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From Giant in the Playground Forum:

Well a Great Wyrm Red breathes hot enough to theoretically liquify[melt] 4 inches of solid iron across an 80 foot or so line. So an 80 foot wall of iron 4 inches thick could be liquified[melted] by a single attack by a Great Wyrm Red.

the end result is that the air breathed out by the dragon [onto the iron reaches] 1520 degrees Kelvin, 1246 degrees Celsius, or 2276 degrees Fahrenheit.

Can an umbrella be used to deflect a breath from the dragon, and also keep its average human user alive?

If so, what materials would it have to be made of? It must still be light enough for an average person to still raise.

Assume the umbrella is a small personal umbrella, with at most 60cm radius, with a surface area of roughly 2.25-2.5 meters squared.

This happens in a low-fantasy world, with some magic, but still has similar physics.

Edit: to better support yes answers, seeing as the d&d dragon’s breath is far more deadly than expected, you can assume the umbrella can activate magic, but only to turn the umbrella into a sphere/semi-sphere around the user.

Edit2: Due to an error made in calculations by the source on the temperature of the breath, we now have some inconsistencies for temperatures mentioned in some answers. We will assume 1246 degrees Celsius for this question (enough to melt ductile and cast iron), as all of the answers mentioning higher temperatures of 1500 degrees scale linearly, so this does not invalidate any of the premises.

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    $\begingroup$ To clarify, do you want the dragon's breath to be 1300-1500 degrees celsius or do you want it to be able to melt through 4" of iron? $\endgroup$
    – Nosajimiki
    Commented Sep 21, 2020 at 14:06
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    $\begingroup$ Edits cannot invalidate existing answers $\endgroup$
    – L.Dutch
    Commented Sep 21, 2020 at 14:17
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    $\begingroup$ It's worth noting that the original question never really gives a timescale. It is implied that the dragon can melt the wall in a single breath, but as many answers have pointed out doing that makes the dragons breath more powerful than a nuclear bomb. I think it's more likely that whoever initially stated that "a dragons breath can melt an iron wall" completely underestimated just how resistant iron is to melting. After all, a flame is easily hot enough to melt ice, but you can hold an icecube in a flame for a couple of seconds and it won't be completely melted. $\endgroup$
    – Turksarama
    Commented Sep 21, 2020 at 23:57
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    $\begingroup$ The first edit makes the question without purpose. The only answer possible to "Can my magic shield protect from dragon breath ?" is "If your want". $\endgroup$
    – Jemox
    Commented Sep 22, 2020 at 10:09
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    $\begingroup$ Perhaps worth noting that the initial assumption given in that GITP forum post, that a great wyrm's fire breath will melt iron was, as explained further down, based on the hit point system used in D&D and how much damage the dragon's breath does vs how much a block of iron can withstand. But this hit point system is notoriously abstract, so you may be drawing unfair conclusions for your proposed low fantasy setting. For example, a level 20 fighter with a CON of 18 could stand in that same fire butt naked and not die as a result. $\endgroup$
    – Steve-O
    Commented Sep 22, 2020 at 15:21

9 Answers 9

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Yes, but...

You can probably construct an umbrella which deflects and protects that which is immediately behind it from immense heat, however, without a thermal protection system like a thermal suit, anyone in the vicinity of such heat would probably roast simply by proximity.

For example, there's NASA's ADEPT program where they developed an unfolding heat shield:

ADEPT

And here it is in a hot-air wind tunnel:

In hot air

Adept's TPS system is mostly constructed out of woven carbon fiber layers with a rigid central shell.

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  • $\begingroup$ Interesting references and pictures, thanks for the answer! $\endgroup$
    – Enthu5ed
    Commented Sep 21, 2020 at 15:48
  • $\begingroup$ Probably the most accurate answer up to now. Goes into what happens to both the umbrella and the person. $\endgroup$
    – Enthu5ed
    Commented Sep 27, 2020 at 5:34
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Umbrella vs a kiloton nuclear warhead.

