Brown Mold vs. The Volcano - Delving into Fantasy Physics


Pathfinder First Edition General Discussion

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What's causing the air motion? The pressure differentials.

The hot air is rising. Where is new air coming from to replace it?

Anywhere it can, nearby. generally, it would come from the edges of the caldera...but now we have a cold air source in the middle of the caldera - the Brown Mold Hemisphere.

The hemisphere's cold air is also being forced out by its own weight. It is descending from above as it cools and grows heavy, and naturally will flow out along the ground as it descends.

You seem to be saying it will be flowing out in all directions, including up. Not true. Hot air will be descending from above, as the cold air falls down and then moves sideways along the floor. The downward motion draws in more hot air from above.

The cold air leaves the hemisphere through pressure and diffusion, and hits the rising hot air around the sphere. The cold air is like a cold, heavy blanket, deflecting the course of the hot air. The temperature and pressure differential is extreme...the hot air is going to attempt to move past the cold air, losing energy as it does so...and pushes the cold air to one side even as it is sucked further out into the lava to replace the rising hot air.
That's sideways movement. There's more hot air at the new location, screaming for the sky, further moving the cold air sideways and up, and at the same time the hot air is being deflected past the cold air's weight as it rises as well.

Sideways movement of the air, endless and extreme pressure differentials...you're going to have a vortex forming. It's going to be very strong because there is NO LIMIT to the amount of air that brown mold can suck the heat out of. As fast as it enters the radius, it gets cold, gets heavy, and drops down to draw more hot air in at multiples of volume. I don't know how much negative pressure going from 1000+ degrees to 40 degrees would create, how much volume the air would lose. The faster the volcano heats the air and sends it drafting up, the faster new air is going to be sucked down into the hemisphere to replace it. The two systems can feed each other furiously.

The winds will be strong. As fast as the volcano can make something hot, the mold can make it cold even faster. The different volumes and weights of the air is going to be creating massive pressure differentials.


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I did say I'd go with the earlier ruling... But the grognard geek in me wants to point out that the mold was first 'discovered' in a set of physics that had no time unit smaller than one minute. To assume that it can instantly chill what was described as happening in a minute is perhaps losing sight of the historical source of the mold as a concept.

I still want my lava-boats, though. :)


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This argument now makes me want to hear about how a party gets to the end of an apocalyptic quest(hyped up by doomsayers/oracles) to find only a patch of brown mold sitting in a volcano is the big bad guy.


Das Bier wrote:
The pressure differentials.

There are no pressure differentials, as I explained in my previous post. The air within the radius increases in density as it approaches lava, keeping the pressure equivalent to that outside. Hot air is rising, but that happens entirely outside of the radius and can easily be accounted for by other hot air moving in response - nothing is affecting the pocket of cold air in particular.

Das Bier wrote:
Hot air will be descending from above

Why? The hot air is lighter than the air beneath it.

Das Bier wrote:
The cold air leaves the hemisphere through pressure and diffusion, and hits the rising hot air around the sphere. The cold air is like a cold, heavy blanket, deflecting the course of the hot air... the hot air is going to attempt to move past the cold air.

Even if the cold air was somehow pushed out of the hemisphere, how on earth would it interact in the way you're describing? The hot air is rising directly away from the sphere, and the cold air is sliding out slowly along the floor, rapidly becoming just as hot as its surroundings. The cold air couldn't be acting like a heave blanket, because it is definitely not coming down from above. The hot air is moving past the cold air to where exactly? The floor?


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OK, so we have to ignore both all convective effects and blackbody radiation effects, and yet somehow we have not kicked physics to the curb.

Right.

I fully don't expect the game to model physics much as too much will bog things down.

However you trying to apply physics to something which ignores physics is flat out silly. You are cherry picking what to ignore and what not to.

There are going to be massive convective forces working in any volcano. The outer area has to be cooler (there is solid rock there which sets a boundary condition). That means a flow of cooler air there into the center which is hotter.

The brown mold sphere says nothing about blocking wind, so the hot air will hit it and immediately cool down. This causes a huge localized pressure drop.

Temperature in the sphere is 32 F or 298 K(to make this easy).

Ambient laval temperature we will take to be 1000 C (1273 k)

As air hits the boundary, the volume drops from 10.5 m^3 per mole to .987 m^3. That is a huge volume change and will drive one hell of a lot of convection.

As you will already have substantial convective flows present in the volcano already, this will drastivally speed it up. The air will get sucked in, and have no place to go but up (yes, the air above will also be hot, but not as hot as to the sides and hence the volume change there will not be large enough to offset the volume change driving the flow in from the sides).


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drbuzzard wrote:

OK, so we have to ignore both all convective effects and blackbody radiation effects, and yet somehow we have not kicked physics to the curb.

