The Hollow World

This post is derived mainly from this thread at the piazza. Here I put it all together describing the problems of a hollow earth in the real world, the problems I have with how it was represented in the hollow world products published by TSR, and my take on a scientifically sound (mostly) representation of the physics and geology of a hollow earth. 

The Science

The laws of physics as we understand them precludes a planetary body that is hollow from existing, and this is the main reason I have not included the hollow world in my version of Mystara so far. 

Let's start by talking about why hollow planets don't exist. Within Astrophysics there is a principle called Hydrostatic Equilibrium. Essentially, this is a balance between gravity and pressure. Gravity pushes all the mass of a planetary (or star) body inward toward a single point, while the substance of the body resists compression and builds up pressure pushing outward away from the center of gravity. When these two forces are equal, there is an equilibrium. If a body is not in equilibrium it will either expand or contract depending on whether the force exerted by gravity or pressure is greater. Hydrostatic Equilibrium is what causes all planets and stars to be spherical in shape.

The reason a hollow world is unstable is because we have removed the material in the center exerting outward pressure, so now there is vacant space into which the rest of the planetary mass will collapse. Now, you may be asking why a solid object would change shape and collapse. The answer to this requires a different way of thinking about solids and liquids. Most of the mantle and core of the earth would normally be called a solid. But we're talking about millions of tons of material here constantly being acted upon by both pressure and gravity and heat (a significant portion of the heat in the Core comes from Radioactive materials). On a geological timescale the mantle and core behave viscously; basically the main body of the earth is a fluid. So when any significant hole is opened up inside the earth, the mass surrounding it would flow into the empty space until a new equilibrium was reached.

The other big issue has to do with tectonic plates. Given that we know Mystara is the same as earth in the late Jurassic period per this post, we therefore should try to use all that we know about earth's geology to define the geology of Mystara. From this we must concede that continental drift occurs. Continental drift is occurs because a number of tectonic plates forming the earth's crust are 'floating' on a super heated layer of the earth's interior that acts somewhat like caramel. This process is dependent on the heat and pressure derived from the layers of superheated liquids and solids deep inside the earth. Specifically the Asthenosphere (upper Mantle) is what causes the tectonic plates to shift and expand and contract; the other geological layers (the crust, mantle, and core) of the earth also impact other features we take for granted such as the electromagnetic field around the earth, the magnetic north pole, and volcanoes. As it is the hollow models created by Thorf require that the density of the material in the different layers would have to be drastically increased to make up for the hollow center to maintain the same observed force of gravity.

The effects of Gravity cause a whole range of other issues. To summarize, even if the hollow world structure is viable, anyone or object at any point inside this hollow space would feel weightless essentially and eventually drift towards the center of gravity.

This would also influence atmospheric pressure. If atmosphere is allowed to flow freely between the outer and inner worlds(such as through the giant polar openings) then the air of the inner world would have a pressure 387000 times that of the outer world (if we assume the outer world still has the equivalent atmospheric pressure as we are familiar with, overall there would be a lot more air in the atmosphere). The solution is fairly simple; there must be barrier of some kind preventing air flow between the two worlds. Essentially this creates two separate atmospheres. The atmosphere on the inside (refer to Solution 2 described below) would settle into a layered strata as described in the canon sources, with the space around the red sun being similar to the vacuum of space.

The Hollow World Setting

As much as I respect Allston and Heard as authors and creators, there are many things in their works that I don't like. Despite this, there have always been certain things about the idea of a hollow earth that appealed to me; I just really dislike the presentation of most of the official hollow world products. To explain away the scientific problems with magic and Immortals in complete disregard of physics is unacceptable to me.

The world shield and the spell of preservation are my two biggest hang-ups with the published hollow world setting. The spell of preservation just seems unnecessary and heavy handed; magic isn't necessary to maintain a cultural tradition. Societies change and remain stagnant for a variety of reasons and to set arbitrary limits on a certain people is lazy design. For a long time, due to the problems of the physics of the hollow world, I've been thinking about just transplanting the general cultures of the hollow world to a corresponding place in Davania or Skothar (Nithia to Arypt, etc). I don't think it was necessary to put these cultures there in the first place when there was space (and places already specifically named after a historical analog) on the outer world. I see the hollow world as an opportunity to explore some of the concepts from Pellucidar by ERB. The inclusion of weird creatures and societies like the beast-men fits in with my idea of the hollow world, but there is no reason for the Immortals to place a magic spell on the whole world so that people will maintain tradition and not try anything new.

