What causes lightning?

Lightning is, at its core, static electricity on a planetary scale. Inside a thunderstorm, updrafts lift small ice crystals upward while heavier graupel and hail fall toward the cloud base. As those particles collide, electrons transfer from the lighter ones to the heavier ones, leaving the top of the storm positively charged and the bottom negatively charged. NOAA’s SciJinks explains that once the charge separation gets strong enough, the air can no longer insulate the layers, and a discharge follows to restore balance.

That discharge is what you see as a flash. The bright channel is air heated to roughly 30,000 K — about five times hotter than the surface of the Sun — in a fraction of a second, according to NASA. The sudden expansion of that superheated air is the shock wave you hear as thunder. Cloud-to-cloud and intra-cloud strikes are the most common types. The dramatic cloud-to-ground strike is the one most people picture, and it happens when a faint, branching “stepped leader” of negative charge descends from the cloud in 50-yard jumps and meets an upward positive streamer rising from the ground, often from a tree, fence post, or person. When the two connect, the main return stroke surges back up the channel at roughly one-third the speed of light. That return stroke is the bright flash you actually see.

About 100 lightning strikes hit somewhere on Earth’s surface every second, NOAA estimates, which is roughly 8 million strikes a day and around 2 billion a year. The U.S. alone averages about 20 million cloud-to-ground strikes per year. Florida and the Gulf Coast are the country’s lightning capitals because warm, humid air there fuels frequent afternoon storms. Globally, the Lake Maracaibo region of Venezuela holds the record, with lightning on roughly 297 nights a year. Lightning can strike more than 10 miles away from the rain of a storm, which is why the National Weather Service warns that if you can hear thunder, you are already within striking range. That gap is sometimes called a “bolt from the blue.”

The safety rule of thumb from NOAA is simple: when thunder roars, go indoors, wait 30 minutes after the last rumble before going back out, and avoid open fields, isolated trees, and tall objects. Buildings are safest because the wiring, plumbing, and metal framing conduct current around you to ground. A hard-topped car is also safe, not because of the rubber tires but because the metal body routes the current around the occupants. If you cannot reach shelter, the lightning crouch with heels together is a last-resort backup, not a real defense. The CDC estimates about 20 to 25 people are killed by lightning in the U.S. each year, and hundreds more are injured, often with lasting nerve or hearing damage.

Lightning also shapes decisions far from the storm. Pilots route around cumulonimbus clouds because turbulence and static discharge can damage avionics. Launch teams scrub rocket flights when storms are within about 10 nautical miles of a pad because a strike to a vehicle full of fuel is catastrophic. Power utilities track strikes closely because direct hits and nearby induced currents can trip breakers, damage transformers, and start wildfires; a single dry-lightning strike is enough to ignite a tree in drought conditions, and lightning-ignited fires account for a meaningful share of western U.S. wildfire starts each year.

The Red Cross and FEMA repeat the same basic guidance: plan around the forecast, treat any rumble as a real risk, and keep moving until you are inside a building or a metal vehicle. If a storm is in the forecast, unplug sensitive electronics, avoid plumbing and wired devices during the strike, and stay off elevated terrain. Most lightning injuries happen not at the height of a storm but during the surprise second or third rumble after people assume the danger has passed. For more on the storms that produce it, see the tornado formation guide and the weather versus climate explainer.