Right.

I fully don't expect the game to model physics much as too much will bog things down.

However you trying to apply physics to something which ignores physics is flat out silly. You are cherry picking what to ignore and what not to.

There are going to be massive convective forces working in any volcano. The outer area has to be cooler (there is solid rock there which sets a boundary condition). That means a flow of cooler air there into the center which is hotter.

The brown mold sphere says nothing about blocking wind, so the hot air will hit it and immediately cool down. This causes a huge localized pressure drop.

Temperature in the sphere is 32 F or 298 K(to make this easy).

Ambient laval temperature we will take to be 1000 C (1273 k)

As air hits the boundary, the volume drops from 10.5 m^3 per mole to .987 m^3. That is a huge volume change and will drive one hell of a lot of convection.

As you will already have substantial convective flows present in the volcano already, this will drastivally speed it up. The air will get sucked in, and have no place to go but up (yes, the air above will also be hot, but not as hot as to the sides and hence the volume change there will not be large enough to offset the volume change driving the flow in from the sides).

Awesome! So, we'd have a doughnut like air circulation effect. Would this develop into a cyclone in a teapot? How fast do you think the winds would end up being with just a single node of brown mold.

What would be the effect if the lava chamber were completely enclosed?


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Avoron wrote:
Das Bier wrote:
The pressure differentials.
There are no pressure differentials, as I explained in my previous post.

There are pressure differentials within the cold sphere of air itself. Let's use a more readily imaginable example: scuba diving. The water is the cold air, at the surface of to water you have 1 atm of pressure. 10m down you have 2 atm. And 20m down 3 atm. The weight of the water above increases the pressure.

Now imagine you had a tower of water. You can do this by running a tap. The water is forced downwards by gravity and the weight of the water above it. Once it hits the sink it can no longer fall down but has to get out the way of the water above. So it goes sideways.

That is what would happen here. The cold air (water) around the mold falls and is pushed sideways and is replaced at the top of the sphere by warm air.

Make sense now?

Avoron wrote:


Das Bier wrote:
Hot air will be descending from above

Why? The hot air is lighter than the air beneath it.

There is a vacuum formed as the cold air falls. This will suck the warm air in from above. You can test this by pushing a door closed really quickly, wind is pushed by the door, but there is also wind trailing the door due to the vacuum of the displaced air by the door.

Make sense?

Avoron wrote:
Das Bier wrote:
The cold air leaves the hemisphere through pressure and diffusion, and hits the rising hot air around the sphere. The cold air is like a cold, heavy blanket, deflecting the course of the hot air... the hot air is going to attempt to move past the cold air.

Even if the cold air was somehow pushed out of the hemisphere, how on earth would it interact in the way you're describing? The hot air is rising directly away from the sphere, and the cold air is sliding out slowly along the floor, rapidly becoming just as hot as its surroundings. The cold air couldn't be acting like a heave blanket, because it is definitely not coming down from above. The hot air is moving past the cold air to where exactly? The floor?

Ok, we go back to the tap example. You can see how quickly water covers the bottom of the of the sink yes? That is what the cold air is doing. If you want to see why it goes out instead of just rapidly heating and rising again (which would cause turbulance and convection and gale force winds anyway) try pouring a bucket of water through a hose into a pot on top of a stove. The fist bit of water to hit the pot boils instantly and re vaporizes, but as you pour more and more water the pot cant keep up and eventually you end up with a layer of water in the pot that is being heated to boiling.

Make sense?

I am asking if I am making sense because my analogies might not be great. I am not trying to be condescending. Just make sure that I am actually explaining in a way that makes sense


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The most important question for this thread is do we name the brown mold 'Joe'?

I think it's self evident - no other name is acceptable.


J4RH34D wrote:
The cold air (water) around the mold falls and is pushed sideways and is replaced at the top of the sphere by warm air.

I get what you're saying about the cool air's weight pulling it down, but I don't think it would create the effects your describing. It all comes back to pressure: in a column of water, the water spreads out at the bottom because the downward pressure of the column is greater than the air pressure keeping it in place. In the brown mold radius, though, the pressure from the outside air is the same as the downward pressure exerted by the cool air inside, for the simple reason that the outside pressure is what determined how much air entered the radius in the first place. Without a pressure difference between inside and outside the radius, even the denser cool air can be kept in a fairly stable hemisphere.

J3RH34D wrote:
The fist bit of water to hit the pot boils instantly and re vaporizes, but as you pour more and more water the pot cant keep up and eventually you end up with a layer of water in the pot that is being heated to boiling.