The World Shield has no explanation; from what I've been able to gather it is treated as a source of gravity, not the center of gravity. It is just explained away as a magical plane, in fact very similar to Spelljammer's gravity plane, which make no sense either. In spelljammer "[gravity] is there at full strength or it is not there at all." (Concordance of Arcane Space, pg 13) Actual Gravity is measured as acceleration and is completely dependent on distance. I understand this is fantasy, but real world scientific terms refer to very specific things; if something else is meant a new/different term should be used (maybe bodily attraction instead of gravity in Spelljammer). My issue with the World Shield is that it solves the problem of gravity by simply saying the problem is solved without explaining where this new nonsensical gravity comes from, and as I said earlier 'Magic' is not acceptable for me.

In my version of Mystara, no matter what physical model I adopt for the hollow world, I don't plan to have polar openings. This is for various reasons, 1) I'd rather not have to deal the mapping issues that would entail; yes it is possible to accommodate the polar openings, I just don't want to do it. 2) If I include the Hollow World I want it to be more difficult to enter/find than just a trip to the north pole(not that that is an easy task either). I think the hollow world should be nigh impossible to reach for an outworlder. It should be a completely unknown and mysterious place. 3) polar openings would probably disrupt the structural integrity. Even implementing Solution 2 described below, hydrostatic equilibrium would still collapse the openings in the end without some other magical explanation. I think, I haven't done the math so I can't say for sure, but it's a likely possibility.

Given that, I may still have entrances at the poles, maybe a mile wide shaft directly over the pole. However, for purposes of separating the two atmospheres the shaft would not go directly to the hollow world; it would probably lead to a system of tunnels with doors or cave-ins through which someone could eventually reach the hollow world, though it wouldn't be a guarantee. I might also place other smaller entrances in other places. To counteract the heat of the mantle which these tunnels would pass though I think they would have to be lined by some material with a high melting point; the tunnels would probably be created by the Annelids and other burrowers like the purple worms.

When I started this project I was pretty firmly in the camp of my Mystara not being hollow; now I consider my Mystara to be hollow because I can create a somewhat scientific model of it, though the peoples inside will probably be different, and the chances of any PCs reaching it are extremely low. Now I'm actually kind of excited to have my version of Mystara be hollow.

Solution 1

We understand at this point that a hollow earth can't exist because the interior of the earth is viscous. In order to support a cavity inside the center of the planet a solid non viscous material is required. If the mass of the earth didn't behave as a fluid, then Hydrostatic Equilibrium would be irrelevant because these principles only apply to fluids. The extremely easy solution is just to make the hollow world be a dead planet, meaning the core and mantle are completely cooled down and the interior has solidified so it is no longer viscous. This would preclude the presence of continental drift though because the movement of tectonic plates is dependent on the presence of the asthenosphere which is itself dependent on the unique conditions of the upper mantle.

An alternative option is to have an inner shell of solid non viscous material that would support all the viscous layers of the rest of the earth so that it wouldn't collapse in on itself. I think it's better explained through an image:

The model that I am using would preclude tectonic plates inside the hollow world because the shell must be deepest layer of the earth. If the model had mirroring layers of mantle and crust on either side of the shell, the interior layers would simply collapse in towards the center the of gravity.

My next step with this is to determine what material the inner shell should be made of and how thick it needs to be. The shell must be able to act as a replacement of the pressure that would normally be in the center of the earth. There are a series of equations that I must iterate to determine exactly what type of material for the shell is best.

First I pick a substance and decide on how thick I want the shell to be. From there I calculate Hydrostatic Equilibrium

dP=-D(h)G(h)dh where P is pressure, D is density, G is gravity, h is height

Then I calculate stress exerted on the shell

Rs=P((R^3-r^3)/(r^3-R^3)) where Rs is radial stress, R is outerradius of the shell, r is inner radius of the shell

Cs=(P/2)(2(R^3+r^3)/(r^3-R^3)) where Cs is Circumferential stress

Equivalent Total Stress=Rs-Cs which must be less than the yield strength of the material comprising this inner shell. This will be a process of trial and error, picking a material doing calculations and figuring out what works out the best.

I worked on some preliminary calculations of this model. My results were discouraging to say the least. It seems that this model isn't feasible without eliminating most of the molten elements so that the shell comprises most of the diameter of the planet. The stress that the material in the shell would have to withstand is orders of magnitude greater than any material I could find numbers for. So, again this could be solved by making the shell out of some magical material that can withstand these enormous pressures, but that kinda defeats the purpose of this whole exercise.

I had my brother to check my math and ran more calculations with different variables on the shell model just to confirm my suspicions. After running the numbers it became clear that without some fictional material such as Scrith or Neutronium, which I would rather not use for various reasons, a hollow world with a viscous mantle was not viable. So in the end I decided not to go with it and explored my options using another model I stumbled across while trying to solve the problems of gravity inside the Hollow World.

