Lightning forms when collisions between ice crystals, graupel, and supercooled water droplets inside a growing thunderstorm strip electrons from one another, building up separated pools of positive and negative charge. When the electric field between those charge regions, or between the cloud and the ground, grows strong enough to overcome the air's resistance, it discharges in a bolt that can reach 50,000°F, hotter than the surface of the sun. Southern California sees this process far less often than the Great Plains or Florida because the region usually lacks the deep, sustained updraft strength that builds a fully charged storm, but when a strong enough cell forms over the mountains or deserts, the physics is identical everywhere on Earth.
How does a storm separate electrical charge?
A thunderstorm becomes electrified through billions of tiny collisions inside its coldest layers. As a storm's updraft strengthens, it lofts supercooled water droplets, ice crystals, and a soft, dense ice pellet called graupel into the same turbulent region of the cloud, typically between about 14,000 and 30,000 feet where temperatures sit well below freezing. When rising ice crystals collide with denser, slower-falling graupel, electrons transfer between them. Laboratory and field research summarized by NOAA JetStream shows that in this temperature range, graupel tends to come away negatively charged while the lighter ice crystals come away positive.
Because the storm's updraft is stronger than the graupel's fall speed only up to a point, the two particle types separate by weight and by the wind field around them. The lighter, positively charged ice crystals get carried higher and toward the top and forward edge of the storm, while the heavier, negatively charged graupel sinks or holds lower in the cloud. Repeated over millions of collisions, this sorts the storm into a rough vertical stack: a positive charge region near the anvil top, a negative charge region in the mid-level, and often a smaller positive pocket near the base.
The ground beneath the storm responds too. As the negative charge region overhead grows, it induces a positive charge to build on the surface below, setting up the field that eventually connects the cloud to the earth.
| Stage | What happens | Approximate speed or duration |
|---|---|---|
| Charge separation | Ice crystal and graupel collisions build stacked charge regions in the storm | Builds over the storm's growth, minutes to tens of minutes |
| Stepped leader | A faint, negatively charged channel steps downward from the cloud in short bursts | Roughly 200,000 mph in ~50-yard steps |
| Upward streamer | A positively charged channel rises to meet the leader from a tall or grounded object below | Triggered in the final tens of yards |
| Return stroke | Once the channels connect, the visible, brilliant flash surges upward along the completed path | About 270,000,000 mph (roughly one-third light speed) |
| Repeated pulses | Additional strokes can reuse the same channel, producing the flicker seen in a single flash | Often 3-4 strokes in well under a second |
How does a cloud-to-ground strike develop?
A visible strike is really two channels meeting in midair, not a single bolt shooting from cloud to ground. It starts with a stepped leader, a faint, negatively charged channel of ionized air that descends from the storm's charge region in short, roughly 50-yard jumps, pausing for a fraction of a microsecond between each step as it feels out the path of least resistance toward the ground. As the leader nears the surface, usually within a few hundred feet, the strong electric field it creates draws an answering positively charged channel, called an upward streamer, up from the tallest or most conductive object nearby, whether that is a tree, a radio tower, or a person caught in the open.
When the descending leader and the rising streamer connect, they complete a conductive path between cloud and ground. The return stroke follows instantly: the visible, brilliant flash that people call lightning surges back up that newly completed channel at roughly a third the speed of light, releasing the stored charge and the heat that generates thunder. A single flash often includes three or four separate return strokes reusing the same ionized channel in rapid succession, which is why lightning frequently appears to flicker rather than fire once. The entire sequence, from the first step of the leader to the final pulse of the return stroke, typically unfolds in well under a second.
Why can lightning strike outside the rain?
Lightning can strike well outside the rain shaft because the charge regions that produce it are not confined to the area of heaviest precipitation. A storm's positive charge region often sits at the anvil top, which can extend miles downwind of the core, and the electric field connecting that charge to the ground does not respect the boundary of visible rain. This is the physical basis for the phenomenon sometimes called a "bolt from the blue," a strike that appears to come from clear or lightly clouded sky when it has actually traveled laterally from a storm's overhanging anvil before curving down to the ground.
The National Weather Service documents strikes occurring 10 miles or more from the parent storm's rain core, which is the basis for the safety guidance to shelter as soon as thunder is heard, not to wait for rain to arrive overhead.
This matters directly for hikers on exposed ridgelines in the San Gabriel or San Bernardino Mountains, where a storm cell visible several miles away can still be an active threat even under a patch of blue sky. It is also why "outrunning the rain" is not a useful safety strategy on its own. The detailed reasoning behind the wait-for-thunder rule and what to do once you hear it is covered fully in lightning safety in Southern California, which this page defers to for personal safety guidance.
When does Southern California get lightning?
Southern California's lightning arrives in three distinct patterns, and none of them resemble the Great Plains thunderstorm season most Americans picture. The most reliable source is the North American monsoon, a summer pattern that pulls moisture north from Mexico and the Gulf of California into the mountains and deserts of the region, most active from roughly July through September. That moisture fuels afternoon and evening storms over the San Gabriel Mountains, San Bernardino Mountains, and desert ranges near Big Bear Lake, where elevation and daytime heating combine to build the strong updrafts lightning requires.
