In non-uniform fields with ac voltages it is common to
encounter corona prior to complete breakdown of the gap. As
the humidity increases from a low to a high value (10 g/m³),
the appearance of the corona has been reported to change
from a stable glow to streamers [5, 6, 9]. Due to the presence
of glow discharges in non-uniform configurations at power
frequencies, the breakdown voltage has been reported to
decrease with humidity. However, if there is no corona prior
to breakdown for power frequencies, it has been reported that
the breakdown voltage increases with humidity at a rate of
0.25 %/g/m³. This is similar to that seen with uniform fields at
power frequency [6]. In contrast, if corona is in the form of
streamers, the breakdown voltage is reported to be either
constant or to slightly increase with humidity [6].