Solution 2

This second solution occurred to me and my brother while we were trying to figure out a way to reintroduce gravity to the surface of the hollow world without using the mysterious magic of the world shield

The solution we came up with in order to allow gravity in the Hollow world is in the realm of pseudo science, yet the principles used should still apply if the details are off. We decided the best way to generate the necessary gravitational force is to replace the portal to the plane of fire (the red sun) with an anti-matter core. This anti-matter would generate anti-gravity counteracting the gravity generated by the mass of the Earth itself. This also requires us to increase the mass of the earth so that the equivalent gravity on the outer world remains the same. This solution comes with a big caveat; we don't really know what anti-matter is, how it interacts with matter, or whether it even has an anti-gravity force associated with it. Everything about it are hypotheses and there is no evidence so far to really support one hypothesis over another. Regardless of this, if it turns out anti-matter doesn't behave in this way, I would simply reason that the nature of the red sun inside the hollow world, whatever it's scientific or magical properties, is a source of anti-gravity such that our equations still hold true.

(After more research, what I call anti-gravity here should be referred to as negative gravity or repulsive gravity because anti-gravity is the absence of gravity. In addition, it is generally agreed upon among the scientific community that anti-matter has normal mass the same as normal matter, while it is likely that dark energy has negative gravitational properties. As stated above the actual substance of the red sun is irrelevant here, just the fact that it generates negative gravity, though I do prefer the term anti-gravity, it just makes more sense to me.)

Introducing the Anti-Matter sun opens up some more options however. Because the forces of gravity and anti gravity are working against each other, they balance out somewhere inside the shell of the earth. A new center of gravity appears as sphere at this point of balance. Based on our latest equations, Gravity achieves 0.00 m/s^2 at 1382 miles below the outer surface of the earth. This center of gravity is similar in some ways , and different in other to the world shield. it is the same in that is the center of gravity, that is where all mass accelerates towards, yet it is different in that the change of gravity is gradual. If a person were 1382 miles below the surface of the planet they would not experience a sudden shift in gravity. They would feel weightless, and within a few miles of this point they probably wouldn't notice the effects of gravity because they are so minuscule. If the shadow elves truly had a Cavern this deep and had lived there their whole lives, once they returned to the surface they would be unable to function; they just wouldn't have the bone density or the muscle mass to exert the kind of forces necessary to even move their limbs.

Another consideration for layers of the earth and tectonic plates, is that with this antimatter sun, the inner solid shell would be unnecessary for structural support. This is because the center of gravity is now within the earth itself, and not at a single point at the center of the hollow world. Therefore, there are two possible models to make the hollow earth structurally sound; one with an inner supporting shell, and another one with no solid shell, but with a completely viscous interior with a center of gravity inside the thickness of the earth which is generated by the competing forces of antigravity from an antimatter interior sun and gravity from the matter which comprises the earth. If one desires the movement of tectonic plates on both the inner and outer surfaces, it is possible to have mirror layers of core, mantle, and crust on either side of of this center of gravity.

Now just for some basic statistics to get an idea of the changes that occur from this implementation.

The real Earth has a mass of 5.97x10^24 kg with an average density of 5.51 g/cm^3.

My model of the Hollow World has an outer diameter of 7917.5 miles(the same as earth's) and an inner diameter of 3790 miles(based on the canon circumference of 11908 miles) giving us a shell 2063.5 miles thick. To maintain the same gravity on the outer world as exists on earth we must compress the same mass of the earth into a smaller volume giving us an average density of 6.18695105 g/cm^3 for the hollow earth.

To calculate the gravitational effect of the anti-matter and the necessary increase in mass, my brother used the shell theorem and put the results in a spreadsheet. What follows are a couple of graphs derived from that spreadsheet; the first shows the normal force of gravity(assuming a uniform density for the sake of simplicity) inside a hollow earth, and the second the effects of gravity with the anti-matter sun inside the hollow world.

This results in the red sun containing 1.37x10^24 kg of anti-matter or negative mass.

This requires the mass of the earth being increased to 7.34x10^24 kg with an average density of 7.60674 g/cm^3

So overall the mass of the earth increased by 123% and density by 138%, but the surface gravity on both the outer and inner worlds maintains a force of acceleration of 9.8 m/s^2 and center of gravity 1382 miles from outer surface, and 681.55 from inner surface.

So I ended up dividing the planet into 7 layers: the Lithosphere, Asthenosphere, Upper Mantle, Lower Mantle, Outer Core, Inner Core, and the World Shield. I didn't include the transition zones simply because it was too difficult to find the mass/density or volume for all the different transitions. I did decide to include a mundane version of the World Shield just to make the math easier for me, but it doesn't have any special magical properties it is just denser material(osmium) at the center of gravity. The other layers have the same composition and properties as found in the real world.