Okay, for the sake of argument, let's assume that everything I said in the above paragraph was complete nonsense and the cool air will sink to the bottom of the hemisphere spread out along the ground. You have cool air sitting on the ground, denser than all of the hot air above it. What you don't have is Das Bier's description of the cool air as a "cold, heavy blanket deflecting the course of the hot air." The denser cold air is moving slowly along the ground, while the hot air is rapidly rising directly away from it.

Eventually, the cool air will be heated up by its surroundings and behave just like any of the hot air around it. Lava has a much higher temperature than a kitchen stove and there's a lot more of it, not to mention that the air has a lower heat capacity than water and won't be expending energy in a phase change, so I imagine this heating will happen fairly quickly. But in the meantime, there's nothing to cause any sort of destructive clash between cool air sitting on the ground and hot air rising away from it.


Some of these discrepancies are showing up because we haven't established where on the volcano the brown mold is. If it's high up, we get closer to tornado-forming conditions than if it's on the floor


Quintain wrote:

Awesome! So, we'd have a doughnut like air circulation effect. Would this develop into a cyclone in a teapot? How fast do you think the winds would end up being with just a single node of brown mold.

What would be the effect if the lava chamber were completely enclosed?

I suspect that short of having a finite element modeling program at our disposal and time to cook up appropriate boundary conditions, you couldn't do even rough calculations. Then again my fluids type classes were a long time ago (decades), so maybe someone more recently familiar might know a quick and dirty approximation method.

I suspect it would get going pretty fast though since we are talking an amazing energy sink in the brown mold physics exclusion zone.


drbuzzard wrote:
Quintain wrote:

Awesome! So, we'd have a doughnut like air circulation effect. Would this develop into a cyclone in a teapot? How fast do you think the winds would end up being with just a single node of brown mold.

What would be the effect if the lava chamber were completely enclosed?

I suspect that short of having a finite element modeling program at our disposal and time to cook up appropriate boundary conditions, you couldn't do even rough calculations. Then again my fluids type classes were a long time ago (decades), so maybe someone more recently familiar might know a quick and dirty approximation method.

I suspect it would get going pretty fast though since we are talking an amazing energy sink in the brown mold physics exclusion zone.

To my knowledge, you still need a big computer to do fluid dynamics.


The Sideromancer wrote:
Some of these discrepancies are showing up because we haven't established where on the volcano the brown mold is. If it's high up, we get closer to tornado-forming conditions than if it's on the floor

I'm envisioning floor/lava surface level.


Someone already gave the math on why there would be pressure.

The difference in the volume of air between 1000 degree air and 40 degree air is a factor of TEN.

Yes, that's right. 10 cubic meters of hot air hits the cold boundary, and INSTANTLY becomes 1 cubic meter of cold air, Avoron.

That's 9 cubic meters of space which has NOTHING in it. that's a vacuum. Nature abhors a vacuum. More hot air charges into the gap, shrinks down at a ratio of 10:1...

You also have to realize that even though the hot air is lighter then the cold air, air pressure still forces air DOWN. Above every single person alive is miles of air pressing DOWN. Hot air rises because the cold air around it is HEAVIER and pushes it UP. But if there is no air below it, it will still fall.

The hot air outside the dome comes screaming down into the dome, forcing its way in...and forces existing cold air OUT, just like a garden hose. There's literally no way to stop the effect...the cold air WILL flow, it will naturally disperse, simply because of the density difference between the hot air outside and its own mass.

And then the sideways movement is going to set up a vortex of temperature and pressure differentials.

It's like a Perpetual Motion machine. That instant 'destruction' of 90% of volume is going to drive a LOT of air pressure, continually drawing in air on one end and forcing it out the other. You could drive a lot of turbines with this kind of eternal wind.


Das Bier wrote:

Yes, that's right. 10 cubic meters of hot air hits the cold boundary, and INSTANTLY becomes 1 cubic meter of cold air, Avoron.

That's 9 cubic meters of space which has NOTHING in it. that's a vacuum. Nature abhors a vacuum. More hot air charges into the gap, shrinks down at a ratio of 10:1...

Here we go again. You're still describing a situation where the brown mold appears out of nowhere, or otherwise cools an entire sphere of air down to 40 degrees instantaneously. This is not what happens. As the mold is brought to the volcano and the temperature of the surrounding air increases, the density of the air within the radius remains higher to compensate. When sitting inside the volcano, the radius is already filled with cold air that is much denser than the air outside and exerts the exact same amount of pressure on the edge of the sphere. There is no pressure differential, because the pressure is continually being equalized. There is no vacuum creation, because the necessary air is already there.


If the mold is on the floor, eventually the radius will be full of cold air. It may only take the volume of the hemisphere, or it may fill the entire chamber to that depth.
If it's on a wall, you get the single most impressive convection cell in existence.
If it's on a ceiling, you likely get a vortex.

