An electric field sounding system, based on corona discharge, was designed to measure the vertical component of the electric fields in a thunderstorm. The decay stage of a thunderstorm that occurred during the night of August 20, 2012, in the Pingliang region, China (35.57 degrees N, 106.59 degrees E, 1620 m above sea level (asl)), was investigated by two balloon-borne electrical soundings. The results of the first sounding showed that the thunderstorm had a tripole charge structure; a lower positive charge region located at temperatures of 3 to 15 degrees C (2.0-4.0 km asl); a middle negative charge region located at temperatures of -3 to 3 degrees C (4.5-5.3 km asl); and an upper positive charge region at temperatures of -10 to -3 degrees C (5.3-6.3 km asl). In addition, there was a negative screen layer at the bottom of the thunderstorm with a depth of about 400 m. The charge density of the middle negative charge region was larger than that of the lower and upper positive charge regions. Influenced by the downdraft of precipitation, each charge region moved down to a lower altitude region. The results of the second sounding showed that the lower positive charge center totally disappeared and only the middle negative charge region (3.7-4.2 km asl) and upper positive charge region (4.2-4.7 km asl) remained. We conclude that the downdraft with precipitation caused the dissipation of the lower positive charge region. Compared with the first sounding result, we found that the charge density increased and the depth decreased for both of the charge regions. (C) 2015 Elsevier B.V. All rights reserved.