Atmospheric Conductivity

the capacity of the atmosphere to conduct an electric current. Atmospheric conductivity is caused by atmospheric ions and increases in proportion to the concentration and mobility of the ions. For this reason, conductivity will rise as ionization increases and as the atmosphere becomes purer and more rarefied; that is, the atmospheric conductivity depends on the meteorological conditions. An increase in water-vapor content, the concentration of dust particles, or the amount of fog and cloud cover will all serve to lower the conductivity. In storm clouds, however, where there is a high level of ionization, the conductivity can be markedly increased. Generally speaking, atmospheric conductivity is lower where industrial pollution is high. But the effect is not limited to cities; even above the center of the Atlantic Ocean the atmospheric conductivity has decreased by a factor of 2 during the last 50 years.

Nuclear explosions increase atmospheric conductivity markedly. The average magnitude of the specific atmospheric conductivity near the earth’s surface is 2.2 × 10^-18 ohm^-1 m^-1. The atmospheric conductivity varies at different points of the earth’s surface and is also dependent on time. Above the continents, the daily fluctuation from the average value is about 20 percent and the annual fluctuation can approach 30 percent. Above the oceans these fluctuations are smaller.

In pure air, the conductivity increases exponentially with altitude. At 6 km, the specific conductivity may reach 13 × 10^-18ohm^-1 m^-1, while at 30 km it can be as high as 300 × 10^-18ohm^-1 m^-1. In the ionosphere, the conductivity is caused by electrons, and its value is many times higher than that in the troposphere. The total atmospheric conductivity in the layer between the earth’s surface and the ionosphere is 0.5 × 10^-2ohm^-1.

Displacements of space charges in the atmosphere [seeATMOSPHERIC ELECTRICITY) caused by air motion and by turbulent diffusion produce effects similar to those created by atmospheric conductivity in an electric field. To characterize these effects, the concepts of convective and eddy conductivity have been introduced.