It has been found statistically that the occurrence of thunder clouds in the neighbouring area of the volcano Asama was significantly less frequent in summers when the volcano had frequent eruptions than in summers when the volcano was less active. The average occurrence frequency of thunder clouds in the former case has been about a half of that in the latter case.
 Results of analysis on this fact suggest that frequent eruptions of the volcano have modified the nature of cumulonimbus and suppressed the occurrence of thunder and lightning through the effect of some cloud-physical mechanism induced by seeding of the volcanic dust particles.