抄録
Lightning location systems, such as UP and LPATS, cover the whole Japanese land at present, However, the winter lightning flash density measured by these systems are less than 0.3 times per km2 per one winter season, which are very small than the appropriate density. The author points out three reasons of this contrariety. (1) The velocity of winter lightning return stroke is less than one third of summer ones. (2) The average channel length of winter lightning discharge is less than one third of summer ones. (3) The attenuation of radiation field may be much larger in winter because the height of a discharge channel (the source of electro-magnetic field) is much lower in winter than in summer.
