In reference to conductivity near the ground, the primary source of ions in the lowest 1 or 2 km of the atmosphere above land. (Cosmic radiation is dominant over the oceans and at higher altitudes. )

The ionizing radiation can take the form of 1) emission of α, β, or γ radiation by radioactive materials in the soil; or 2) emission from gaseous radioactive daughter products (radon) that emanate from the soil. The α radiation emitted by materials in the ground is absorbed in the lowest few centimeters of the atmosphere, β radiation penetrates a few meters, and γ radiation a few hundred meters. The ionization produced by radioactive gases in the atmosphere is highly variable and depends on the rate of emission from the soil and also on atmospheric dispersion. ²²²Rn, which has a half-life of 3. 8 days, is produced by the uranium decay series. ²²⁰Rn (thoron) is produced by the thorium decay series and has a half-life of 54 seconds.