How dangerous would lightning REALLY be in a (especially medieval) combat situation?

Question

Lightning is one of the token magical attacks that are there in every fantasy book; something like "[Mage] called down bolts of lightning on the pike-formation", is less common than only something like "He threw fireballs at the charging cavalry."

Calling down lightning upon your foes seems to get less and less devastating as a series goes on. So I want a realistic estimate on how dangerous lightning can really be.

Details: Assume that lightning can be called down on any place within throwing, or arrow, or even trebuchet range (So if you can throw a rock there, you can call down lightning there). I don't know whether how frequently it can be called down is an important enough factor or not, so as a random baseline assume it can be done once every 30 seconds, and this can be done for let's say one hour at the very maximum.

EDIT: There is a metallic rod that must be thrown to call down lightning. Upon being summoned, the lightning directly hits that metallic rod

So how dangerous would it be? Would it just contribute to more attrition? Or would it require a complete restructuring of strategy for the army that's facing an army which can call down lightning?

Note that I do not consider this to be a clone of this, as it asks in a very specific context while I ask in a very general one.

Answer 1

So devastating that it would change warfare in pretty much the same way modern artillery did: Nobody would field armies in closed formation anymore, but instead in more or less dispersed, smaller and mobile units.

Your link already describes the actual effect of the lightning strike. Your edit with regard to the rod only changes the nature of application:

A 4th century roman ballista already had a reach of over a kilometer, which means that your specification would essentially provide a medieval ballista with the raw firepower of modern artillery and an even more profound psychological effect since a lightning strike is much more terrifying than an explosion. Being able to fire it every 30 seconds is enough to make fielding an army in regular closed formation suicidal, even if the enemy has only one ballista-mage-combo, especially considering that soldiers are people and not videogame-units which do not care about swathes of their comrades being annihilated by thunderous lightning every 30 seconds.

Obviously there are other applications made possible by your specification: You could for example use metallic rods basically as mines, making passage through an area prepared with rods suicidal.

What exactly the nature of warfare would evolve into depends on other variables: The amount of mages, the amount of access to such mages across several factions, the quality of the available siege engines etc...

Answer 2

Lighting that strikes a metal rod wouldn't be all too dangerous because since this rod is planted in the earth, any lighting that reaches it would be immediately grounded. Through the same principle that lighting rods on houses work, soldiers or other humans standing near the rod when lighting struck would probably be fine--although they might suffer temporary deafness/blindness.

Lighting is most dangerous to people when humans are part of the "path of least resistance" (outside of secondary effects like exploding shrapnel from struck trees or superheated material). For example, someone standing in a flat field during a thunderstorm is more likely to get struck by lighting because human flesh is slightly more conductive than air. That means instead of traveling a couple more feet through air to reach the ground, the lighting bolt travels through the person, likely killing them.

Answer 3

Impacts

Sound

Per here, lightning generates about 1470 psi (100 atmopsheres) of overpressure.

Deafening:

Per CDC, 45 psi is enough to rupture ear drums, causing permanent hearing loss.

Sound will attenuate a little better than radius-squared, but just using that for a quick model. Assuming $r_0$ is 1 meter, ${1470 \over 45} \approx 32 {r_0}^2 \rightarrow r_0 = 5.7$ meters radius. (37 feet diameter)

Knockdown:

The typical human cross-section is 3,168 square inches. Typical human body weight is 100 pounds-mass. That means typical human strength is $poundmass \times gravity = 100 \times 32.2 = 3,220$ pounds.

Converting pressure to force $ = pressure \times area$. Just glancing at the human cross-section and weight in pounds-force, about 1 psi looks like enough force to knock a person down.

Using same radius-squared attenuation, a lightning stroke will very likely knock people down out to $\sqrt{1470} = 38$ meters radius. (250 feet diameter)

How Effective is That in War?

Let's take a look at the Battle of Hastings for some sense of scale. A well-placed lightning strike among the enemy, with a blast diameter of 38 meters (about 38 yards) will very likely flatten an entire unit (about 137 men).

Even more importantly, flattening a unit every 30 seconds opens several holes in the shield-and-spear walls protecting the back lines from cavalry charges.

Or, conversely, flattening a cavalry charge before it can do any harm.

