Air ionization produced by natural ground radioactivity, mainly by radon emanating from the Earth's crust, is a primary source of ions in the planetary boundary layer (PBL) of the atmosphere over land [Hoppel et al., 1986]. These ions provide the air conductivity responsible for fair weather vertical electric current in the global electric circuit (GEC), a system of stationary electric currents between the ground and ionosphere driven by global thunderstorm activity. This activity is considered an electric generator of the potential difference between the ground and ionosphere (200–600 kilovolts), and the return downward current closes the circuit in the areas of fair weather [Roble and Tzur, 1986]. The upper closing limit of this circuit is of the altitude of 60 kilometers. At altitudes higher than 60 kilometers, cosmic rays and solar radiation also contribute to air ionization.