Lightning is a giant electrical discharge. In a thunderstorm cloud, electric charges build up until the electric field becomes strong enough to force a path through the air. That path becomes a super‑hot, glowing channel of ionised air (plasma) — what you see as lightning.
Step 1: Storm clouds separate electric charge
Inside a thunderstorm (cumulonimbus) cloud, strong up‑ and down‑drafts toss around water droplets, ice crystals, and soft hail (graupel).
As these particles collide, charge gets transferred. The exact microphysics is complex, but the typical outcome is:
- negative charge concentrated in the lower/middle part of the cloud
- positive charge concentrated higher up
Step 2: The ground gets “charged” too
The cloud’s negative region repels electrons in the ground below, leaving the surface relatively positively charged. (Nothing has to touch — it’s electrostatic induction.)
Step 3: Air normally insulates… until it can’t
Air is usually a good insulator. But if the electric field becomes strong enough, it can start ripping electrons off molecules, creating charged particles. Once that happens, the air becomes much more conductive.
Step 4: A stepped leader starts the path
Many cloud‑to‑ground strikes begin with a faint, branching, downward-moving channel called a stepped leader. It travels in jumps, searching for an easier conductive route through the air.
Step 5: An upward streamer meets it
As the leader approaches the ground, objects on the ground (trees, buildings, even the ground itself) can launch an upward streamer. When the leader and streamer connect, a continuous conductive path is formed.
Step 6: The bright flash is the return stroke
The brilliant part you see is usually the return stroke: a powerful surge of current travelling back up the ionised channel. This heats the air to roughly tens of thousands of degrees Celsius, causing the intense light and rapid expansion of air.
Why does thunder come after lightning?
Thunder is the sound of that explosive expansion of super‑heated air (a shock wave that becomes a sound wave).
You see lightning first because light travels vastly faster than sound. Sound moves through air at about 343 m/s (varies with temperature), so thunder arrives later.
Quick distance trick
Count the seconds between the flash and the thunder:
- 3 seconds ≈ 1 km away (rough rule of thumb)
- 5 seconds ≈ 1 mile away
Different types of lightning (fast overview)
- Cloud-to-ground (CG): strike connects cloud and ground.
- In-cloud (IC): discharge happens within the cloud (very common).
- Cloud-to-cloud: between different charged regions/clouds.
Lightning safety (worth being direct)
- If you can hear thunder, you’re close enough to be struck.
- Go indoors (substantial building) or into a metal-roofed car.
- Avoid isolated tall trees, open fields, and water.
Sources
- Met Office (UK) – Thunderstorms / lightning basics: www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/thunderstorms
- NOAA National Weather Service – Lightning Safety: www.weather.gov/safety/lightning
- Encyclopaedia Britannica – Lightning: www.britannica.com/science/lightning
