Corona inception voltages and breakdown voltages at non-uniform fields depend on the configuration characteristics and conductor surface irregularities of the power transmission system as well as ambient weather conditions. Asymmetrical non-uniform electric field configuration of rod-plane electrode systems have been the reference field distribution for corona phenomena in atmospheric air due to local confinement of ionization phenomena around the stressed electrode. The objective of this study is to analyze corona inception voltages and breakdown voltages of rod-plane electrode systems for severe operating conditions in terms of humidity and the ambient temperature. The study covers both the AC and the DC experiments where 20 g/m3 of absolute humidity was kept constant at 29 oC of ambient temperature during the tests. The aim is to observe the development of corona phenomena for the worst ambient conditions, especially by the coastal areas. Quite a large range starting from 10 cm to 100 cm of gap spacing were scanned to cover AC and HVDC applications.