Even ones made of unobtainium and blessed by all the gods will be vaporised in an instant.

Lets take a look at that wall melting, and try to work out the power in the dragons breath

So, melting point of iron is 1538 degrees C

Lets assume Earth global average temperature at the spot the attack happened (15 degrees C).

Irons specific heat capacity is 444 J/kgC. Irons density is 7874kg/m^3

Your wall that was melted was 24.384m ('80ft') * 10cm ('4 inches') * ...? Assuming the wall is a standard medievil fortification the wall height was 2.5m minimum. (But it could be up to 6m). We're going 2.5m here just to give the umbrella the best chance.

So we have $24.384 \cdot 0.1 \cdot 2.5 = 6.1 m^3$ of iron.

The wall weighs 48031.4kg.

So to heat that wall by one degree, you need 21325941.6 joules of energy.

... But we need it heat it up by 1523 degrees.

So the energy contained in the breath is: 32479409056.8 joules!

That number means nothing, so I suggest refering to Wikipedia's helpful "Orders of magnitude" page, which shows us that:

  • This is just under a kiloton nuke.
  • This is bit over a Massive Ordnance Air Blast Bomb, the largest non nuclear bomb ever created.
  • This is a bit over the impact of an A380 at cruising speed - 560 tonnes at 511 knots

Also remember this is just the energy that hit the wall - the breath also heated up the air, the ground, the dragon, and the poor people behind it.

(Edit: Corrected a maths error, I had originally calculated this as over the strength of a Hiroshima bomb, it's actually about an order of magnitude below this.).

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    $\begingroup$ This is a very conservative calculation. ~1kt is just the energy that is absorbed by the wall, but since this is an uncontained heat source most of the heat from the dragon's breath will be wasted meaning you are probably looking at a multi-kiloton thermal output. $\endgroup$
    – Nosajimiki
    Commented Sep 21, 2020 at 14:04
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    $\begingroup$ What about a fridge? $\endgroup$
    – Aron
    Commented Sep 22, 2020 at 1:40
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    $\begingroup$ Also, whilst your energy calculation is correct, you're comparing it to the energy released in a blast. That's not the case for a breath weapon. We could probably assume the breath lasts 6 seconds. The energy in the melted iron will only be released over half an hour or so as it cools. There's the question of how the energy is transferred into the wall; given the mechanic of breath weapons is that they don't affect squares they don't hit, we should probably assume some kind of magical mechanism, rather than it being physics (radiation / conduction) of our world. $\endgroup$
    – Dan W
    Commented Sep 22, 2020 at 14:07
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    $\begingroup$ You forgot about the latent heat of fusion (Fe: 13.8kJ/mol). If you want to melt the iron after it's been heated to the melting point, you'll need to add another 1.19x10^10 J of energy to your calculations, which puts it almost exactly at the MOAB power level. $\endgroup$ Commented Sep 22, 2020 at 19:41
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    $\begingroup$ Your kiloton conversion is wrong 32479409056J = 0.007762kT. I think you mixed up joules and kilojoules maybe? $\endgroup$
    – Nosajimiki
    Commented Sep 23, 2020 at 14:45
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No you can't

The breath is simply too hot. If it hits the umbrella and it holds, you'll still be exposed to incredible heat from all sides where the breath goes past you. It is able to liquify 4 inches of metal on an 80 feet surface! Assuming steel, that is 1510C or 2750F! But it's much, much hotter, as it can liquify 4 inches deep. In. A. Breath. The flash heating will probably cook you alive. Where it hits the ground or surroundings it'll likely liquify the material, likely giving off lots of toxic funes that'll rob you from breath or poison you, likely both. This will also add to the heat flying around you.

The umbrella might not protect you fully as well, making you in even more dire straights.