This may not kill anyone, but it creates precious seconds of neutralizing a force -- giving allies time to close, bypass, or engage stunned enemy soldiers without resistance.

How much control of the territory does this allow?

Morale In a Battle of Hastings-like situation, a lightning-thrower (pending easy getting of the required metal rods to the targets) could knock down all of the English forces, at least once, over a span of 25 minutes.

The real battle was an all day affair. In a whole 16-hour day or beating down the enemy every 30 seconds, a single lightning-thrower could smack down the entire English army nearly 40 times.

Neutralizing the benefit of fighting from high ground

A typical soldier runs a 4 minute mile (440 yards per minute). By providing cover, a lightning-thrower can either keep enemy archers down (literally) while friendly forces advance on the high ground.

At the moment of contact between your force and the enemy, you can knock a hole in the shield wall, allowing friendly forces to walk past the no-longer interlocked shields or set spears of defenders.

Allowing maneuver

Flipping the script as the surrounded (or nearly-surrounded) English forces, a lightning-thrower could effectively halt a small force attempting to flank.

If retreat is required, cavalry or fast infantry moving to cut-off your retreat can be stopped.

Answer 4

Consider that Wood was one of the strongest and most easily available materials medieval kingdoms had to work with, not to mention the easiest to shape into whatever you wanted. From giant drawbridges to artillery and barricades, wood was used everywhere at that time. The one big disadvantage is that it is highly flammable. So if your magician(s) threw their rod like a spear at the gate of the castle and it burst into flames, that would certainly send a statement. Soldiers watching their hope of defense crumble into ash. Terrifying. This sorcerer would be the most powerful piece on the battlefield with this ability but it also creates a weird dynamic.

This Mage would have to be close enough to the target to hit it without a chance for missing. If he is close enough to hit (artillery, the main gate of a castle, barricades) than the enemy soldiers are not far behind. So the strategies that unfold are these:

(A) Extreme Stealth

For the mage, to get close to the target and avoid the enemy soldiers, he would need to be very stealthy. A transport spell or an illusion would work in this case.

(B) Siege Warfare

With the introduction of this element, siege warfare would give the advantage to the one with the castle. the attackers would be restricted to hiding in the woods to avoid being hit by the lightning. if the attackers had the mage, he wouldn't be able to get close enough to the gate to blast it, because of the nature of the open field.

(C) Longer Construction Time

To avoid the impediment of wood being a target, construction would take longer if the army wanted to make things out of stone. Going to a quarry, having it carved, moving it back.

(D) Larger armies would cover a larger range

As AuronTLG mentioned, the risk of splash damage would be enough to deter armies from coming together in one group. But the lightning (unless severely altered by the mage) would only strike one person at a time. It would give shock to anyone within 4-6 feet. Being spread out more, these platoons could cover a larger distance, whether on patrol/outposts or defending a point of interest such as a capital city.

So, in the end, it goes back to fire.

Answer 5

How DANGEROUS? Not that dangerous apparently: https://www.google.com/amp/s/www.businessinsider.com/what-to-expect-when-you-survive-lightning-2016-4%3famp

90% survive. "Survive" does not mean they'll be doing anything on the battlefield that day. Its unlikely that you'll be struck directly since the rod is what attracts the ligtning and it gets grounded quickly, but there are other ways to get struck.

https://www.weather.gov/safety/lightning-struck

The people close to the metal rod, within a foot or so, can get hit by side-flashes. That means the people in immediate vicinity of the rod are in danger, unfortunately it is likely that any rod striking nearby will already have either killed the people or caused them to move away.

Upon a strike electricity will travel through the ground, and the most common way for people to get "struck". Electricity will go up one foot, course through the body and exit the other foot. I couldnt quickly find an actual range for this, but at the bottom of this page you can see that it can be a wide range indeed:

https://www.weather.gov/safety/lightning-science-ground-currents

There do exist multiple-strike incidents, where a single lightningbolt hits several people or livestock. There's even a picture in the last link of several livestock that were hit and killed while hiding underneath a tree. Livestock and thus Cavalry is more likely to be killed as the lightning needs to travel longer to reach the other end of the body. The AOE isnt going to be zapping everyone in that area when you look at where it travelled though.

