In this model, the thundercloud forms a positive electric dipole as shown in Fig. 1 and Fig. 2; that is, a primary positive charge region is found above a primary negative charge region. By the end of the 1930’s, Simpson and
co-workers [ 121, [ 131 had verified this overall structure from measurements made with sounding balloons inside clouds and had also identified a small localized region of positive charge at the base of the cloud. Subsequent measurements of electric fields both inside and outside the cloud have confirmed the general validity of this double-dipole structure [ 141-[24].