How much will the humidity affect the breakdown voltage of air at STP.
What makes you think it has any effect at all?
Humid air will have a much greater number of ions that can more easily facilitate a current through it.
Also, the breakdown voltage of dry air is 30kV/2.54cm.
I have a marx generator that has a maximum voltage of 55kV sparking over a distance corresponding to 100kV in dry air .
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