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  • $\begingroup$ Interesting perspective, so you believe regardless of umbrella materials, the user will likely be cooked alive by the raw heat from the surroundings? $\endgroup$
    – Enthu5ed
    Commented Sep 21, 2020 at 12:51
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    $\begingroup$ @Enthus3d yes definitely. en.m.wikipedia.org/wiki/Laser_cutting#Power_consumption. To cut through 6.4mm (1/4 inch) steel, you need about 25Kw power. That is for cutting, and takes more than a breaths time. Liquifying a whole block in a breaths time in insanely, over the top powerful. You cannot battle that with a mere umbrella, whatever material it's made from. $\endgroup$
    – Trioxidane
    Commented Sep 21, 2020 at 13:01
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    $\begingroup$ @Enthus3d Consider this: a plasma cutter used to cut steel creates an arc that is ~20,000°C and they can only go through a few millimeters of steel at best. For a dragon's breath to cover a large conical area that means you are heating a large portion of the room you are in to well over 20,000°C. So, the room you are in will have to be heated to many times as hot as the surface of the sun to get that effect $\endgroup$
    – Nosajimiki
    Commented Sep 21, 2020 at 13:21
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    $\begingroup$ @Enthus3d At those temperatures, I can't imagine a situation where any space is a good place to fight a dragon. Even getting close enough to that flame breath to shoot an arrow at the dragon would mean looking straight into something bright enough to burn your retinas and probably cause at least surface level burns... when Trioxidane says "much, much hotter" he's not exaggerating. $\endgroup$
    – Nosajimiki
    Commented Sep 21, 2020 at 13:57
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    $\begingroup$ @PeterCordes Hmm, that makes sense. I suppose we might be able to explain away the incongruent parts by attributing it to the magic of the dragon $\endgroup$
    – Enthu5ed
    Commented Sep 23, 2020 at 5:46
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Frame Challenge: The premise of the original thread is wrong

The original theory is as follows:

A pure iron wall has 30 HP per inch of thickness and Hardness 10. A Great Wyrm Red has a breath weapon that does 24d10 Fire Damage in a 70 ft. cone. Maximum damage is 240, which is halved against objects for 120, which is then dropped by 10 because of Hardness. So it's technically a little less than 4 inches thick that can be liquified. A 70 ft. cone has a maximum straight horizontal line in it of something like 80 ft., and everything in the AOE takes full damage.

Which someone disputed as

4 inch thick Iron has 120HP and hardness 10, and a Great wyrm breath weapon does 24d10 damage in a 70' cone which averages at 132 damage.

However fire would do half damage to objects (I personally wouldn't say that iron walls are particularly vulnerable to fire) which after hardness would be 56 damage so a basic MM great wyrm red dragon would probably heat the metal thoroughly but not completely destroy it.

I would go with the second method with averaged damage (as commonly you don't roll and use the average in a lot of DnD situations for sake of speed) in which case it would heat the metal but not destroy it meaning the breath is much less than expected heat wise.

~600 C is a lot less destructive than the initial 1243 C. However what is commonly stated in the RPG stack is DND is not a physics simulator thus, we can also assume that these measurements were not really thought out in terms of physics in general.

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  • $\begingroup$ Good answer, I did want to mention the fact that the iron melting is related to the results of a 24d10 dice, and the example of the wall of iron is at the maximum potential, but it's difficult to convert between a full and half value directly, so I thought we could use the example directly. Perhaps should have made this part of the premise. $\endgroup$
    – Enthu5ed
    Commented Sep 21, 2020 at 18:01
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Make the umbrella with something akin to the Shuttle thermal protection system

The black HRSI tiles provided protection against temperatures up to 1,260 °C. [...] These tiles were such poor heat conductors that one could hold one by the edges while it was still red hot. [...] The HRSI tile was composed of high purity silica fibers. Ninety percent of the volume of the tile was empty space, giving it a very low density (140 kg/m3)

For comparison water has a density of 1000 kg/m3.

Just be sure that the deflected breathe doesn't bounce back on some other nearby surface.

I can't find information on their price per square meter, but from the answer to this question I infer it is not prohibitively high, if jewelers can use it as soldering base.

