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believe the answer you're looking for has to do with the pressure-altitude-voltage derating related to corona discharge, which increases (gets worse) with higher altitude and peaks at around 80-100,000' Typically this is considered to occur at votlages above 300 V peak, but be sure you are accounting for all transients and internal voltages.
The seeding effect might still persist there, but it is not associated with a decreasing trend of the unseeded conditions. It is apparent that the effects of cloud seeding and air pollution in the low-elevation areas are more limited because of the lack of an orographic enhancement to the rainfall.