Lightning, why do you need to wait for the flash to reach you?

Question

Why would a person about to be struck by lightning have to "wait" for the actual flash to reach them before getting hit?

As the electric field exists regardless of whether or not the charge in the atmosphere is moving and thus once a high enough potential is reached it should ionize the cells in your body regardless of whether or not the "flash" has reached you yet.

Was my initial assumption that you have to wait before the flash reaches you before you feel it wrong? As I have never actually seen someone get hit.

Another question: Why doesn't the flash appear at all points at the same time? The electric field exists everywhere so why doesn't it ionize the atoms at the same time (or at least the speed of light) when there is a high enough PD between the clouds and ground? Is it because the electric field is stronger at the top then the bottom so ionizes it first at the top?

Answer 1

You don't have to wait for the flash to get hit. However, the flash occurs so astonishingly close to getting hit that no biological system is going to be able to distinguish them.

The flash is an effect caused by the flow of electrons, heating the air. If there are no electrons flowing, there's nothing heating the air, but also no electrical "hit." Technically the electrons are flowing through you before the flash occurs, but for all intents and purposes, it's instant. Technically it takes the lightning a few millionths of a second to heat up the air to 50,000F, so if you have a system that is operating in the millions of samples per seconds range, you could detect the lag between the flow of electricity and the generation of heat. At any slower speed, the flash is an indication that the current has begun flowing very fast in that particular region of space.

As for why it doesn't appear at the same time, that's a fundamental reality of electricity. While we sometimes think about electrical circuits as just wires, voltage sources, and resistors, there are also inductive and capacitive elements. When working with really fast signals like a lightning flash, these play a part. It's possible to transfer a large amount of charge from one section of the air to another without a path to ground, because the capacitance of the air lets it store a charge. For a simplified handling of this, one could look at transmission lines, which are similar but don't have the complication of dealing with ionizing gases (or at least you really hope your transmission line doesn't have ionizing gases. It's typically a bad day for you when that happens!). These can be looked at to understand how electricity which "travels at the speed of light" can result in slower propagation of the effects that we are observing.

Of course, once a path to ground is "found," the resulting circuit lights up into a lightning flash extremely quickly. It's not instantaneous, but dang is it fast. Only the exploratory phase is slow, as we see in some of my favorite lightning videos. Here's one which shows a bunch of branching feelers, which precede the strike, and here's one which shows a nice leader, which actually forms the lightning strike.

Also, potentially of interest is the Marx generator, which is a high voltage generator built on similar concepts of ionizing gasses. This naturally has the advantage of being implemented with bulk components (resistors and capacitors) so that you can see what the structure looks like, and it also strikes twice in the same place (especially if that "same place" is Mehdi "Electroboom" Sadaghdar's fingers)!

Answer 2

The previous answer is not quite correct.

The real reason is that there doesn't exist a strong field until the lightning flash gets there.

The field in the clouds is strong and the field on the ground are weak. A pathfinding group of charges called a "leader" branches its way towards areas of lower/higher potential (depending on the charge - incidentally leaders are normally negative).

They go into areas that may be high field or low field, but what creates the giant strike is connection to a conductor (i.e. the ground). This is why intracloud lightning has less dramatic flashes (not called strikes unless it hits the ground).

These leaders obviously bring regions of locally very high field with them, so the high field does not exist until this part of the lightning reaches you.