What happens inside a single flash
A lightning flash is over in a few tenths of a second, too fast to see as anything but a flicker. Slowed down, it is a sequence: a faint spark that feels its way down, a spark that rises to meet it, and a wave of light that travels from the ground up.
Before
Inside a thundercloud, where the air rises fast and the temperature is between −15 and −25 °C, small ice crystals collide with soft hail. The crystals come away with a positive charge and are carried to the top of the cloud. The hail keeps a negative charge and stays in the middle and lower part.
The ground under the storm takes on a positive charge in answer. The air in between is an insulator. It holds until the difference becomes too large.
0 to 9 thousandths of a second
A thin channel of negative charge comes out of the base of the cloud. It is called a stepped leader, and it is far too faint to see with the eye. On this clock, zero is the moment it sets off.
It branches as it goes, and each branch is feeling for a way down.
21 thousandths of a second
The leader does not know where the ground is. Its tip senses only the charge within about 50 metres, so it jumps about that far, pauses, and jumps again in whichever direction looks best from there. That is why lightning is jagged.
Counting the pauses, it comes down at about 320,000 kilometres an hour. This one has three and a half kilometres of zigzag to cover, and that takes it 39 thousandths of a second.
Just before contact
When the leader is within about a hundred metres of the ground, the charge beneath it is strong enough to pull sparks upward from whatever stands tallest: a tree, a pole, a roof.
Several things send one up. Only one will be met. Here it is the tree.
39 thousandths of a second
The leader and the rising spark meet a few tens of metres above the tree. For the first time there is an unbroken path of charged air from the cloud to the ground.
Everything up to here was preparation, and none of it was bright enough to see.
19 millionths of a second later
The negative charge stored along the channel drains into the ground, starting at the bottom. The part of the channel that is draining is the part that lights up, so the brightness races upward, from the tree to the cloud.
It climbs at about 100,000 kilometres a second, a third of the speed of light, and reaches the cloud in about 35 millionths of a second. This is the flash you see. To the eye it seems to come down.
56 millionths of a second later
A typical first stroke carries a current of about 30,000 amperes. Within a few millionths of a second the air in the channel reaches about 28,000 °C, five times the temperature of the surface of the Sun.
The branches light up as well, but only this once.
59 thousandths of a second
Air heated that fast cannot expand in time, so for an instant it is under enormous pressure. It bursts outward as a shock wave. Within about ten metres the shock has weakened into an ordinary sound wave. That sound is thunder. Twenty thousandths of a second after the flash, it is about seven metres from the channel.
Every metre of the channel makes its own. The long rumble you hear afterwards is the sound arriving from farther and farther up.
79 thousandths of a second
The light fades within a few hundredths of a second. The channel stays hot enough to conduct for a moment longer, and there is usually more charge left in the cloud.
98 thousandths of a second
About 60 thousandths of a second after the first stroke, new charge starts down the old channel. This time there are no steps and no branches. The path is already made, and the spark, called a dart leader, runs down it far faster than the first one did.
When it reaches the ground, a second wave of light climbs the channel.
220 thousandths of a second
A typical flash has three to five strokes down the same channel, about 60 thousandths of a second apart. This one has four. The eye cannot separate them, but it can tell that the light is unsteady. That is the flicker.
About four flashes in five have more than one stroke. The most ever counted in a single flash is 26.
260 thousandths of a second
From the first step of the leader to the last stroke, this flash took about 220 thousandths of a second. Most flashes are over within a few tenths of a second.
500 thousandths of a second
The clock has reached half a second. The light of the flash crossed the whole landscape at once. The thunder, moving at about a third of a kilometre a second, has covered some 160 metres.
Someone a kilometre away has seen everything and heard nothing. For them the thunder is still two and a half seconds off: three seconds for every kilometre.
That was one flash. Counted from orbit, lightning flashes about 44 times every second somewhere on Earth, so in the half second you have just watched there were about 22.
Most of them never reach the ground: there are five to ten flashes inside the clouds for every one that strikes. Each light here is one flash, placed where satellites counted lightning over two decades.
Nearly four flashes in five happen between 30 degrees north and 30 degrees south, and lightning is about ten times as frequent over land as over the sea.
The busiest place on Earth is Lake Maracaibo in Venezuela, with 233 flashes a year over every square kilometre. Second, with 205, is a spot near Kabare in the east of the Democratic Republic of the Congo. Of the 500 places with the most lightning, 283 are in Africa.
Added up, that is nearly 1.4 billion flashes every year.
Each one is over in less than half a second.
The flash in this scene is not a recording. It is one typical flash of the commonest kind, negative from cloud to ground, drawn from the numbers below: steps of 50 metres, a leader moving at about 320,000 kilometres an hour, four strokes 60 thousandths of a second apart. Its shape comes from a random draw, and its path of three and a half kilometres gives the 39 thousandths of a second on the clock. Real flashes vary widely around these figures. The tree, the pole and the barn are to scale; the bar in the corner shows the scale. The flashes on the globe are placed at random in proportion to the satellite record, 44 a second, and are not a record of particular flashes.