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  • $\begingroup$ Great answer! Do you have a rough idea of how much it would cost for 2.5 meters squared of that material? $\endgroup$
    – Enthu5ed
    Commented Sep 21, 2020 at 12:44
  • $\begingroup$ Found it. Silica Fiber cost is very reasonable, at $1-10 per kg although high purity fibers might be more expensive. Most of the cost would come from engineering such an umbrella initially, I presume. $\endgroup$
    – Enthu5ed
    Commented Sep 21, 2020 at 12:49
  • $\begingroup$ Good addition to the answer, it’s an interesting read. $\endgroup$
    – Enthu5ed
    Commented Sep 21, 2020 at 12:52
  • $\begingroup$ So it doesn't provide protection at iron melting temperatures. $\endgroup$ Commented Sep 22, 2020 at 11:56
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What if the umbrella were made from the esophaguses of a Great Wyrm Red Dragon? The umbrella might well survive and be found smoking, deep in the wall's crater directly behind where the player -used- to be.

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  • $\begingroup$ So... the umbrella would survive but the user won’t still? Sad :(. Maybe if you could prove the esophagus of the red dragon also provides shock resistance? XP $\endgroup$
    – Enthu5ed
    Commented Sep 22, 2020 at 2:18
  • $\begingroup$ Well, the tag did say the question was to be science-based. (wink) $\endgroup$ Commented Sep 22, 2020 at 2:34
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The Wall Would Explode Before it Melts

You can not "melt" the wall in any thematically satisfying and meaningful since of the word. Ash's energy calculations do a good job of expressing how much thermal energy it would take to melt the wall, but they do not tell the whole story.

Here are a few practical points that it misses:

  1. He is off by a few order of magnitude when converting J to Kilotons. By his calculations, it would be a 0.007762kT blast, or roughly the energy force of a really big conventional explosive missile.

  2. High energy fires condensed into a small area create explosive forces that blast the steel away as you "melt" it. Basically you are doing this to the wall, but at about 1,000,000:1 scale. This means there are forces at play which may not require you to fully melt the metal to remove it.

enter image description here

  1. When shooting fire across a room, you create more heat than is absorbed by the target.

With these 3 factors added together it is REALLY hard to say how accurate a simple thermodynamics equation will be; so, instead I will assume that a dragon's breath would probably have a similar thermal efficiency to an Oxyacetylene torch, then we can do some calculations to figure out how much energy is actually going into the metal.

It was surprisingly hard to find the energy efficiency of an Oxyacetylene torch, but after running through a long rabbit hole of various sources I've found that:

  • Oxyacetylene torches produce ~1585.26 kJ/cuft of fuel
  • You can cut a 1/4" channel through about 10.9 ft of 1/2" thick steel per cuft of fuel
  • This means you need about 1163.49 kJ/cuin of steel to heat/blast your way through the wall.
  • Using a 90" tall wall you have a total volume of @ 345,600 cu inches to remove.
  • This requires a total energy of ~402,103,380kJ. Compared to Ash's equations, this means you are actually outputting about 12.38 times as much energy as it takes to just melt the steel to do so by heat blasting it. While this is still no nuclear bomb, it is roughly equivalent to blasting the wall with 10,544 liters of burning jet fuel. That is 17.6 liters per sqft.

So, to answer your question, with a normal umbrella having an 8-10sqft surface area, standing in the dragon's spray means you getting pelted with the equivalent of ~140-180 liters of high velocity jet fuel. Even if the Dragon's breath failed to ignite, it would probably buckle your umbrella under the sheer force of getting hit by whatever fuel it it breathing at you. If your umbrella were able to take the force of getting hit by the dragon's breath, you would then have to be able to survive the explosive force of that much fuel combustion in such a small place. Even if you could survive that, you are then looking at a sudden wave of heat that is going to be hotter than the surface of the sun coming at you from all angles.