Edit: extra thought: in a sudden appearance, the air would effectively implode into the volume of the mold's influence, rather than having a specific direction of movement


Avoron wrote:
Das Bier wrote:

Yes, that's right. 10 cubic meters of hot air hits the cold boundary, and INSTANTLY becomes 1 cubic meter of cold air, Avoron.

That's 9 cubic meters of space which has NOTHING in it. that's a vacuum. Nature abhors a vacuum. More hot air charges into the gap, shrinks down at a ratio of 10:1...

Here we go again. You're still describing a situation where the brown mold appears out of nowhere, or otherwise cools an entire sphere of air down to 40 degrees instantaneously. This is not what happens. As the mold is brought to the volcano and the temperature of the surrounding air increases, the density of the air within the radius remains higher to compensate. When sitting inside the volcano, the radius is already filled with cold air that is much denser than the air outside and exerts the exact same amount of pressure on the edge of the sphere. There is no pressure differential, because the pressure is continually being equalized. There is no vacuum creation, because the necessary air is already there.

If physics worked the way you seem to imagine, there would be no such thing as convection.

However physics doesn't work that way, and we do have convention.


drbuzzard wrote:

If physics worked the way you seem to imagine, there would be no such thing as convection.

However physics doesn't work that way, and we do have convention.

I'm not in any way denying the existence of convection, and I'm not sure where you'd be getting the idea that I am. I simply doubting that the convection occurring in this instance would be abnormally extreme, create unnecessary pressure differentials, or cause a vortex to form.

Exactly what part of my characterization do you think would undermine the existence of convection processes? The idea that a sphere of cold air traveling into higher temperature surroundings will equalize pressure by maintaining a higher density? In what way does this contrast arise?


What you seem to be trying to say is that the cold air would remain absolutely motionless and held in place.

That's not going to happen. It's going to settle and spread out...outside the bubble. Gravity and flow, you know.

And then new air comes in from above, experiences a horrendous volume drop, and sucks more air down into the bubble.

IF the air outside the bubble was cold, what you want to happen would...absolutely nothing.

What the described situation is, would instantly start a convection system and funnel of movement.

I'm just wondering WHY you think the cold air in the bubble wouldn't move. Do you think it is held in there by the very, very light air outside, that is howlinig towards the sky and DRAWING it out of the bubble as it does so? I mean, seriously, the air is already in furious motion outside the bubble, and new air has to come from somewhere. It's going to come from the bubble. And this temperature differential engine is simply going to drive the system incredibly fast as it does so. It'll be coming down to fill the vacuum faster then it is being squeezed up into the sky.


Convection happens when you have hot fluid below cold fluid. Brown mold at the bottom of a volcano will reduce the convection in the air and the magma is so viscous that violent enough convection to destroy the mold is unlikely.

If you don't already get tornadoes in normal volcanoes you won't get them when you make the magma surface colder.


nah it'd get destroyed by the wind motion. It's just mold, and it would be setting up some pretty severe winds.


Das Bier wrote:
nah it'd get destroyed by the wind motion. It's just mold, and it would be setting up some pretty severe winds.

Without cold air overlaying warm air you don't get those winds.


The bubble of cold described by the rules does not block wind.

A volcano will have convection going on. It has to. The boundary condition of solid rock at the outside cone has to be cooler than the liquid rock.

That means air will flow from the outside towards the center of the volcano. There is no escaping that fact. Put your brown mold anywhere in the caldera and it will experience air flow.

As there will be air flow, the moving air, as it enters the physics exclusion zone will compress. It has to if we are not ignoring PV=nRT. That creates at vacuum at the boundary and will suck in more air. Unless we kick physics to the curb (as you do), nothing else can happen.

Now as we suck in air from the edges it has to go somewhere. While it is lower temperature than the air around it, the ambient temperature around the physics exclusion zone is going to be highest around the edges closest to the magma. Thus that is where the air flow in will have to be highest as the greatest pressure differential is generated. Hence we will create a plume of cooler air (as counter intuitive as that might seem). Air will rise up out of the physics exclusion zone and then turbulently mix to the sides and rush back down, heat up off the magma pool and continue the flow.

You have a massive convective flow driver which will run until it freezes the whole volcano.


drbuzzard wrote:

The bubble of cold described by the rules does not block wind.

A volcano will have convection going on. It has to. The boundary condition of solid rock at the outside cone has to be cooler than the liquid rock.

That means air will flow from the outside towards the center of the volcano. There is no escaping that fact. Put your brown mold anywhere in the caldera and it will experience air flow.

As there will be air flow, the moving air, as it enters the physics exclusion zone will compress. It has to if we are not ignoring PV=nRT. That creates at vacuum at the boundary and will suck in more air. Unless we kick physics to the curb (as you do), nothing else can happen.

