The prebreakdown phenomena of impulse corona, corona propagation and leader propagation in the standard rod gap in air have been studied for spacings of up to 30 cm. The variations occurring in the measured quantities of corona-current amplitude, time of occurrence, corona-propagation time, corona speed, number and length of steps in the leaders, leader-propagation time and leader speed are correlated to the variations in the conditions of gap length, applied voltage, humidity and irradiation. The results of the prebreakdown phenomena have been shown to explain the characteristics of impulse breakdown of the rod gap.