Two kilometres above your head, inside a towering thundercloud, ice crystals the size of sand grains are smashing into soft hail pellets at 40 metres per second. Every collision rips electrons free. The cloud is building a charge that will shortly punch through a billion volts of air resistance and incinerate a channel to the ground at five times the surface temperature of the sun.
The short answer
Lightning happens when colliding ice particles inside a storm cloud separate electrical charge—positive on top, negative in the middle. When the voltage difference between cloud and ground exceeds roughly 100 million volts, the air’s insulating capacity fails and electricity arcs across the gap as a lightning bolt.
The engine: updrafts and ice
Every thunderstorm that produces lightning needs three ingredients: moisture, instability, and violent updrafts. The updrafts are the engine. According to NOAA’s Earth System Research Laboratories, air inside a mature thunderstorm can rush upward at 10 to 40 metres per second—faster than a highway-speed car driving straight up.
That speed matters because it keeps ice particles suspended and colliding. At the altitude where temperatures drop below freezing (typically 3 to 8 kilometres up), supercooled water droplets freeze onto small particles to form graupel: soft, 2-to-5-millimetre hail pellets that look like tiny Styrofoam balls. Smaller ice crystals, meanwhile, form higher and colder in the cloud.
The updraft carries everything upward in a chaotic mixer. Graupel pellets, being heavier, fall back down through the rising column of ice crystals. The collisions transfer electrons. Graupel acquires a negative charge; ice crystals gain a positive charge. The updraft then separates them vertically: positively charged ice crystals are swept to the cloud’s top, while negatively charged graupel sinks toward the middle and lower regions.
You now have an atmospheric battery.
How charge builds to the breaking point
The electrical structure of a thunderstorm isn’t simple, but the basic picture is a layered cake: positive charge near the top (around 8 to 12 kilometres), a strong negative charge in the middle (4 to 7 kilometres), and a smaller pocket of positive charge near the base. The National Weather Service documents that this charge separation can build potential differences of 100 million to 1 billion volts between the cloud and ground.
Air is an excellent insulator under normal conditions. It takes roughly 3 million volts per metre to break down air’s resistance and force a conductive path through it. But when a storm cloud accumulates hundreds of millions of volts across a vertical column several kilometres tall, the math works out: the insulation fails.
What happens next happens fast.
The lightning stroke
A lightning bolt begins not with a single dramatic leap, but with an invisible feeler. A “stepped leader”—a branching channel of ionised air—descends from the cloud in 50-metre spurts, each step taking about a microsecond. The leader is faint, invisible to the naked eye, and zigzags as it searches for the path of least resistance.
When the stepped leader gets within 100 metres or so of the ground (or another charged region in the cloud), the electrical field becomes so intense that a “streamer” of positive charge leaps up to meet it. The moment they connect, a complete circuit forms.
Then comes the return stroke. A wave of electrons races upward along the ionised channel at roughly one-third the speed of light—about 100,000 kilometres per second. This is the brilliant flash you see. NASA confirms that the current in this stroke can exceed 200,000 amperes, and the channel heats to around 30,000 Kelvin in less than a millisecond—five times hotter than the sun’s surface.
Most “flashes” you see are actually three to five individual return strokes riding the same channel in rapid succession, happening so fast they blur into one.
Lightning’s twin: why thunder follows
Thunder isn’t a separate phenomenon. It’s the sound of lightning—specifically, the acoustic shockwave created when air along the lightning channel heats explosively.
When 30,000-Kelvin plasma suddenly appears in a channel a few centimetres wide and several kilometres long, the surrounding air expands outward faster than the speed of sound. That supersonic expansion creates a shockwave that propagates as a pressure wave—thunder.
You see the lightning instantly because light travels at 300,000 kilometres per second. Thunder, moving at the speed of sound (about 343 metres per second at sea level), takes roughly three seconds to cover one kilometre. Count the seconds between flash and rumble, divide by three, and you know how many kilometres away the strike occurred. This is not folklore; it’s straightforward physics, and it works.
Close lightning—within a few hundred metres—produces a sharp crack. Distant lightning rumbles because the sound waves from different parts of the channel arrive at slightly different times, and because temperature and wind layers in the atmosphere refract and scatter the sound. Beyond about 15 kilometres, thunder typically fades below audibility.
Not all lightning reaches the ground
Only about a quarter of lightning strikes are cloud-to-ground. The rest occur within the cloud (intra-cloud lightning) or between clouds. You often see the cloud illuminate from within—so-called “sheet lightning”—which is just intra-cloud discharge seen from a distance. The physics is identical; the only difference is where the charge imbalance happens to be greatest.
Tall structures and mountaintops do get struck more often, but the old rule that “lightning always hits the tallest object” is misleading. Ground conductivity, moisture, and the geometry of the stepped leader all play a role. Tall structures like the Empire State Building are struck dozens of times per year, but plenty of shorter structures nearby are also hit.
The satisfying answer to “why”
Lightning happens because ice. Updrafts smash ice crystals into graupel, electrons move, charge separates, voltage builds, air breaks down, and a billion-volt arc closes the circuit in a flash hotter than any furnace on Earth. Thunder is the sound of that flash heating air faster than it can get out of the way.
The whole sequence—from the first graupel collision to the final return stroke—takes seconds to build and milliseconds to release. Globally, that happens about 100 times per second. Right now, somewhere over tropical Africa or the Amazon or the South China Sea, a thunderstorm is separating charge and preparing to light up the sky.
FAQ
How hot is lightning?
Around 30,000 Kelvin (roughly 54,000°F or 30,000°C). That’s about five times hotter than the surface of the sun, though nowhere near the sun’s core.
Can lightning strike the same place twice?
Yes. Tall structures like the Empire State Building are struck dozens of times per year. The idea that lightning avoids places it’s already hit is myth—electrical discharge follows physics, not memory.
Why does thunder rumble instead of just making one sound?
A lightning channel can be several kilometres long. Sound from the near end reaches you first; sound from the far end arrives later. Wind, temperature layers, and echoes further scatter the sound into a rumble. Close strikes, by contrast, produce a single sharp crack because the entire channel is roughly the same distance from you.
What’s the difference between lightning and household electricity?
Both are flows of electrons—electricity. Household current is regulated (120 or 240 volts, relatively steady), while lightning is an uncontrolled discharge of 100 million volts or more lasting a fraction of a second. Same fundamental phenomenon, wildly different scale and behaviour.
If you’ve ever wondered why storms smell different afterward, trace amounts of ozone form when lightning splits oxygen molecules—but that’s another story. For now, the next time you count seconds between flash and thunder, remember: you’re timing the gap between light-speed and sound-speed, and measuring the distance to a 30,000-Kelvin spark born of ice.
Written for general interest and accuracy-checked, but not a substitute for specialist sources.