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We're starting off with a Japanese mythical creature, the Yuki-Onna. Now the specifics of this being vary quite a bit from one tale to another, so let me be specific what this being is about. Keep in mind that this is my version, and may therefore deviate from popularly held versions.

Yuki-onna are ghosts of women who died during snowstorms. They're beings that begin being corporeal but can learn to 'phase through' other things (temporarily becoming incorporeal). They are essentially frozen corpses, so their core temperatures are well below -50 degrees Centigrade. Their lungs are filled with air of that same temperature, so they don't have 'ice breath', they just cool the air as they breathe out. Their diets and circadian rhythm is unimportant for this question, so I'll skip that.

What I want to know is: how are they affected by a fireball type spell? Say they breathe in the flames, does the temperature of the fireball affect whether they 'melt'?

The problem I am having is this. Ice (frozen water particles) should melt in layers, which would then need even more energy (heat) to vaporize. So, in theory, Fire shouldn't be 'super effective' against ice. It would take too long to be effective.

So, if I hazard a guess, I would say that there would be a insulating layer of ice on the skin of the Yuki-Onna. The flames would first have to get through this layer (via the same ineffective methods of heat transference), before melting and burning through skin, then blood, internal organs, etc.

Would fire therefore be the most effective method to combat beings of this nature? Or would the airways allow for the heat to circulate into the core and heat up quicker?

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    $\begingroup$ Such creatures get mostly double damage for fire-based attacks. Such a creature is unimaginable in the real world. I think rpg.stackexchange.com would be a better match for this question. $\endgroup$ – Gray Sheep Mar 5 '17 at 16:05
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    $\begingroup$ So World Building SE is only for Real World questions? @MorningStar Please tell me you're not serious. There was an entire series on reality check mythical creatures! $\endgroup$ – Fayth85 Mar 5 '17 at 16:07
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    $\begingroup$ Faythe what would make one answer any better than another answer? Please provide some constraints. $\endgroup$ – James Mar 5 '17 at 16:28
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    $\begingroup$ I do not have enough knowledge in thermodynamics to properly answer this. However xkcd has a what if involving a flamethrower and snow. In short you would need a really big and powerful fireball to kill this ice creature. Fire is more effective on things that can burn then things that can melt. $\endgroup$ – Anketam Mar 5 '17 at 16:59
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    $\begingroup$ The TL;DR on this is that the ice creatures get really, really (wait for it) steamed. $\endgroup$ – Catalyst Mar 5 '17 at 17:01
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The problem I am having is this. Ice (frozen water particles) should melt in layers, which would then need even more energy (heat) to vaporize. So, in theory, Fire shouldn't be 'super effective' against ice. It would take too long to be effective.

What you're describing is ablative armor where the protection comes from the armor eroding away. This is used on real spacecraft as a heat shield (just not made of water).

enter image description here

Ablative armor works by carrying away the energy of the attack in lots of tiny fragments. A microscopic outer layer heats up, vaporizes, and is blown way taking the energy along with it. Water works fairly well at this because it has a very high specific heat, meaning the amount of energy needed to raise its temperature: 4.2 J to raise 1 gram of liquid water by 1 C. Ice is about 1.9 J/g/C.

Phase changes also take energy. Going from water ice to a liquid water takes 333 J/g. From liquid water to vapor is a whopping 2257 J/g. This is why we still use steam to turn generators, it contains a lot of energy. This is also why steam scalds are so bad.

So to take 1g of -50 C ice to 100 C water vapor takes...

-50 C -> 0 C     |     50 C * 1.9 J    |   95 J
Solid -> Liquid  |     333 J           |  333 J
0 C   -> 100 C   |     100 C * 4.2 J   |  420 J
Liquid -> Gas    |     2257 J          | 2257 J
-------------------------------------------------
Total                                    3105 J

That is roughly 1 Watt-hour (ie. 1 Watt for 1 hour or 60 W for 1 minute) or roughly the kinetic energy of an Olympic hammer throw... for ablating a single gram of -50 C ice.

To put that in perspective another way, a typical stovetop burner uses 1500 J/s and so could ablate 1 gram of -50 C ice every 2 seconds. This is why boiling water on a stovetop takes so long.


The problem with ablative armor is its protection eventually wears out. The question then becomes how much energy does this fire attack have? Does it have enough to ablate all the ice? If so then their goose is cooked. If not, then they'll be -- mostly -- fine.

A quick puff of flame isn't going to do more than vaporize the outer layer of frost; and if their chill is magical it will rebuild that layer with water vapor from the surrounding air. While a sustained flame will wear away the layer of ice and start scorching flesh and presumably doing damage.

So really fire against a frost being would be very ineffective until all the ablative ice vaporizes, then it's just as effective as fire is on flesh. If you're playing an RPG this could be modeled as a pool of HP that absorbs fire attacks and slowly replenishes itself. When it's gone, fire does full damage.

