Minecraft wiki entry for Thunderstorm, associated text: Lightning strikes randomly and creates fires (only on normal and hard difficulty) in a 2 block radius where it strikes. Such fires act normally, igniting all flammable materials, detonating TNT, and even activating nether portals. The lightning itself, however, is not destructive and does not destroy blocks. While most fires are extinguished by the rain, areas that block rain can allow the fire to spread, and any netherrack, magma blocks, soul sand, or soul soil lit by lightning is not extinguished by the rain.