Tests of influence mechanism explanations for thundercloud electrification have been attempted by modifying the initial atmospheric electrical conditions beneath some convective clouds. The modification was accomplished by the release of negative space charges into the air from elevated wires connected to a high-voltage power supply. Anomalous distributions of charge were observed in some clouds with low bases that grew over the charge source. The results of the experiment suggest that influence mechanisms may be operative in the electrification of our mountain clouds and that, normally, the natural, positive space charges in the sub cloud air may initiate the electrification processes. ¿ American Geophysical Union 1989