In figuring the depth of each layer I calculated the percentage of the total radius of the earth that each comprised, and then applied that percentage to the distance between the surface and the center of gravity (another option I explored but didn't go down was to have each layer maintain the same depth as in the real earth but cutting them off at around the center of gravity so portions of the normal core and mantle would be superseded by the world shield). Then I finagled the numbers to get the final mass I needed which was calculated from the real mass of earth plus the effects from the anti-grav sun of the hollow world.

This is a chart with exact measurements for each layer:

Layer	Depth(km)	Thickness(km)	Mass(kg)	Temperature( C )	Density(g/cm3)	Volume(Km3)	Composition
Lithosphere	0	38.91	5.33E+22	200-600	2.7	1.97E+10	granite/basalt
Asthenosphere	38.91	58.37	9.61E+22	1500	3.3	2.91E+10	liquid rock
Upper Mantle	97.28	150.15	2.83E+23	500-900	3.9	7.25E+10	crystalline rocks and hydroxide water in lower transition zone
Lower Mantle (Mesosphere)	247.43	820.57	1.69E+24	2000-4000	5	3.37E+11	semi-solid iron/magnesium/silicon
Outer Core	1068	832.05	2.68E+24	2730-4230	10.7	2.50E+11	liquid nickel/iron
Inner core	1900.05	322.48	1.02E+24	3000-6650	13.5	7.53E+10	solid nickel/iron
World Shield	2222.53	1.97	9.61E+21		22.6	4.26E+08	osmium
center of gravity	2224
Inner core	2224.5	107.6	3.06E+23	3000-6650	13.5	2.26E+10	solid nickel/iron
Outer Core	2332.1	432.2	8.50E+23	2730-4230	10.7	7.94E+10	liquid nickel/iron
Lower Mantle (Mesosphere)	2764.3	426.04	3.09E+23	2000-4000	5	6.17E+10	semi-solid iron/magnesium/silicon
Upper Mantle	3190.34	77.61	3.75E+22	500-900	3.9	9.62E+09	crystalline rocks and hydroxide water in lower transition zone
Asthenosphere	3267.95	31.83	1.26E+22	1500	3.3	3.81E+09	liquid rock
Lithosphere	3299.78	21.22	6.74E+21	200-600	2.7	2.50E+09	granite/basalt
hollow world	3321	1097				1.19E+11
Center	6370

It occurs to me now that I should probably do a similar analysis for the layers of the atmosphere inside the hollow world based on the increase of anti-gravity the closer you get to the red sun. Because of the exponential increases in anti-gravity the atmosphere would essentially give way to vacuum at a much lower altitude than the outer world meaning there is only a thin layer of breathable air near the surface of the hollow world. I will leave that as an exercise for some other time.

Also in my model I plan to not have the curving polar openings(I will probably keep an entrance at the poles, but they will be smaller and harder to find akin to Journey to the Center of the Earth) creating a physical barrier, but if you want to maintain the polar openings as per canon then the air would have the highest pressures in the center of the gaps where the center of gravity is (maybe this is the deal Urt made with the immortals, it created the polar openings in its shell to facilitate the exchange of atmosphere, see below). 

On Megaliths

I also have some idea of how all this can tie into the nature and life cycle of the megalith called Urt. This requires some revision from the Immortals set, but I think it still maintains the central idea presented there.

A megalith is different from a standard planet in a structural nature as described above. A naturally formed planet follows all the forms and patterns that we currently know about planetary bodies, while a megalith is hollow on the inside but appears to be a normal planet from the outside. The red antimatter sun in the center of the hollow is the heart of the megalith. The floating continents inside can be equated to different organs and they are held aloft and in their orbits through unknown forces which the megalith controls. These continents are rendered virtually uninhabitable by humanoids because of the intense gravity at high altitudes and the thin atmosphere.

There are a few option for defining the the role of the planetary mass of the megalith. This could be the standard explanation offered in the Immortal set, but I'm leaning toward something else. The main mass of the 'planet' forms a protective shell, and the mantle and crust, etc are simply matter that has accreted on the outside of the shell during the active phase of the megaliths lifecycle (by the way, the active/dormant phases should be measured in the millions of years at a minimum). During the early stages of a megalith's active phase they seek out(yes they are capable of voluntary movement while active) matter which heats up from impact and pressure and gradually cools down forming a crust and assimilating into the different layers of the shell. So during a megalith's active phase they may have grown significantly. After the crust has cooled down and stabilized it will stabilize it's orbit around a stable star and begin cultivating life on their outer surface.

