Also to consider is air pressure. The breakdown voltage decreases with
decreasing air pressure, and to a slight extent with increasing
temperature.

"Reference Data for Radio Engineers" has a graph of breakdown for a
needle gap and for smooth surfaces. For the needle gap (pretty much a
worst case) at sea level (760mm Hg), it's a bit over 25kV peak at 1
inch spacing, and just over 3kV at 0.1 inches. For comparison, a
couple smooth 10" diameter balls spaced with a 1" gap between them will
exhibit a breakdown greater than 80kV. Plates for an air-variable high
voltage cap should have smooth, rounded edges. Looks like the
handbook's table assumes sharp corners, and/or gets its safety factor
from an assumption of rounded edges. It's usually not much of a
problem, but the higher dielectric constant (rel. to air) of the
ceramic or other insulation in a capacitor can create locally higher
fields that cause trouble.

Cheers,
Tom

(Great quote about hammers, Mike!)