Now as we suck in air from the edges it has to go somewhere. While it is lower temperature than the air around it, the ambient temperature around the physics exclusion zone is going to be highest around the edges closest to the magma. Thus that is where the air flow in will have to be highest as the greatest pressure differential is generated. Hence we will create a plume of cooler air (as counter intuitive as that might seem). Air will rise up out of the physics exclusion zone and then turbulently mix to the sides and rush back down, heat up off the magma pool and continue the flow.

You have a massive convective flow driver which will run until it freezes the whole volcano.

wouldn't the force of the flow mold an earthen shell in the boundary that would eventually muck up the airflow?


drbuzzard wrote:
A volcano will have convection going on.

Obviously, there's going to be some convection, natural movement of the air in the volcano from one portion to the next. I'm not denying that. But there's no reason for massive amounts of cold air to exit the radius at once, leaving only a vacuum that pulls in hot air from another direction. Yes, there will be airflow throughout the volcano as a whole. Some air will enter the hemisphere from upwind, and will immediately cool down. Some air will then exit the hemisphere downwind, and will quickly heat up. The pressure within and outside the radius is still more or less constant: nowhere is this process creating vacuum-like pressure differences, and nowhere is it occurring at tornado-force speeds.

Das Bier wrote:
I'm just wondering WHY you think the cold air in the bubble wouldn't move.

What keeps the hemisphere from losing shape entirely, you ask? The weight of the cool air itself and the pressure of the hot air in the surrounding environment. Das Bier, you seem to be thinking of a hemisphere of fluid as a shape that cannot be maintained, that must immediately collapse upon itself. And this is often true - I'm picturing a hemisphere of water in normal air, which will shortly become nothing but a puddle. But it only remains true when the pressure of the outside air is substantially less than that of the hemisphere itself, which is not the case here. You are correct in stating that the hot air in the volcano is "very, very light," but, as I believe has been pointed out, it is also very, very hot, and the increase in temperature provides a proportional increase in pressure that compensates for the increased density within the mold's radius and preventing the hemisphere from collapsing like a heap of water. But the point is fairly moot anyway, since any air pushed down by its own weight would, as you yourself described, spread out along the ground away from the mold - with no violent conflict of any sort with the hot air rising away above.


Avoron wrote:
drbuzzard wrote:
A volcano will have convection going on.

Obviously, there's going to be some convection, natural movement of the air in the volcano from one portion to the next. I'm not denying that. But there's no reason for massive amounts of cold air to exit the radius at once, leaving only a vacuum that pulls in hot air from another direction. Yes, there will be airflow throughout the volcano as a whole. Some air will enter the hemisphere from upwind, and will immediately cool down. Some air will then exit the hemisphere downwind, and will quickly heat up. The pressure within and outside the radius is still more or less constant: nowhere is this process creating vacuum-like pressure differences, and nowhere is it occurring at tornado-force speeds.

at a certain level Temperature change = pressure change, they are inseparable concepts.

Scarab Sages

Does the size-doubling effect of fire within 5 feet also extend to gases that are as hot as fire, but aren't currently combusting? If so, then a patch brought into a volcano should rapidly fill the caldera. (This is assuming that its cooling effect is instantaneous, and thus the mold cannot be heated to ignition temperature).


M1k31 wrote:
Avoron wrote:
drbuzzard wrote:
A volcano will have convection going on.

Obviously, there's going to be some convection, natural movement of the air in the volcano from one portion to the next. I'm not denying that. But there's no reason for massive amounts of cold air to exit the radius at once, leaving only a vacuum that pulls in hot air from another direction. Yes, there will be airflow throughout the volcano as a whole. Some air will enter the hemisphere from upwind, and will immediately cool down. Some air will then exit the hemisphere downwind, and will quickly heat up. The pressure within and outside the radius is still more or less constant: nowhere is this process creating vacuum-like pressure differences, and nowhere is it occurring at tornado-force speeds.

at a certain level Temperature change = pressure change, they are inseparable concepts.

Not if density can be different to accommodate.


KarlBob wrote:
Does the size-doubling effect of fire within 5 feet also extend to gases that are as hot as fire, but aren't currently combusting? If so, then a patch brought into a volcano should rapidly fill the caldera. (This is assuming that its cooling effect is instantaneous, and thus the mold cannot be heated to ignition temperature).

By raw, the answer to this is no. You have to have open combustion, not just super-heated air.


Dot.


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Avoron wrote:
Das Bier wrote:

Yes, that's right. 10 cubic meters of hot air hits the cold boundary, and INSTANTLY becomes 1 cubic meter of cold air, Avoron.

