How is clearance affected by altitude (air pressure)?
Well, to start with, "creepage" has nothing to do with the air at all &mdash; it relates to currents flowing in surface contaminants.

"Clearance" has nothing to do with the conductivity of air, but rather its ability to prevent an arc from occurring. Paschen's Law provides some insight into this issue. I got there by searching "air arc voltage vs pressure" &mdash; there are other useful resources there, too.

Clearance has less to do with the working voltage of the circuit, and more to do with the level of expected transients. Yes, a circuit with a given clearance will break down more readily at high altitude for a given level of transient, but this is rarely an issue that requires a design change.