Burrowers(annelids, purple worms, etc.) are part of the biological systems of the megalith. They have a symbiotic relationship with the megalith and can be compared to the bacteria in the human gastro-intestinal system. They consume any radioactive matter that has accreted in the shell of the megalith and convert this into energy which fuels the heart of the megalith, thus allowing the megalith to remain active.  A megalith will remain active usually until the all significant sources of radioactive matter in the shell have been consumed.

Towards the end of the active phase a megalith will use the last of its energy to leave its solar system on a trajectory towards a source of fuel, and then become dormant until it reaches that destination. When the megalith becomes dormant the 'heart' cools down to a point that the anti-gravity generated by it is practically zero causing the shell to collapse inward under its own weight. While dormant the megalith will not act in any way(no involuntary volcano eruptions or earthquakes), though it may be awakened by drastic changes in its environment such as disruptions to it's path or significant impacts on it's shell. After lying dormant for a period of time, it's energy reserves will be replenished, the heart will become active again and the megalith repeats the cycle.

Urt is a special case, normally the interior of the megalith is uninhabitable with no atmosphere whatsoever, but the Immortals in the hollow world council have persuaded Urt to allow life to grow within the shell as well as on it's crust. Urt carefully regulates the temperature and gravity and atmosphere of the inner world to best support life. Also some Immortals suspect Mars of being a dormant megalith that was captured by the gravity well of the sun. Venus is thought to be a normal planet because of the chaotic and hostile nature of it's atmosphere. Megalith's tend to carefully manicure their surface to encourage the evolution of life, and all evidence points to an uncontrolled surface environment on Venus.

Degrees of Difficulty

So listening to a the revisionist history podcast by Malcolm gladwell, and he brings up the subject of degrees of difficulty as used in judging professional divers. I think it is relevant to any rpg using Difficulty Checks or assigning Target Numbers to tasks. The difference between the difficulty of doing a thing or doing a thing well are also discussed,  and I think that is extremely relevant to being a DM; it's easy to start DMing, but really hard to be a good DM. The relevant part of conversation starts at about the 15 minute mark

The Games We Played . . .

Just felt like showing off what my father had available when he ran games for me and my siblings. If you want to more in depth discussions about these, you can listen to our interviews here: anchor.fm/dndnduncanites

the rule sets,  my dad used the cook/marsh expert set mostly, while I used the Mentzer book when I started dming. My dad allowed mystics and druids at 1st level,  we used weapon mastery and the other rules from the companion and master set starting from level 1, we never did get any characters up to companion levels.

the supplements, ac2 had an adventure we played quite a bit, and ac7 also had an adventure which we never played. Didn't really use the provided character sheets, we had a master sheet that was a photocopy of the one provided in the basic set with some modifications. Some of the items in marvelous magic saw occasional use, but the 2 screens and the creature catalog(more for the index than anything else) saw the most use and are literally falling apart. 

the basic modules.  Sword of justice is from Dragon magazine and is just as iconic to me as B1 or B2. B10 was the beginning of a series of adventures that led to the founding of the Dragon City. 

The expert modules. Isle of dread was a favorite, we really only used the blue one, so the orange is almost like new still. X4 and X5 led to a long excursion into the Western deserts. My dad ran O1 at least once for each of us kids. Lots of fun memories watching others play through it while we knew all the spoilers.

the high level modules we either never played or only did as one-shots.

The version of the Known World my dad was subsequently quite different than what was presented in the gazetteer line as he only had these modules. My version of Mystara/ the Known World is a continuation of where my dad left off, with some modifications based on my personal preferences.  I do use material from the gazetteers and other published material but the results of our early adventures takes precedence over all else.

Weather Generation

I've intended to create weather generation tables for each climate and game mechanic effects of weather for awhile now. I originally intended to do something like is presented in B10 Night's Dark Terror, with a % roll of 4 possible conditions each for clouds, rain, temperature, and wind. I had extrapolated from the calendar in the modular to create a chart that covered the whole year. Well, I've decided not to use that model. 

While rereading the OSE books, it occurred to me that a partial weather table was already built into the expert game with the variable wind conditions chart and that the chart could be repurposed for other weather conditions. So here I present four weather tables that use the 2d6 roll and descriptions of how PCs are affected by the weather.