The only way I see your hero surviving this is if he is wearing and entire suit made out of starlite or similar material. Starlite can protect a person from 5 minutes of sustained blow torch heat; so, it can probably protect you from the heat of a dragon's breath as long as you can solve for how to make it durable enough not to crack under the explosive force... also you're hero will need to be super human tough to survive the explosive forces being transferred through the armor.

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  • $\begingroup$ Good considerations, it's not the temperature, but the energy transfer that we cannot guard against? $\endgroup$
    – Enthu5ed
    Commented Sep 21, 2020 at 14:46
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    $\begingroup$ Well you'd have to guard against both, but yes. Just making a heat proof bubble around yourself won't save you. $\endgroup$
    – Nosajimiki
    Commented Sep 21, 2020 at 14:49
  • $\begingroup$ Understood, thanks :) $\endgroup$
    – Enthu5ed
    Commented Sep 21, 2020 at 14:50
  • $\begingroup$ Large dragons breath slowly. $\endgroup$
    – Bergi
    Commented Sep 22, 2020 at 0:57
  • $\begingroup$ @Nosajimiki: Your analysis might actually show that the breath weapon doesn't need to be as energetic to do 120 HP of damage to the wall. If some of the damage comes from a fireball-like blast, it can damage / destroy walls without having enough energy to simply melt every kg of metal in them. $\endgroup$ Commented Sep 23, 2020 at 2:25
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This very much depends on the mechanism of how the breath works.

If it's heating by radiation (i.e. some kind of heat ray), then @Ash's answer above gives a convincing 'No'.

However, this kind of answer also prevents the dragon surviving the breath, so we should probably conclude this is not what the rules intended.

It may be better to assume it's more like a flamethrower. Flamethrowers work by squirting out a flammable substance, which is ignited. Thus the heat is generated not at a point, but across the target area, and not instantly, but over a period of time as it burns.

Obviously a very hot flamethrower, something like thermite perhaps.

Would this work?

Thermite has an energy density of 20Mj/L

Let's consider a 10cm square of 4" thick wall. Let's for convenince, metricise this to a 10cm cube of iron. this weighs 7.8kg.

Irons specific heat capacity is 444 J/kgC, and we need to raise it 1523C

444 * 7.8 * 1523 = 5.2MJ.

So a 2.5cm thick layer of thermite could melt through 10cm iron (assuming all the heat goes into the iron). Let's say 50% goes in, and we need a 5cm layer.

So (handwaving away how the dragon stores and fires large quantities of thermite), the breath weapon could be thermite-based.

Can an umbrella deflect this?

A standard umbrella can easily deflect thermite before it is ignited.

It's not impossible for an umbrella to deflect lit thermite, though obviously it'll need to be thicker and heat resistant – it doesn't have to withstand the heat indefinitely, just long enough for the thermite to be deflected. Some kind of fibreglass cloth is likely?

If this leaves you standing in a small clear patch in a large field of thermite, your life expectancy won't be great. But if this protects you from a stream of thermite which otherwise splashes down some distance away, then there's no reason you can't survive.

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  • $\begingroup$ Good answer, however I am not certain what form the dragon breath takes either; the most canonical detail is that the dragon uses an organ called the draconis fundamentum to pass magic into its lungs before breathing out the fire. $\endgroup$
    – Enthu5ed
    Commented Sep 24, 2020 at 11:36
  • $\begingroup$ Most likely you could equate it to a propane torch, I believe. $\endgroup$
    – Enthu5ed
    Commented Sep 24, 2020 at 11:42
  • $\begingroup$ I believe it’s very difficult to melt steel (and I’m assuming iron is pretty similar) with a propane torch. You can do it with a propane forge (with forced air, but even so that’s slow and relies on insulation to keep the heat in) or propane/oxygen, but there’s a reason oxyacetylene is used for welding - and even so it doesn’t melt 4” steel. I imagine a propane-air fireball is visually what the designers have in mind, but those are used in films because they look cool and aren’t actually that dangerous. $\endgroup$
    – Dan W
    Commented Sep 24, 2020 at 14:27
  • $\begingroup$ oh yes, but torches can reach 2000 degrees C as well, potentially you could think about it as a scaling up, but with some other type of combustible. $\endgroup$
    – Enthu5ed
    Commented Sep 24, 2020 at 14:28
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    $\begingroup$ You wouldn’t particularly want air in it - it wants to be dense. I’d imagine it might be mixed into some type of gel for lubrication, and held in a sac somewhere. Squeezing the sac with a muscle would propel it out of his mouth. Or he could probably cough up smaller globs and spit them out if he wanted to be more accurate for a smaller target. So similar to a skunk, but from the mouth. The mix would either be self igniting (pyrophoric) in air (e.g. the addition of some white phosphor), or the dragon would have a separate smaller fire ability (like a pilot light!) to ignite it. $\endgroup$
    – Dan W
    Commented Sep 24, 2020 at 16:00
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Supposing your umbrella forces an air current in front of it to shield the user.

