It seems a bit paradoxical, but the physics does indeed tell us that when staring around atmospheric pressure the voltage that a particular fixed gap can withstand does go down as the pressure decreases.
Eventually (but not on the earth's surface) you get to a low enough pressure that the voltage starts to rise again, getting to its highest overall value when you reach a vacuum.
The reason for this depends on the mean free path of an ionized air molecule moving under the voltage gradient and how many additional air molecules it will hit and ionize along the way. Lower pressure equals longer mean free path equals higher energy when it hits another molecule which means more ionization.
Eventually (but not on the earth's surface) you get to a low enough pressure that the voltage starts to rise again, getting to its highest overall value when you reach a vacuum.
The reason for this depends on the mean free path of an ionized air molecule moving under the voltage gradient and how many additional air molecules it will hit and ionize along the way. Lower pressure equals longer mean free path equals higher energy when it hits another molecule which means more ionization.
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