In this paper a methodology is presented to quantify the year-to-year variability of the lightning outage rate of transmission lines based on the stochastic and climatological variations in lightning incidence. The stochastic variations can adequately be described by the Poisson distribution, and the year-to-year variation in the lightning ground flash density by a Log-Normal distribution function. Based on data from ALDIS and EUCLID it is suggested that the standard deviation of Ln(Ng) typically falls between 0.3 and 0.6 and a median of 0.423.