Cloud Cover

2 Fog 

no precipitation; roll 1d4 to determine density of the fog: 

1) visibility limited to 1/2 mile; chances of getting lost increase by 1/6; chances of being surprised increase by 1/6; apply a -1 to To Hit rolls for ranged attacks

2) visibility limited to 400 yards; chances of getting lost increase by 2/6; chances of being surprised increase by 2/6; apply a -2 to To Hit rolls for ranged attacks

3)  visibility limited to 200 yards; chances of getting lost increase by 3/6; chances of being surprised increase by 3/6; apply a -3 to To Hit rolls for ranged attacks

4)  visibility limited to 50 yards; chances of getting lost increase by 4/6; chances of being surprised increase by 4/6; apply a -4 to To Hit rolls for ranged attacks

3-5 Clear 

apply a -1 to next Cloud Cover roll; no precipitation; for every hour the skin of a PC is exposed, they take 1d4 damage from sunburn

6-8 Partly Cloudy 

apply a -4 to the Precipitation table

9-11 Cloudy 

apply a +1 to next Cloud Cover roll; apply a -1 to the Precipitation table

12 Overcast 

apply a +2 to next Cloud Cover roll; roll on the Precipitation table; chances of getting lost increase by 1/6

Precipitation

After the type of precipitation is rolled for it may be necessary to determine what time of day the precipitation occurs; this can be done by rolling randomly by the hour or 4-hour watch, or may be determined as the DM sees fit. The DM may also want to roll on this table more than once a day, at every hourly interval, 4-hour watch, or directly after one precipitation type ends.

The PCs may take precautions to remain dry; this may consist of waterproof clothing, taking shelter under partial cover (large trees) or indoors. If the clothes a PC is wearing become completely soaked, that PC suffers the effects of one Temperature grade lower than reality.

Catching Cold: If a PC catches a cold they suffer a -1 on all To Hit rolls, a -1 INT, and a -1 CON. Also any PC with a cold cannot sneak or hide because of their constant sniffling. A cold will last for 2d6 + 2 days; on the last day another CON check is must be made or the cold persists for another 2d6 + 2 days. For each week a PC suffers from a cold there is a cumulative 10% chance that they will contract pneumonia. A PC who is sick with pneumonia suffers an additional -4 STR, -4 DEX, -1 WIS, and -1 CON. Pneumonia lasts for 1d10 + 10 days, at which point the PC must make a CON check or die.

Freezing Temperatures: If the temperature is Cold, Frigid, or Sub-Zero, then precipitation may manifest as snow, hail, freezing rain aka sleet, or a mixture of freezing rain and hail. 

    The amount of snowfall resulting from any level of precipitation is listed as Snow (x"). Snow 3-12" deep will decrease movement by 10/20/30/50; Snow 12-24" deep will decrease movement by 20/40/60/100; Snow greater than 2' deep will decrease movement by 30/60/90/150. 

    Hail will cause damage to any exposed PCs, this is denoted as Hail (x/time).  

    Sleet will cause a PC to suffer the effects of one Temperature grade lower than reality if their skin is exposed to the freezing rain.

    A mixture of freezing rain and hail cause the normal effects of sleet, but only half the normal damage caused by hail.

    Roll a 1d20 and refer to the table below to determine what type of frozen precipitation occurs

Temperature Grade    Rain    Sleet     Mix        Hail    Snow

Cold                               1-15    16-17    18-19       20          -

Frigid or Sub-Zero            -            -           -         17-20    1-16

Thunderstorms: apply +1d4 to Winds table; apply +4 to frozen precipitation roll; 1% chance per hour an exposed PC is struck by lightning causing 8d6 damage to anyone within 10 feet, this is increased to 5% if the PC is holding or wearing metal

2 No Precipitation 

3-5 Sprinkles 

1% chance of Thunderstorm; lasts for 1d6 x 5 minutes; can stay dry if shelter under partial cover; visibility limited to 1/2 mile; chances of being surprised increase by 1/6; must succeed on a CON check or catch Cold; fire is extinguished on a 3/6; Snow (light dusting); Hail (1/30 minutes)

6-8 Showers  

apply a +1 to Winds table; 20% chance of Thunderstorm; lasts for 2d6 x 10 minutes; can stay dry for up to 70 minutes if shelter under partial cover; visibility limited to 400 yards; chances of being surprised increase by 2/6; must succeed on a CON check each hour or catch Cold; chances of getting lost increase by 1/6; fire is extinguished; Snow (1"); Hail (1d4/30 minutes)

9-11 Downpour  

apply a +2 to Winds table; 80% chance of Thunderstorm; lasts for 1d6 +1 hours; completely soaked in 10 minutes; visibility limited to 400 yards; chances of being surprised increase by 2/6; must succeed on a CON check each hour or catch Cold; chances of getting lost increase by 1/6; fire is extinguished; movement decreased by 1/4; Snow (2d6"); Hail (1d8/10 minutes)