If your handle is hollow and pushes the cold air behind you up and out around the outside of the umbrella, one could hope the forced convection will reduce heat transfer sufficiently. Basically, we're making an air curtain in front of the umbrella.

This requires a little bit of machinery or magic inside the umbrella but it seems you have no other choice if this is to work.

Convection formula: P = dQ/dt = h * A * (T − T0) Therefore E = h * A * (T − T0) * Δt

A human can withstand temperatures of 42°C and the breath is at 1246°C meaning T - T0 ≈ -1200°C.

Let's suppose the Wyrm's breath lasts Δt = 45s. Its a big thing and a long human breath is about 15s (I timed myself).

A = 2.5 m^2

Taking into account the calculations made by others on this thread, we have an E = 30 GJ energy transfer. And yeah, there is latent heat of fusion and dissipated energy but a few numbers were taken for worst case scenarios and constraints have been eased since so this is a good order of magnitude. BUT this energy is spread over a 70-ft = 21.3 m cone according to https://www.d20pfsrd.com/bestiary/monster-listings/dragons/dragon/chromatic-red/red-dragon-great-wyrm/ So we are only taking 100 * 2.5 / 21.3^2*pi = 0.175 percent of that because the rest is not directed towards us. We get a final E = 0.00175 * 30 GJ ≈ 50 MJ.

For air on air conducto-convection, we have empirically h = 10.45 − v + 10√v according to https://physics.info/convection/

This gives us v - 10√v = 10.45 - E / (A * (T − T0) * Δt) = 10.45 - 5*10^7 / (2.5 * -1200 * 45) ≈ 380 m/s

So √v = (10 + √(100 + 4*380))/2 ≈ 25 √(m/s) therefore v ≈ 625 m/s. That's a bit under twice the speed of sound in air.

I don't know how permissive your magic/machinery system is but it's theoretically possible. Plus, you can probably modify many these numbers to make it more favourable, especially if the heavy constraints on total energetic output are eased only slightly (a slightly smaller wall means orders of magnitude less energy and therefore orders of magnitude less speed for the air curtain). For example, if you double the breath time, you almost halve the required speed of your air curtain.

The physical model is not perfect but it works well enough to get an idea of the situation and gives a nice proof of concept.

Hope this helps!

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  • $\begingroup$ +1 for going into some of the details here, just one thing, what is to stop the latent heat from nearby the user from roasting them? $\endgroup$
    – Enthu5ed
    Commented Sep 26, 2020 at 23:40
  • $\begingroup$ Fair point. Although I can't quantify this, the idea would be that the air curtain keeps the hot air on the other side of the umbrella, leaving the air behind you relatively cold ... at least long enough for you to get out of there. This buys you a good while but there is only so much one can do. $\endgroup$
    – Bob
    Commented Sep 28, 2020 at 12:18
  • $\begingroup$ Understood, the curtain of air, would certainly help, I suppose with enough magic it could definitely help insulate the user regardless of the situation, just that it may be difficult for an average person to get their hands on a similar umbrella. $\endgroup$
    – Enthu5ed
    Commented Sep 28, 2020 at 14:49

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