That's 9 cubic meters of space which has NOTHING in it. that's a vacuum. Nature abhors a vacuum. More hot air charges into the gap, shrinks down at a ratio of 10:1...

Here we go again. You're still describing a situation where the brown mold appears out of nowhere, or otherwise cools an entire sphere of air down to 40 degrees instantaneously. This is not what happens. As the mold is brought to the volcano and the temperature of the surrounding air increases, the density of the air within the radius remains higher to compensate. When sitting inside the volcano, the radius is already filled with cold air that is much denser than the air outside and exerts the exact same amount of pressure on the edge of the sphere. There is no pressure differential, because the pressure is continually being equalized. There is no vacuum creation, because the necessary air is already there.

>There is no pressure differential, because the pressure is continually being equalized.

Unfortunately it can't be equalised over the whole sphere. If you assume that the pressure is exactly equal at the top of the sphere, it will be different at the bottom, because 30ft of cold air weigh more than 30ft of warm air, and thus pressure change(rho(density) times g(acceleration due to gravity at that spot) times height difference(30ft)) would be different inside the sphere from that of the outside. If you assume the pressure is exactly equal at the bottom of the hemisphere, it will be different at the top. You just can't have your cake and eat it too:either temperatures are different, or pressure gradient is the same.

As for how to make a quick and dirty calculation of the wind's strength, I imagine something can be achieved if you assume that the patch along with the volcano is a machine doing work using carnot's cycle, with patch being the cooler reservoir and volcano being the heater. All work done by it would be converted into kinetic energy of the wind, and you could probably get a function giving you the stable air speed based on the energy loses in the system due to friction. Result should be accurate within one order of magnitude, I think, which would be good enough for your purposes.


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Back checking the rules, although the mold is much colder than the air, lava itself would still be hotter than the mold is cold (3d6 nonlethal/round cold vs. 2d6 lethal/round fire). Unfortunately, we won't be getting lava boats this easily.


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Klara Meisson wrote:
Unfortunately it can't be equalised over the whole sphere. If you assume that the pressure is exactly equal at the top of the sphere, it will be different at the bottom, because 30ft of cold air weigh more than 30ft of warm air, and thus pressure change(rho(density) times g(acceleration due to gravity at that spot) times height difference(30ft)) would be different inside the sphere from that of the outside. If you assume the pressure is exactly equal at the bottom of the hemisphere, it will be different at the top. You just can't have your cake and eat it too:either temperatures are different, or pressure gradient is the same.

Acknowledged, that's a perfectly legitimate point. I ran the numbers, though, and if my calculations are correct the pressure difference between the top and the bottom is only a couple hundred pascals - hardly enough to cause tornado-force winds. And any air that does seep out from the bottom will still be spreading out harmlessly along the ground until it heats up, not violently clashing with the hot air rising away from it.


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Avoron wrote:
Klara Meisson wrote:
Unfortunately it can't be equalised over the whole sphere. If you assume that the pressure is exactly equal at the top of the sphere, it will be different at the bottom, because 30ft of cold air weigh more than 30ft of warm air, and thus pressure change(rho(density) times g(acceleration due to gravity at that spot) times height difference(30ft)) would be different inside the sphere from that of the outside. If you assume the pressure is exactly equal at the bottom of the hemisphere, it will be different at the top. You just can't have your cake and eat it too:either temperatures are different, or pressure gradient is the same.
Acknowledged, that's a perfectly legitimate point. I ran the numbers, though, and if my calculations are correct the pressure difference between the top and the bottom is only a couple hundred pascals - hardly enough to cause tornado-force winds. And any air that does seep out from the bottom will still be spreading out harmlessly along the ground until it heats up, not violently clashing with the hot air rising away from it.

Wind speed is measured in meters per second, not pascals, so I fail to see your point. Pressure is a driving force in this case. Since it isn't balanced by anything, it will just keep ratcheting the speed up, and up, and up...

Tornadoes, as it has been stated before, form from temperature differencials of 10-20 degrees. This is 2 orders of magnitude bigger. There will absolutely be windstorms near that patch of mold.


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So.

Drop a patch of brown mold ("Joe") into a volcano.

What I've gotten from this conversation is that this will generate a) a lava-boat, and b) a lava tornado.

I can see literally no downside to adventurers doing this. *cough*


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Avoron, you're ignoring the movement implications.

yes, the hot air has its own pressure.

It's also GOING AWAY. As in, exiting upwards.

and as it does so, it's reaching out and grabbing for cooler air to come in below it.

And there happens to be an unending supply of cool air right in the middle of this thousand degree furnace to supply it all.

Imagine you have a door in front of you. On the other side of the door, the atmospheric pressure is 1/10th of that in front of you. This is a differential worse then going from the top of a mountain to the bottom...or being in most airplanes.