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    $\begingroup$ Nice answer. I think the Leidenfrost effect should also be considered. The water that evaporates expands and "blows" away the flame greatly reducing heat transfer making it less effective. $\endgroup$ – Christoph Mar 6 '17 at 8:30
  • $\begingroup$ Since people are essentially bags of water, their outer layers can be considered ablative armor as well. Except for the added heat of fusion, any fireball which turns a human into (dessicated) crispy critters will evaporate an ice being of the same mass. $\endgroup$ – WhatRoughBeast Mar 6 '17 at 17:25
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    $\begingroup$ Solid water (ice) specific heat is not equal to liquid water (water) specific heat. At std conditions, it is 2.108 kJ/kg-K solid, 4.187 kJ/kg-K liquid, 1.996 kJ/-kgK vapor. $\endgroup$ – Mindwin Mar 6 '17 at 18:05
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    $\begingroup$ @WhatRoughBeast I don't know about you, but I keep my water on the inside. If the water in my body is boiling that's not ablative armor, that's dying. $\endgroup$ – Schwern Mar 6 '17 at 18:21
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    $\begingroup$ @KRyan "Would a fireball that can turn a human to ash melt an equivalent mass of ice?" wasn't the question, but let's answer it real quick. The answer is: not necessarily. The human body has a specific heat of 3.5 J/gC, less than water, so it takes less energy to raise its temperature (it's only 70% water) than water. Water in a human is at body temp, the ice is at -50 C and it's ice. Extra energy to raise it to 0 C, transition to water, and raise to body temp. The ablative effect is reduced on a human because its water is locked under skin and inside cells. Finally, fat burns adding energy. $\endgroup$ – Schwern Mar 6 '17 at 20:40
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Ice has a large enthalpy of fusion

The specific heat of ice is 2.05 kJ/kg*K. That means it takes so many kilo-joules to raise a kilogram of ice by one degree kelvin.

The enthalpy of fusion of ice is 333.5 kJ/ kg. Therefore it takes so many kilo-joules to melt a kilogram of ice.

For a mythical ice creature, losing and kilogram of ice might not be that important, assuming it has a way to make up the lose by freezing water out of the air or something. If this is the case, then the 333 kJ blast wouldn't really be life-threatening.

Its hard to melt that much ice

Now 333 kJ of heat energy is not that much energy in the long run. But it still represents a lot of energy for one attack. That is roughly the complete discharge from a lead-acid battery. That means if you discharged a car battery through this creature, it would probably not be killed (can't say the same about you or me).

To put this in more fireball related terms, this is 10% of the blast of 1kg of TNT. If a block of explosives went of about a foot away from this creature, and blasted in all directions, the amount of heat energy that this creature absorbed would be around 333 kJ, and would not be deadly. If 1kg of TNT went off 1 foot from you....well....condolences to your family.

Conclusion

Melting ice is hard. Better off doing what most people do to get rid of ice, hit it with a shovel. Let springtime and the sun take care of the melting.

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You are describing that the air she inhales is magically cooled, and that the ice being is magically cooled against normal room temperatures.

So why can’t it combat oven temperatures as well? It’s a matter of capacity, not general ability.

Just as we heat our flesh and can tolerate some cold environment, yourcreature will have some active countereffects against some increased temperature. This can be tireing.

Just as we can reach into a hot oven and grab a foil tray but are burned instantly if touching a solid metal tray, and fireballs remove the hair from my arm but don’t otherwise injur in such a short exposure, your creature can be overwhelmee with heat if delivered fast enough.

Hot air might not do it. Use hot water, or hot metal objects.

I suggest that the best weapon is steam. Steam will scald you far worse than boiling water, because of the phase change heat. This nicely counterballances the huge amount of heat needed to melt the ice!

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I'm not certain a fireball would do much more than surface damage to a creature like this in most instances. something like a fireball (in my mind) is pretty much just fire. To cause lasting damage, you need a reliable and steady means of thermal transfer. Boiling Water would do much more damage much more quickly because of the specific heat of water as opposed to the specific heat of air. This is to transfer heat damage through the skin and clothes.

There is one thing about a fireball that might work though. Lungs are pretty fragile. Time a fireball to go off during the Ice creatures "inhale". Cell layers in the lungs are maybe one or two layers thick, so the fire should be able to crisp a large quantity of lung tissue in a short period of time. The problem is: Is you mage skilled enough to launch and place the fireball well enough to catch the Yuki-Onna on the inhale. Not very likely. Go with Boiling Water, or even better, a Glob-O-Lava for max damage.

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Well, if by fireball you actually mean a sphere of nothing but temperature gently surrounding your target, then yes, heat is going to take too much time to be an effective weapon. But if you are thinking in the more usual form of fireball, a blast of flames akin to a deflagration, then you surely can blow the Yuki-Onna out.

The key, as with any explosive, is not the heat nor the raw chemical energy of the explosive, but the sudden raising pressure blow when the thing goes off - gasoline is more energetic than TNT, and butter more than gasoline, but you have way more probabilities of surviving a pound of butter burning besides you than an pound of TNT :D.

Ice is hard, and so, very fragile. It should shatter in a thousand pieces when smashed by a 3 psi push or above.

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In addition to already existing answers, please remember that the layer of ice being melted away is analogous to human skin and flesh. Thus this creature would effectively still sustain damage, though it would not instantly be killed.

Depending on your setting this creature would or would not feel pain from the damage caused - a corpse should not feel pain, though.

I would suggest using fire or any other source of heat will still be an effective method to combat this creature.

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