Point at a thunderstorm and describe what you see: a bolt drops out of the cloud and hits the ground. That description is backwards. In a cloud-to-ground flash, the bright stroke — the one you see, the one in every photograph — starts at the ground and travels up.
The downward part is real; you just can't watch it. The sequence runs in an order the unaided eye can't follow, which is why nearly everyone narrates it wrong. The physics is laid out in the field's standard reference, Rakov and Uman's "Lightning: Physics and Effects" (Cambridge University Press, 2003), and NOAA's public lightning-science pages walk through the same sequence.
The dim part goes down
A flash begins with a stepped leader: a faint channel that works its way down from the cloud in short, branching steps, largely invisible to the unaided eye. This is the one piece of the story that matches the folk version — something really does descend from the sky — and it's the piece nobody sees.
As the leader approaches the surface, the ground stops being passive. An upward connecting leader rises to meet the descending one. Where they join, the main event begins.
The bright part goes up
At that junction the return stroke fires, starting at the ground and propagating up the established channel toward the cloud. The return stroke carries most of the current, and it produces almost all of the light a person sees. The flash you've been watching your whole life is the ground's reply, not the sky's attack.
It's also absurdly fast: roughly one third the speed of light, about 100 million meters per second. That figure wasn't worked out on a chalkboard — it was measured directly, with photoelectric instruments and streak cameras, during the Thunderstorm Research International Program. Idone and Orville published the velocities in the Journal of Geophysical Research in 1982.
One correction, because the fun version of this fact usually overshoots: the whole discharge does not travel upward. The leader genuinely goes down; it's the visible return stroke that goes up. "Lightning strikes upward" is exactly as wrong as "lightning strikes downward." The flash is a round trip, and your eye only gets a ticket for the second leg.
Very tall towers strike first
Tall structures push this further. For very tall structures, many or most flashes are upward lightning: the discharge begins with an upward leader that starts from the structure's own top and grows toward the cloud. The tower isn't struck from above at all — it initiates its own flash. Researchers have even photographed multiple upward leaders rising simultaneously from tall structures.
Not every tall-structure flash starts this way. But the image is worth keeping: a broadcast mast on a stormy night isn't always a victim. Sometimes it reaches up and starts the conversation.
The bias behind this page: I build Curio, an iOS app that serves five short curiosity stories a day, and the return stroke is exactly the kind of story that ends up in it. If a daily handful of these sounds good, it's Curio on the App Store. If not, you still get to narrate the next storm in the right direction.