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In a battle, a mage uses a spell that summons a bolt of lightning from the heavens to strike whatever the mage desires.

The lightning lands smack dab in the middle of a tightly packed formation of men, similar to a phalanx. What happens?

-How many people would die of this?

-How many would be injured?

In its current iteration, the main effect this magic has on the battle is not the direct damage, but the disorientation caused by the light and sound of a lightning strike, which leaves the formation vulnerable, which is another valuable follow-up question to ask: How long are the soldiers blinded by this lightning, assuming they looked right into it, and how long would it take for their hearing to recover?

Edit: For the purposes of this question, the men in the formation will be wearing chainmail hauberks over gambesons, and conical steel helmets. They will have kite shields, and arming swords. Their footwear will generally consist of simple shoes/boots of various materials, with the richer soldiers who can afford it wearing iron greaves (though these soldiers are in the minority)

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    $\begingroup$ This is very story based. That apart, effects of lightning strongly depend on several factors which you are not providing. This is simply unanswerable, if it doesn't get closed. $\endgroup$
    – L.Dutch
    Apr 14, 2020 at 12:11
  • $\begingroup$ This question, as it is have real life asnwers on the internet. Death and casualty similar to hand granade, blindness and deaftness similar to flash granade. $\endgroup$ Apr 14, 2020 at 12:16
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    $\begingroup$ Add in some hail and you could use en.m.wikipedia.org/wiki/Black_Monday_(1360) as your inspiration. $\endgroup$
    – DrMcCleod
    Apr 14, 2020 at 12:23
  • $\begingroup$ @L.Dutch-ReinstateMonica Sorry, I'm not at all knowledgable about this sort of thing. What are some of the factors that I should be providing here? $\endgroup$
    – KaiGuyMBK
    Apr 14, 2020 at 12:24
  • $\begingroup$ @DrMcCleod I knew why it was rinning in my head "we had something like that yesterday". 13 of April. $\endgroup$ Apr 14, 2020 at 12:31

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If your magician can summon a real lightning strike that you normally only find in a thunderstorm, then the effects are profound.

Range

Such a strike has millions of volts. It can kill anybody within a few meters of distance, and severely hurt and throw off their feet anybody within tens of meters distance. The closer you are to the center, the more severe the effects. There was a lightning during a soccer game. The lightning hit the ground not a man, but all players were thrown to ground and unable to play afterwards.

Medieval Armor

Chainmail might not protect so much as most of the victims will have the strike entering by one foot and exiting by the other while the charge disperses over the ground. Again, it's not the flashy effect, it's the electricity. The farer away you have your feet from each other, the heavier the consequences for you.

The Shock

After you have been tasered, you don't get up easily and you can have after effects for hours. Those poor warriors will have the same. So if it happens during battle, the opposing army will have an easy day. This will be the main effect for most of the victims - those who are up to some two or three dozen metres away.

The film effect

The one person in the center who is hit directly, if his helmet is made of steel, might have a molten cap afterwards. This is deadly for sure - steel is a bad conductur, so it heats a lot, melts, drips liquid steel. Induction Cookers work because steel gets incredible hot when forced to conduct. Worse, it also doesn't make a faraday cage if there is no connection between the steel parts, or rather, the connection is provided by the person inside the steel parts; accounting for cooked and burned-out parts of the body after the strike.

Formations

If people are in a dense formation like a shield wall or phalanx, they are even touching each other. This would make everything worse, for both they have no distance to the impact and the entire formation might even provide a better conductor than the wet ground. If your magician can direct a lightning strike into the middle of such a formation and he hits someone, the entire formation goes down, and deaths could go up into the hundreds. Even when they are not touching skin to skin, being so close to the Center of the strike will render the men severely hurt and at least unable to fight.

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  • $\begingroup$ I dunno, wouldn't full armour act as a Faraday cage? $\endgroup$
    – DrMcCleod
    Apr 14, 2020 at 12:40
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    $\begingroup$ It would. But Iron and steel are exceptionally bad conductors. So if it is not really massive, it would melt in the current, spraying your head with molten steel droplets. $\endgroup$
    – Anderas
    Apr 14, 2020 at 12:51
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    $\begingroup$ And, afterthought, if it is not fully electrically connected, the current takes detours through your body everywhere where a connection is missing. Fun? No. $\endgroup$
    – Anderas
    Apr 14, 2020 at 12:53
  • $\begingroup$ We do have many examples of people surviving being struck by lightning however. Quite a bit of luck is involved, it would seem. $\endgroup$
    – Ryan_L
    Apr 14, 2020 at 16:33
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    $\begingroup$ Lightning is extremely high-voltage, low-amperage. What happens to those it hits is very dependent on the route it takes through the body. $\endgroup$
    – jdunlop
    Apr 14, 2020 at 16:37
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In the modern day, chain mail is used to protect people from electrical currents. High voltage electrical workers may wear chain mail, and there's a video of a man being struck by artificial lightning while wearing chain mail. Even over long periods of time, the man does not seem to be injured, and the chain mail does not seem to be appreciably heated.

