Topics Earth, weather and climate

Why does lightning happen?

Lightning happens because a thunderstorm cloud separates electric charge, and eventually the air can no longer keep the two sides apart. Positive charge gathers in one region, negative in another, and when the difference grows large enough, a channel of air breaks down and current surges through it. That surge is the flash.

What makes it interesting is how much is still unsettled. Scientists agree that ice is central: small ice crystals, soft hail called graupel, and supercooled water droplets collide inside strong updrafts, and charge moves between them. Exactly how that transfer works, and which processes matter most, is still debated. Even how a spark gets started in the first place has open questions.

An episode would walk through that story from the ground up: what charge is, how a storm cloud becomes a battery, what a bolt does to the air around it, and why thunder follows. bre's hosts are AI, so they can be wrong, and you can press Talk to ask a question mid-episode, like why you see the flash before you hear the boom.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. A giant static shockStart with ordinary static electricity, the spark from a doorknob, and why a thundercloud is the same idea at enormous scale.
  2. Ice, hail and updraftsInside a storm, ice crystals, soft hail and supercooled droplets collide while air rushes up and down. Charge is transferred, and the lighter and heavier particles tend to end up in different parts of the cloud.
  3. What we still argue aboutScientists still debate the details of charge transfer and how a spark first starts. This chapter separates what is well established from what is open.
  4. How a bolt actually formsA faint channel steps toward the ground, a connection is made, and a bright return stroke races back up. Most of what your eye sees is that second part.
  5. Heat, light and thunderThe channel heats the air to several times the temperature of the sun's surface in an instant. The air expands violently, and that shock wave is thunder.
  6. Where lightning strikes and common mythsWhy tall, pointed and isolated objects are often struck, why lightning can jump between clouds, and which popular sayings about it do not hold up.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Cal. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Let's start with the question under the question. Lightning is a spark. But a spark between what and what? There's no wire up there.

  2. CalAI host

    Short version: it's a giant static shock, like a balloon on hair, except the cloud does it with ice. The air is a decent insulator, so charge piles up until the air finally gives way.

  3. breAI host

    And the ice part is where it gets fun. Inside a storm you've got ice crystals, soft hail called graupel, and supercooled droplets, all smacking into each other.

  4. CalAI host

    Hold on, how does that actually work? Why would bumping into each other sort charge into top and bottom?

  5. breAI host

    Roughly: when particles collide, charge transfers between them. Lighter ice crystals tend to get carried up, while heavier soft hail falls or hangs lower. Scientists still argue over the details, and I'd rather say that than fake certainty.

  6. CalAI host

    So the cloud ends up a bit like a battery. Positive on one end, negative on the other, and a lot of air in between.

  7. breAI host

    Right, and air only holds out for so long. Then it breaks down, and current finds a path.

  8. CalAI host

    And that path heats the air hard enough to make a bang you can hear miles away. I want to get to that part.

Questions people also ask

Is lightning hotter than the sun's surface?
Yes, by a wide margin, though only in a thin channel and only for an instant. Estimates put a lightning channel near 30,000 kelvins, while the sun's surface is about 5,800 kelvins. These are rough figures. That sudden heating makes the flash and the shock wave we hear as thunder.
What causes thunder?
Thunder comes from the lightning channel heating the air extremely fast. The air expands explosively, sending out a pressure wave that we hear as a boom or rumble. Light travels much faster than sound, so you see the flash first and hear thunder afterward.
Can lightning happen without rain?
Yes. Lightning can occur in volcanic ash clouds, and storms can produce strikes some distance from their rain. A storm's charged regions can extend well beyond where rain is falling, so a strike is not limited to the spot under heavy rain.
Do scientists fully understand how lightning starts?
No. The broad picture is well established: storm clouds separate charge and air breaks down. But how electric fields inside clouds become strong enough to start a spark is still being studied, and researchers debate several ideas.

Related topics

More: all 300 topics, earth, weather and climate, or the longer reads on /learn.

bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.