Minecraft wiki entry for Thunderstorm, associated text: When lightning is to strike, random X and Z coordinates within the chunk are chosen, and the block just above the highest block that is liquid or obstructs movement is chosen for the lightning strike. If a lightning rod is nearby, it strikes the rod instead. Then if there are any living entities that can see the sky in a 3×h×3 region from 3 below the target block up to the world height, one such entity is selected at random and the lightning target is moved to the block the entity stands in.