With plate armor, the conductive surfaces are larger, and thus tend to be more conductive. Furthermore, there are less jumps between armored portions that could cause problems.

Gaps between armor is something that is designed against, even in the relevant time period. Any gaps are holes that an enemy can wedge open with a sword. Furthermore, people wear leather between metal armor and their skin, which is an additional layer of insulation.

According to Wikipedia, steel structures can function as part of lightning rods.

Lightning will preferentially flow through the armor, and where it is unable to, might flow through the leather. It is extremely unlikely it will flow completely through the leather, into the body, and then back out through the leather.

Steel is a relatively good protection against lightning. For instance, it is recommended to shelter inside, for example, a car during a lightning storm. Lightning does not cause the car's body to melt, and it is even less likely to melt the less resistive (because of the higher cross-section area) steel armor.

If they are touching each other, the resistivity will further decrease, thus reducing the effect of lightning.

The thunder is around 120 decibels, equivalent to standing at the front of a rock concert for less than a second. Deafening is possible, but unlikely.

Lightning is brighter than the sun, but only for a few milliseconds. This may be enough to temporarily blind someone looking at it (and it is certain to blind someone should the lightning strike directly in the eye). However, there seems to be a dearth of information on the typical brightness of a natural lightning strike, so it is difficult to determine how long a person may be blinded. However, lack of eyesight is likely to occur for at least an eighth of a second (blink time), and it is more likely for temporary blindness to occur should the lightning strike at night – though it is essential to note that the troops will likely be more scattered at night.

In summary, lightning will likely be more effective as a psychological weapon or as a tool to frighten horses than as a direct damage-dealer. Injuries from electricity and secondary heating effects are likely to be minimal to none, injuries from sound are possible but short-lived, and injuries from light are likely, but hard to quantify.

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  • $\begingroup$ This was more or less my thought, but I wanted to point out that the OP mentions gambesons which could be anywhere from 6-20 layers of linen between the chainmail and skin. This would probably offer way better insulation even than leather. $\endgroup$
    – Nosajimiki
    Oct 1, 2020 at 19:16
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Clearly the lightning most likely kills the individual it strikes. It may kill others in the immediate area. There are a few cases in real life of multiple individuals being killed by one strike, and more with groups of animals. There's a video online of 23 cows lying against the same fence being killed by a strike.

A lot depends on the exact geometry of what happens, but if there is physical contact between people with reasonable conductance, you could see a lot of casualties. If they are insulated though, or not actually in contact (and they might not be in a phalanx with proper spacing) it may only go through one person.

Flashblindness would last a few minutes at most but probably would not be a major effect. In daytime the flash effect would probably be minor -- it's very hard to dazzle people on a sunny day without light sources powerful enough to cause damage. And this would only affect those looking the wrong way (helmets and visors may provide accidental protection).

'Close thunder' - the supersonic shockwave before it decays to acoustic speed and becomes thunders - would blow out the eardrums of anyone within a few metres producing deafness lasting several days at least.

The EM pulse form the lightning may also kill unlucky individuals nearby with cardiac arrest but this is rare.

Coincidentally enough, I just wrote this piece about the catastrophic effect of lighting on the English Army in 1360 -- https://www.theguardian.com/news/2020/apr/17/weatherwatch-deadly-storm-king-edward-english-french-troops

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  • $\begingroup$ Haha I didn't even think about a phalanx! Poor guys, if they are in a tight formation like that, the deaths might go in the hundreds. $\endgroup$
    – Anderas
    Apr 14, 2020 at 13:16
  • $\begingroup$ In ancient Greece, the shields that were used in a lot of phalanxes were meant to be used in formations, meaning that the shields interlocking/overlapping with each other was an important part of actually making the phalanx work. If all the shields were made of a conductive metal... I shudder to imagine how devastating that might be! $\endgroup$
    – KaiGuyMBK
    Apr 15, 2020 at 1:00
  • $\begingroup$ The Greek shield was known as a Hoplon (hence Hoplite) and was apparently usually made of wood and covered in bronze. The charge may have flowed through the metal to the next shield without harming the holder...or it may not... $\endgroup$ Apr 15, 2020 at 7:34
  • $\begingroup$ If the equipment is made of bronze and interlocked then it is the best protection you can think of. However, in the middle ages they already used the cheaper and lighter steel for equipment. $\endgroup$
    – Anderas
    Apr 15, 2020 at 10:09
  • $\begingroup$ theguardian.com/news/2020/apr/17/… $\endgroup$ Apr 19, 2020 at 19:14

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