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In China external discharge accidents caused by heavy rain is growing. However, the rain effect is not considered in atmospheric correction for external insulation design in IEC 60071‐1(Edition 8.1, 2011) or domestic standard. Hence, a series of experiments are implemented with positive switching impulse, a typical kind of overvoltage of power grid, to analyse the discharge performance of rod‐plane short air gap under rain conditions. The effects of rain and its intensity, rain conductivity and ambient temperature on the discharge voltage are explored, and the theory behind them, which can help in the development of discharge mechanism of air gap under rain condition, are respectively given. The results show that under the experimental rain condition, the discharge voltage can be up to −10.9, −9, −8.5, −3.2 and −2.3% lower than under dry condition for clearance distance of 0.1, 0.2, 0.3, 0.4 and 0.5 m, respectively. Besides, it decreases with increase in rain intensity, rain conductivity and ambient temperature, respectively. Among them, the rain intensity is the most significant one, next is temperature, whereas rain conductivity is the minimum and trifling. Finally, a correction formula for breakdown voltage (positive switching impulse) of rod‐plane air gap under rain condition is proposed for reference. [ABSTRACT FROM AUTHOR]