Other injuries from people not directly hit are often temporary deafness and ruptured eardrums

So best use: anti-cavalry artillery, then just a low-lethality artillery weapon against ground troops. Low-lethality if you are smart that is. A single strike incapacitates a lot of soldiers, and letting them live shows mercy while burdening the enemy supply train with wounded. If you are nasty however you can strike a second time, likely killing anyone lying down from the first strike.

Compared to a mortar or canister shot a lightning bolt might be underperforming. But the psychological and aural effects (everyone nearby deafened) are definite bonuses. Its ability for repeat strikes and prior seeding are its best advantages. Its an artillery piece, so you'll be able to copy-paste the tactics for and against artillery pieces. Combined with mine warfare.

Answer 6

If you can call a lightning strike to wherever you want as long as you have seen the rock... well, calling a lightning strike on a rock 10 meters away is for you as if you have been tasered through the legs.

Horses go down. Then they panic and run away.

Men go down. Tasers can have after-effects like cramps and spasms for hours after the strike, depending on the voltage.

Imagine that happening to your army. Imagine reacting by changing the formations so that it doesn't happen to hundreds or thousands but only to a few or a dozen. Basically you get Napoleon's war tactics. That's so different from Medieval that yes, I would call it devastating for your setting.

Except, except if you have just two or three magicians per state. Artillery is nice, but if you have just two or three cannons in the state, it doesn't change the war.

Answer 7

I don't know that this has been tested scientifically, but a suit of metal plate armor may work as a Faraday cage, insulating knights from lightning. A Faraday cage is any metal encasing that can redirect lightning around something you want to protect. This is actually why it is safe to be in a car during a thunderstorm. It is a common misconception that the rubber tires protect you. The lightning simply hits the roof, goes around the sides, and arcs to the ground.

Like other answers have mentioned, the real effect may be largely psychological. Especially if the defending army is largely uneducated in matters of electrical engineering, which was not understood at all in our civilization until the mid 1800's.

Answer 8

Let me tell you about two very real circumstances

My wife and I once lived outside of Austin, Texas. The lightening storms there have breath-taking beauty. But, there is a downside.

1. I exercised each night after work by walking a loop of about 5 miles. We lived out in the boonies where it was very rural and very unpopulated. The fog would regularly be so dense that I couldn't see much more than 50 feet in front of me. I loved it — except once. I never saw the lightning flash save as an after-image in my mind. My best estimate was that it was about one-quarter of a mile away (1,300 feet or 396 meters). The bang deafened me for a solid 10-15 minutes. The concussion from the strike knocked me flat on my back and seriously winded me.

2. A second experience had to do with our house. We think the strike was within a tenth of a mile (530 feet or 160 meters). It sounded like it would blow out the windows, but the only damage was phone and electrical wires in the walls being burned to a crisp (I had to replace them), a centronix-style parallel cable (that dates me) that was burned up, and the parallel port on the printer that was fried. That cable was the one you need to worry about, it was only six feet long.

A disclaimer...

We're treating "lightning" as if all lightning is alike. It isn't. Some strikes are itty-bitty and would serve only to scare the snot out of someone. Others are life-altering blasts. You don't mention how much control your magic users have over the strike. I'm assuming they're at least what I've personally experienced.

My conclusion?

If you're trying to inject reality into your scenario, you need to use lightning in the same way a modern military uses artillery: as a supporting action at a considerable distance. Unless you change the rules of your magic, lightning is nondiscriminatory. It'll affect friend and foe alike. Used in the middle of a battlefield (as in, at the range a human could throw a javelin) it would bring the entire battle to a halt.

Most RPG-style uses of lightning treat it more like a directed energy weapon. It's generally focused on a single target and delivers damage based on the strength of the spell, from a mild tickle up to a small pile of carbon for remains. The more "real life" you make it, the harder it is to justify using it. A thrown rod is a nice idea... if you're on the top of a hill. Electricity flows from the highest charge point (somewhere up in the sky, that's the magic part, right?) to the lowest charge point. Sometimes that's something high. Sometimes not. But a rod isn't enough to make a difference electrically. If there's a nearby body of water or it was raining nearby before the battle, that's where the lightning will go — rod or not.

If your rules of magic make the rod always the lowest charge point then there's really no point to asking this question. Make the lightning do what you want with the damage that you desire.