Cloud-to-ground lightning frequency is one of the most important factors to establish the rational insulation design of power transmission systems. Ten-year lightning occurrence data 1992 through 2001 with lightning location systems of electric utilities in Japan have been collected and analyzed. As a result, it is found that the annual number of the lightning strokes varies at the level of about 0.6 million on an average, but it is small in 1993 and extremely large in 2000 and 2001. This is considered due to the abnormal meteorological conditions in summer. Seasonal variations and annual variations of lightning occurrence have been clarified from the seasonal and annual lightning stroke frequency maps made by the above data. It is shown that the variations are different from those of thunderstorm days. Furthermore, comparison between the thunderstorm days observed by the lightning location systems and thunderstorm day data collected by the Japan Meteorological Agency is made.

No abstract