A lightning bolt can heat the surrounding air to roughly 30,000 kelvin, several times hotter than the surface of the sun, and the main return stroke travels at up to a third of the speed of light. The explosive expansion of air heated that quickly and violently is what produces thunder. All of this begins with something far less dramatic: collisions between tiny particles of ice inside a thunderstorm cloud, gradually building up an electrical charge imbalance large enough to rupture the insulating properties of air itself.
Building the Charge
Inside a mature thunderstorm cloud, strong internal updrafts and downdrafts carry ice crystals and larger, softer ice pellets called graupel past each other at high speed. Collisions between these particles transfer electrons, and evidence suggests smaller ice crystals tend to come away positively charged while larger graupel particles tend to come away negatively charged. Because the smaller, lighter crystals get carried higher by updrafts while the heavier graupel sinks lower, this sorting process leaves the upper part of the cloud predominantly positively charged and the lower part predominantly negatively charged, creating a large-scale electric field within the cloud that continues strengthening as the storm develops.
When Air Stops Being an Insulator
Air is normally an effective electrical insulator, but every insulator has a breakdown voltage beyond which it can no longer resist an electric field, at which point it suddenly permits current to flow. As charge separation inside a storm cloud intensifies, it also induces an opposite charge on the ground beneath it — the negatively charged cloud base causes the ground below to become relatively positively charged. When the electric field between cloud and ground becomes strong enough, a channel of ionised air called a stepped leader begins working downward from the cloud in short, discrete segments, each roughly 50 metres long, branching as it goes and searching for the path of least resistance toward the ground.
The Flash Goes Up, Not Down
As the descending stepped leader nears the ground, the intensifying electric field causes upward-moving streamers of charge to rise from tall or pointed objects below — trees, buildings, sometimes people standing in open ground. When a descending leader connects with an ascending streamer, it completes a conductive channel between cloud and ground, and an intense pulse of current, the return stroke, surges upward along that channel at a meaningful fraction of the speed of light. This detail surprises most people: the visible flash of a lightning bolt, though it appears to travel downward, is actually dominated by current flowing upward from ground to cloud along the path the leader has already carved out.
A Process Still Being Measured
Despite lightning being one of the most immediately visible natural phenomena on the planet, meteorologists tracked by organisations including the National Weather Service continue to refine exactly how the initial charge separation process works inside a cloud, since directly measuring conditions inside an active thunderstorm remains genuinely difficult and dangerous. What is well established is the overall structure of the event: a slow, largely invisible build-up of charge through millions of microscopic ice collisions, followed by a discharge lasting a fraction of a second, releasing more energy in that instant than the process took hours to accumulate.
A single storm cloud does not stop at one bolt. Most storms produce repeated discharges, some striking the ground and some remaining entirely within or between clouds, and a single flash often contains several rapid return strokes travelling along the same ionised channel in quick succession, which is part of why lightning sometimes appears to flicker rather than flash once. Globally, lightning strikes the ground somewhere in the world roughly 40 to 50 times every second, a reminder that a process built from something as small as colliding ice crystals is, in aggregate, one of the most continuously active physical phenomena on the planet's surface.