Tukuk-OSInside a big cumulonimbus, supercooled water, ice and graupel (soft hail) collide in violent updrafts, and those collisions separate charge: lighter ice carries positive charge to the top, heavier graupel drags negative charge to the middle and base. The ground below becomes positively charged by induction. When the field grows strong enough — millions of volts per metre in places — a faint stepped leader of ionised air probes down from cloud in stages, and a streamer rises from the ground to meet it. Contact closes the circuit, and the return stroke — the flash you actually see — races back up the channel at around a third of light speed.
The channel briefly reaches about 30,000 °C, roughly five times the Sun’s surface, and the air around it detonates outward in a shock wave that decays into the sound we call thunder. Light and sound travel the same path at wildly different speeds, which is the counting trick: three seconds per kilometre (or five per mile) between flash and bang gives the distance. If the gap shrinks as storms pass, the storm is approaching you.
Safety facts that survive the folklore: lightning does strike the same place twice — the Empire State Building is hit dozens of times a year; rubber-soled shoes and car bodies are safe because of the metal shell’s Faraday-cage path around occupants, not because rubber insulates anything at lightning voltages; and the “30/30 rule” (go indoors if thunder follows a flash within 30 seconds, stay 30 minutes after the last thunder) exists because lightning can kill far from rain — up to 10–15 kilometres from a storm’s edge in what people call “bolt from the blue”. The only short answer: when thunder roars, go indoors.