Now, Open that door.

What happens?

The air pressure behind you drives you into that opening. This is a pressure rupture, exactly like you'd get if you popped a hole in an airplane. The difference is, this pressure is NOT going to equalize. Why? Because on the other side of the room, there's a vent where a furnace is blowing hot air out. it's sucking the cold air out of the room, heating it up, and sending it skywards. Except the area of the furance is, say, a hundred times that of the cold air room. Its demand for cold air is huge and also unending.

So, in every direction from that hemisphere, hot air is screaming for the sky, and the cold air is being sucked out of the hemisphere to replace a huge volume of air. The draw will be strongest closest to the lava, and follow the natural flow of the heaviest, coldest air to flow out of the sphere.

at the top of the sphere, hot air is entering and instantly losing 90% of its volume, creating that massive vacuum as more air rushes in to fill the area...exactly like a breached airplane hull. Except, there's no end to incoming or outgoing demand.

Just watch a disaster movie where someone pops a hole in the hull of an airplane, and see how strong that draw can be.Airplanes have a fixed amount of air, and eventually pressure will equalize. That is not going to be the case here.

Cold air forces hot air up. gets heated up, draws more in behind it. Hot air hits the cooldown threshold, loses massive amounts of volume, creates a pressure differential of massive size.

Boom, you have an endless pressure cycle that requires no fuel but the temperature of the volcano.

You probably have the airflow OUT of the hemisphere occupying a greater area then the inflow, since incoming air is going to compress so significantly as it crosses the temperature threshold. But still, the masses of air movement are going to be pretty significant. Nature abhors those vacuums.


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Tacticslion wrote:

So.

Drop a patch of brown mold ("Joe") into a volcano.

What I've gotten from this conversation is that this will generate a) a lava-boat, and b) a lava tornado.

I can see literally no downside to adventurers doing this. *cough*

I prefer to call it "Roberto". And we are oversimplifying here-mold would need some protection, for example, like being sealed in an adamantine casing. Air in the volcano isn't actually atmospheric air, so density will be different(added sulfur should increase it, I imagine). Lava around the patch won't be still, and considering that there will be serious winds there, small droplets will likely be torn away from the surface, be brought inside the cooling sphere, solidify there into tiny stones, then be carried forth by the wind, so it's not just a tornado you will see-it will be a lava tornado with stone shrapnel in it.

One thing is certain:it won't be boring to watch.


Tacticslion wrote:

So.

Drop a patch of brown mold ("Joe") into a volcano.

What I've gotten from this conversation is that this will generate a) a lava-boat, and b) a lava tornado.

I can see literally no downside to adventurers doing this. *cough*

Klara Meison wrote:

I prefer to call it "Roberto". And we are oversimplifying here-mold would need some protection, for example, like being sealed in an adamantine casing. Air in the volcano isn't actually atmospheric air, so density will be different(added sulfur should increase it, I imagine). Lava around the patch won't be still, and considering that there will be serious winds there, small droplets will likely be torn away from the surface, be brought inside the cooling sphere, solidify there into tiny stones, then be carried forth by the wind, so it's not just a tornado you will see-it will be a lava tornado with stone shrapnel in it.

One thing is certain:it won't be boring to watch.

You realize that there is nothing in this post that contradicts my take away*.

It only means that now adventuring parties have to do it. For Science.

>:D

* This could be because my takeaway is purely fueled by the rule of "stupid-cool" instead. But, you know, I can at least pretend to be scientific about it.


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I will be sending a brown mold infected mummy into the next active volcano I encounter in a game ... for research purposes.


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mmm, spinning brown mold mummies! For science!


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The Sideromancer wrote:
Back checking the rules, although the mold is much colder than the air, lava itself would still be hotter than the mold is cold (3d6 nonlethal/round cold vs. 2d6 lethal/round fire). Unfortunately, we won't be getting lava boats this easily.

Can you cite your references toward the lethal/non lethal values? The ones I found on the PRD under environmental effects of lava-heated air were lethal/minute and the brown mold is non-lethal per round.

I want to be accurate if at all possible.


Das Bier wrote:
Imagine you have a door in front of you. On the other side of the door, the atmospheric pressure is 1/10th of that in front of you. This is a differential worse then going from the top of a mountain to the bottom...or being in most airplanes.

For the umpteenth time, there is no pressure differential. The air inside the radius is colder, but it is also denser - there is more air packed into a smaller space. The pressure inside and outside of the radius is still exactly the same.