12 Drizzle 

5% chance of Thunderstorm; lasts for 24 hours, at the end of 24 hours there is 2/6 chance that Overcast clouds and Drizzle continue for another day; completely soaked in 30 minutes; visibility limited to 400 yards; chances of being surprised increase by 2/6; must succeed on a CON check each hour or catch Cold; chances of getting lost increase by 1/6; fire is extinguished; movement decreased by 1/4; Snow (1d4"); Hail (1d4/30 minutes)

 

Wind Speed

2 Calm

apply a -4 to next Wind Speed roll; Sailing speed reduced to 0; oarsmen gain 3 times normal fatigue

3 Faint Breeze

apply a -3 to next Wind Speed roll; Sailing speed reduced by 2/3

4 Gentle Breeze

apply a -2 to next Wind Speed roll; Sailing speed reduced by 1/2; fog or smoke is cleared in 1 hour

5 Brisk Breeze

apply a -1 to next Wind Speed roll; apply a -1 to the Temperature Grade table; Sailing speed reduced by 1/3; fog or smoke is cleared in 30 minutes; apply a penalty for ranged attack rolls of 0/-1/-2 for short, medium, and long ranges

6-8  Fresh Breeze

apply a -1 to the Temperature Grade table; fog or smoke is cleared in 10 minutes; apply a penalty for ranged attack rolls of -1/-2/-4 for short, medium, and long ranges; fire is extinguished on a 3/6

9 Strong Breeze

apply a +1 to next Wind Speed roll; apply a -1 to the Temperature Grade table; Sailing speed increased by 1/3; fog or smoke is cleared in 1 minute; apply a penalty for ranged attack rolls of -2/-4/-8 for short, medium, and long ranges; fire is extinguished

10 High Wind

apply a +2 to next Wind Speed roll; apply a -1 to the Temperature Grade table; Sailing speed increased by 1/2; fog or smoke is cleared in 1 round; apply a penalty for ranged attack rolls of -4/-8/-16 for short, medium, and long ranges; fire is extinguished; movement(land) against direction of wind decreased by 10/20/30/50

11 Gale

apply a +3 to next Wind Speed roll; apply a -2 to the Temperature Grade table; Sailing speed doubled; fog or smoke is cleared immediately; apply a penalty for ranged attack rolls of -8/-16 for short and medium ranges, missiles will not reach long ranges; fire is extinguished; movement(land) against direction of wind decreased by 30/60/90/150; flying creatures cannot stay aloft more than 10 minutes; Take on Water (10% if seaworthy, 20% if coastal) resulting in movement reduced by 1/3

12 Storm

apply a +4 to next Wind Speed roll; apply a -2 to the Temperature Grade table; Sailing speed tripled; fog or smoke is cleared immediately; apply a penalty for ranged attack rolls of -16 for short ranges, missiles will not reach medium or long ranges; fire is extinguished; movement(land) against direction of wind decreased by 60/120/180/300; flying creatures cannot stay aloft; Take on Water (10% if seaworthy, 20% if coastal) resulting in movement reduced by 1/3, coastal vessels that roll 21-80% are swamped and sink; if in sight of land when storm hits may find safe harbor on 2/6; if encounter land while being driven by storm, 75% chance ship is wrecked

Wind Direction

2 180 degrees 

3 135 degrees left

4 90 degrees left

5 45 degrees left

6-8 Previous/predominant direction

9 45 degrees right

10 90 degrees right

11 135 degrees right

12 180 degrees

Temperature Grade

A PC may mitigate the effects of temperature by putting on or taking off clothing. Any armor or mild winter clothing will increase the effective temperature of a character by 1 grade; heavy winter clothing will increase by 2 grades; and shedding clothes to the point of near nudity will decrease by 1 grade.

Hypothermia: If exposed to cold temperatures a PC must make a CON check at a specified interval of time, denoted as Hypothermia (x units of time). A failed check increases the degree of hypothermia. These are:

Mild - apply a -1 to STR, DEX, and WIS; gain 1 fatigue, gain fatigue at double the normal rate

Moderate - DEX and WIS are halved; gain 4 fatigue, gain fatigue at double the normal rate

Severe - WIS reduced to 3; gain 10 fatigue, gain fatigue at triple the normal rate; 50% chance of passing out immediately and progressing to the next stage 

Apparent Death - unconscious and completely unresponsive, may still be saved if given expert care

Death - the character dies from hypothermia and there is no hope of revival 

Frostbite: If the bare skin of a PC is exposed to cold temperatures for an extended period of time the extremities of that PC will become frostbitten. The time it takes for frostbite to set in is denoted as Frostbite (x time). Each extremity exposed, the hands, the feet, or the face, will receive 1 light wound each when the given time has passed. When the time exposed is doubled each wound will become serious. When the time has tripled each wound will become critical and any adjacent areas, such as the arms or legs, will also receive a light wound from frostbite. When the time has quadrupled the frostbitten extremity will be dead and irretrievable and adjacent areas will continue to suffer the effects of frostbite until the PC is dead or given treatment.

