For inhomogeneous fields like in many
practical insulation configurations, the switching-impulse
breakdown voltage of a given air-gap usually decreases
with the decrease of atmospheric humidity. A correction
procedure recommended by IEC is commonly used for
converting the measured voltage or the test voltage from
non-standard to standard reference atmosphere, or vice
versa. The applicability of the humidity correction on
relative uniform gap is not clear. In this paper results
from newly performed laboratory test on air gaps of
different uniformity under different atmosphere are
presented. Comparison is made between the outcomes of
the IEC recommended corrections for these results.