A second pattern is cool-season convection, when an unusually cold, unstable upper-level low moves through in fall, winter, or spring and destabilizes the atmosphere enough to spark isolated thunderstorms, sometimes reaching the coast and the Los Angeles Basin itself. The third and rarest pattern is a landfalling tropical remnant, when the decaying moisture from an eastern Pacific hurricane or tropical storm reaches Southern California and interacts with monsoon-like instability, occasionally producing a more widespread lightning event than either seasonal pattern alone.
The through-line across all three patterns is that lightning in this region is a mountain-and-desert phenomenon far more often than a coastal or basin one. Areas like Downtown Los Angeles and Pasadena see thunder on only a handful of days most years, while the high country sees it far more routinely each summer. A full breakdown of how often each part of the region actually gets thunderstorms, with the seasonal and geographic data behind it, is covered in how rare are LA thunderstorms, which this mechanism-focused page defers to rather than duplicating.
What happens when lightning hits sand, a car, or a house?
Lightning striking dry sand can fuse it into a hollow, branching glass tube called a fulgurite, formed when the bolt's heat, often estimated near 50,000°F, instantly melts silica along the current's path underground before the surrounding sand cools it back into solid rock. A strike to a hard-topped vehicle is survivable for occupants inside not because of the rubber tires, a persistent myth, but because the metal shell of the car acts as a Faraday cage, conducting the current around the outside of the vehicle and into the ground rather than through the people inside.
A strike to a house typically follows the path of least resistance through plumbing, wiring, or metal ductwork, which is why the National Weather Service advises avoiding contact with corded phones, wired electronics, and running water during a storm even while sheltering indoors.
A less obvious hazard is what happens to open ground itself. When current spreads outward from a strike point across the earth's surface, it creates a voltage gradient called ground current or step voltage, meaning two feet planted a stride apart can sit at different electrical potentials and complete a circuit through a person's body, even without a direct hit. This is one reason open beaches and exposed ridgelines carry real risk during a storm even for someone who never sees a bolt strike nearby. Full personal safety guidance, including where to shelter and how long to wait, is covered in lightning safety in Southern California.
Why is thunder so loud?
Thunder is the sound of air violently expanding. The return stroke heats the air in the lightning channel to roughly 50,000°F in a few dozen microseconds, far faster than the air can expand smoothly, so it instead explodes outward as a shockwave that steepens into the sound wave heard as a crack or rumble. A sharp crack usually means the strike was close and the shockwave reached the listener while still coherent, while a longer rumble typically comes from a more distant strike, where the sound has echoed off terrain, or from a very long, sprawling channel where different segments are different distances from the listener.
Because light is essentially instantaneous over these distances and sound travels at roughly one mile every five seconds, counting the gap between a flash and its thunder gives a rough distance estimate, which is also the basis for the NWS wait-time guidance covered below.
Can airplanes get struck by lightning?
Commercial airplanes are struck by lightning more often than most passengers realize, commonly estimated at around once per aircraft per year of service, and they are engineered specifically to handle it. An airplane's aluminum or conductive composite skin acts much like a car's metal shell, conducting the current around the outside of the fuselage and discharging it from a wingtip or tail static wick rather than letting it pass through the cabin or critical systems.
Modern aircraft carry certified lightning protection for fuel tanks, wiring, and avionics precisely because strikes are a routine, expected part of flying through or near convective weather, which is also why pilots and air traffic control actively route around the most active storm cells when possible rather than treating a strike as an emergency in itself.
What is the 5 second rule for lightning?
The rule states that for every five seconds between seeing a lightning flash and hearing its thunder, the storm is roughly one mile away, based on sound traveling at about 1,100 feet per second while light arrives effectively instantly. The National Weather Service treats this as a rough distance estimate, not a safety threshold. Their actual guidance is simpler and more conservative: if you can hear thunder at all, you are already within striking range, and the correct response is to get to substantial shelter immediately rather than to do the math first.
What should you do when thunder begins?
The moment thunder is audible, the correct response is to move to substantial shelter, meaning a fully enclosed building with wiring and plumbing or a hard-topped vehicle with the windows up, not a picnic pavilion, a tent, or a covered but open-sided structure. The National Weather Service recommends the 30-30 guideline as a practical version of this: if the gap between flash and thunder is 30 seconds or less, head to shelter immediately, and once inside, wait at least 30 minutes after the last audible thunder before resuming outdoor activity, since a storm's charge regions and its ability to produce a bolt from the blue can persist after the rain itself has moved on.
Crouching in an open field, once commonly taught, is no longer recommended as an adequate substitute for real shelter; it reduces height but does nothing to protect against ground current or a direct strike. Because this pattern plays out differently for hikers on an exposed San Gabriel ridgeline than for beachgoers at the coast, the practical, location-specific version of this guidance, including where the nearest real shelter typically is, lives in lightning safety in Southern California.
Southern California's lightning is rare enough that many residents never learn the mechanism behind it, but the physics inside a monsoon cell over Woodland Hills or the San Bernardino Mountains is the same charge separation, stepped leader, and return stroke that powers a Plains supercell. Before heading into the mountains or desert during storm season, check current radar, lightning detection, and any active National Weather Service warnings on WeatherEscape's Los Angeles forecast pages, and treat audible thunder as your signal to move toward real shelter, not a rain shaft, as the marker of danger.