In the IPC 2221 is a table that specifies the required trace spacing. One thing that I find very surprising is the jump once the elevation is above 3050m. E.g. For an external conductor (uncoated) at 500V
The jump seems very abrupt. Why does IPC 2221 suddenly require a factor 5 more clearance? What is the rational?
It comes from the Paschen's Law.
The following graph is a Paschen curve taken from this research.
The graph may look a bit awkward to make a conclusion as "required clearance increases with altitude."
The horizontal axis represents the distance between the two conductors in open air. Starting from a conductor-to-conductor distance of 0.1 mm and an altitude of up to 50 kft (~15250 m) the effect becomes more pronounced. For example, for a distance (clearance) of 0.2 mm, the breakdown voltage (the voltage enough to generate an arc) is about 2 kV at sea level and drops to 750 V at 10 kft (~3050 m).
A similar curve can be obtained by taking the horizontal axis as air pressure (decreases with altitude) for a constant conductor-to-conductor spacing.
IPC tables also include a correction factor so the numbers shown on the tables may not match to the graph above (or Paschen's experimental results).
It's more of a one (or two, in this case) size fits all strategy. If you don't want to deal with calculating the exact factor for the altitude at which you'll be operating, then the values given will cover you in all circumstances, albeit in a somewhat more cumbersome fashion above 10000 feet.
IEC 60664-1 gives correction factors for altitudes up to 20km (snippet here if you don't want to pay $345 just to peek). You can see that the IPC values given ensure adequate clearance up to ~12km which covers all land on earth (Mt. Everest = 8849m).
And as always, IPC will let you substitute different values and remain in compliance. They just need to be agreed upon between the customer and the designer.