Would there be a massive pressure differential if the mold appeared out of nowhere? Yes, just like there would be if you suddenly opened the door on an airplane in mid-flight.
But would there be one if the mold actually moved into the volcano and is now sitting there motionless? No, just like there wouldn't be one if you opened the door on a car after driving up a mountain. The pressure inside the car is exactly the same as the pressure outside, so there is no violent interaction between the two.


Well, you can cook above lava faster then you can cook above a barbecue...


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Quintain wrote:
The Sideromancer wrote:
Back checking the rules, although the mold is much colder than the air, lava itself would still be hotter than the mold is cold (3d6 nonlethal/round cold vs. 2d6 lethal/round fire). Unfortunately, we won't be getting lava boats this easily.

Can you cite your references toward the lethal/non lethal values? The ones I found on the PRD under environmental effects of lava-heated air were lethal/minute and the brown mold is non-lethal per round.

I want to be accurate if at all possible.

Later in the environment chapter, under lava effects

CRB, Lave effects wrote:

Lava or magma deals 2d6 points of fire damage per round of exposure, except in the case of total immersion (such as when a character falls into the crater of an active volcano), which deals 20d6 points of fire damage per round.

Damage from lava continues for 1d3 rounds after exposure ceases, but this additional damage is only half of that dealt during actual contact (that is, 1d6 or 10d6 points per round). Immunity or resistance to fire serves as an immunity or resistance to fire, lava or magma. A creature immune or resistant to fire might still drown if completely immersed in lava (see Drowning).


Title wrote:
BROWN MOLD VS. THE VOLCANO
Ckorik wrote:

The most important question for this thread is do we name the brown mold 'Joe'?

I think it's self evident - no other name is acceptable.

Tacticslion wrote:
Drop a patch of brown mold ("Joe") into a volcano.

Joe vs. The Volcano.

Klara Meison wrote:
I prefer to call it "Roberto".

Roberto Volcano?


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Avoron wrote:
Das Bier wrote:
Imagine you have a door in front of you. On the other side of the door, the atmospheric pressure is 1/10th of that in front of you. This is a differential worse then going from the top of a mountain to the bottom...or being in most airplanes.

For the umpteenth time, there is no pressure differential. The air inside the radius is colder, but it is also denser - there is more air packed into a smaller space. The pressure inside and outside of the radius is still exactly the same.

Would there be a massive pressure differential if the mold appeared out of nowhere? Yes, just like there would be if you suddenly opened the door on an airplane in mid-flight.
But would there be one if the mold actually moved into the volcano and is now sitting there motionless? No, just like there wouldn't be one if you opened the door on a car after driving up a mountain. The pressure inside the car is exactly the same as the pressure outside, so there is no violent interaction between the two.

But you have admitted that there is pressure differential, like, 3 posts ago.


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Quintain wrote:
The Sideromancer wrote:
Back checking the rules, although the mold is much colder than the air, lava itself would still be hotter than the mold is cold (3d6 nonlethal/round cold vs. 2d6 lethal/round fire). Unfortunately, we won't be getting lava boats this easily.

Can you cite your references toward the lethal/non lethal values? The ones I found on the PRD under environmental effects of lava-heated air were lethal/minute and the brown mold is non-lethal per round.

I want to be accurate if at all possible.

It doesn't really matter either way, since all it means is that cold from brown mold is less dangerous for humans than lava is. But brown mold doesn't slowly cool down the area around it through it's cooling power(in terms of joules per second), it cools it with magic, instantly, and to a set temperature no matter what.


Avoron wrote:
there is no pressure differential.
Klara Meison wrote:
But you have admitted that there is pressure differential, like, 3 posts ago.
Avoron wrote:
the pressure difference

Is there a disconnect in terminology or something? That might explain it.


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The Sideromancer wrote:

Later in the environment chapter, under lava effects

CRB, Lave effects wrote:

Lava or magma deals 2d6 points of fire damage per round of exposure, except in the case of total immersion (such as when a character falls into the crater of an active volcano), which deals 20d6 points of fire damage per round.

Damage from lava continues for 1d3 rounds after exposure ceases, but this additional damage is only half of that dealt during actual contact (that is, 1d6 or 10d6 points per round). Immunity or resistance to fire serves as an immunity or resistance to fire, lava or magma. A creature immune or resistant to fire might still drown if completely immersed in lava (see Drowning).

Ok, that is direct lava contact, not a hot environment as a result of lava heating the air. That's where we are seeing the difference.

I think we should take a apples to apples (cold air vs hot air) comparison, imo.

Can any of you physics guys speak towards the rate of lava-hardening by being exposed to the colder air of the brown mold?


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Das Bier wrote:
Well, you can cook above lava faster then you can cook above a barbecue...

Real life story: we had a family die trying that trick with lava from Mt. Etna back in the 90's. The food absorbed toxic fumes from the lava while cooking.

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