Minecraft wiki entry for Thunderstorm, associated text: For each loaded chunk, every tick there is a 1⁄100,000 chance of an attempted lightning strike during a thunderstorm. From this probability, if ≈201 chunks are loaded (from a radius of 128 blocks from the player to the center of each chunk) then 90% of the time up to 5 lightning strikes occur in the world each minute, with an average of approximately 2.4 lightning strikes each minute.