Rapid changes in cross-polarization discrimination (XPD) of Ka-band satellite radio wave signals caused by thunderstorm events are presented. Their yearly statistics and yearly variations are discussed using the co-polar and cross-polar signals received for the past 17 years from 1990 to 2006. The number of thunderstorm events and rapid changes in XPD shows large yearly variations in a time scale of about five or six years. These long-term yearly variations may be related to periodic cycles of the equatorial climate, such as El Nino and La Nina, which tend to bring weak and strong convective activities, respectively, to the summertime weather in Japan.