discharge occurs slowly and constantly as "bleedoff" throughout the world, but a fast discharge—an electrical arc—happens frequently. These arcs are lightning strokes. Numerous thunderstorms are in progress on our planet at any given moment, helping to limit the charge between the ground and the upper atmosphere.

The atmospheric capacitor maintains a constant charge of about 300,000 volts, which is typical of high-tension utility lines. The average current that flows across the atmospheric capacitor is about 1500 amperes. Hence our atmosphere is constantly dissipating about 450 megawatts of power—the equivalent electrical usage of a medium-sized city.

Most of the current flow between the upper and lower atmosphere takes place within cumulonimbus clouds. They concentrate the charge, increasing the local voltage. They also tend to reduce the space between opposite electric poles in the atmospheric capacitor, creating a particularly attractive place for discharge. A typical thundershower discharges about 2 amperes of current, averaged over time. At any given moment, there are 700 or 800 thundershowers in progress on our planet. A current of 2 amperes per thunderstorm may seem small, but this current does not flow continuously. It occurs in brief, intense surges. A single lightning discharge lasts only a few thousandths of a second. Therefore, the peak current is extremely large.

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Responses

  • andrea
    Is cumulonimbus clouds touches ionosphere?
    6 years ago
  • mantissa
    What is the potential difference between earth and ionosphere?
    6 years ago

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