The discharge characteristics of air gap subject to switching impulse voltage is a critical design criterion in selecting the equivalent wind speed for Extra and Ultra High Voltage（EHV/UHV） transmission lines. Existing research work focuses on the discharge characteristics under standard switching impulse voltage

while the long frontwave switching impulse voltage generated by the EHV/UHV transmission lines is rarely considered

especially under the heavy rain condition. We simulated four different precipitation intensities with four different rainfall conductivities in the laboratory. The conductor-tower and rod-plane air gaps with three different length were tested under a switching positive impulse voltage. The results show that the U50 of the rod-plane air gaps is reduced by 11.2% due to the increased precipitation rate and the U50 of the conductor-tower ones is reduced by 7.45%. The U50 of the rod-plane air gaps will reduce by 3.32% due to the increased rainwater conductivity and by 1.95% due to the reduction of conductor-tower distance. In addition

an electrostatic simulation has been conducted to study the influence of water droplets presenting in the electric field. The results show that the larger size the water droplet is

the greater distortion the electric field will have. It is also observed that the water droplets have more impact on the rod-plane air gap rather than the conductor-plane air gap.