Heat Stroke: If is exposed to high temperatures a PC must make a CON check at a specified interval of time, denoted as Heatstroke (x units of time). A failed check means a character collapses and is disoriented, their WIS and INT being halved, there is also a 50% chance the PC will pass out at this time. If not treated within 30 minutes they will begin losing 1 point of CON every 10 minutes, and die when their CON reaches 0.

2 Sub-Zero

apply a -4 to next Temperature Grade roll; Hypothermia (1 minute); Frostbite (1 turn)

3 Frigid

apply a -2 to next Temperature Grade roll; Hypothermia (3 minutes); Frostbite (3 turns)

4-5 Cold

apply a -1 to next Temperature Grade roll; Hypothermia (1 turn)

6-8 Pleasant

9-10 Warm

apply a +1 to next Temperature Grade roll; double water intake

11 Hot

apply a +2 to next Temperature Grade roll; double water intake; gain 2 times normal fatigue; Heatstroke (1 hour)

12 Scorching

apply a +4 to next Temperature Grade roll; double water intake; gain 4 times normal fatigue;  gain 1 fatigue from resting for 1 hour; Heatstroke (30 minutes)

Wilderness Exploration and Journeys

 For a long time now I've wanted to develop a robust rule-set for traveling on long journeys as opposed to the classical hex crawl of D&D. What I mean by a journey is when the PCs want to go from city A to City B and they take the King's Road or some such. They aren't mapping or getting lost or doing any kind of exploration. They just want to get to the destination, and it is anti climactic to just say you travel for eight days and you get there, or they travel for 3 days, random encounter, travel, for 2 days, random encounter, travel for 3 days and they arrive. It never feels like they actually traveled. 

So towards that end, I've been looking at the house-rules of other people, and most recently examining The One Ring and its 5e adaptation Adventures in Middle-Earth. I was working on converting it wholesale when it hit me: long journeys and detailed wilderness exploration don't need separate rule-sets. Sure I can have a robust travel ruleset that expands on the normal D&D rules by giving PCs tasks of scouting, or foraging, etc, but I can use the same method for both aspects. The only real difference between the two modes of travel is the navigation, or getting lost. Just skip that step when the PCs are on a road or river. Resolving all the travel tasks every day, instead of trying to figure every 3 days or 5 days like TOR, can give the journey a sense of length. Every single day matters. 

I'm also of the opinion that wilderness exploration should not be done on a day by day basis, and even the 4 hour watch may be too long of a time frame, because the PCs are exploring and don't have a destination in mind; if they did they would be following a road or river. But that's beside the point. Journeys require the same day to day resolution of tasks as wilderness exploration, discounting navigation rolls.

Movement Rates for Old School Essentials

Go to the Delvers and Denizens Index

So due to the absurd movement rates provided in classic D&D I'm finally putting forth my own take at revising it for realistic game use. I have done this before for D&D back in high school but I don't particularly like the rates I came up with then, and I also came up with standard rates for my own rpg system based on statistics of real world athletes, but this is for D&D which is an abstract system and I want something that plugs into the D&D rules already there.

I think everyone agrees that moving 120 feet every ten minutes is ridiculously slow if we assume the PCs are actually moving, that is 0.14 miles per hour. Sheesh! Someone didn't do some basic math. So I think we need a basic walking speed that is different. The base encounter speed of 40 feet per round is 2.7 miles an hour which is reasonable for the average person, so lets use that. the other huge problem is the given speed for running during a pursuit in the wilderness (40x3x3(because yards instead of feet) which is 360 ft per round, which can be maintained for 5 minutes! That is 24.5 miles per hours, which is humanly impossible. Usain Bolt reaches an average of like 21 or 22 miles per hour in the 100 meter dash(its been a while since I look up the stats), and he can definitely not do that for 5 minutes. So the running speed needs to be paired down. After talking to my brother, who ran cross country and track in high school and college, and doing some math I came up with different rates of speed for the unencumbered man.

Searching: 120 feet per turn. Causes 1 Fatigue point after 1 hour.

This is the speed at which PCs explore dungeons at a cautious rate, looking for traps, secret doors, listening at doors, mapping, etc. When traveling at this rate a roll to find secret doors and traps is automatically made, and the players do not have to specify that they are searching. This could be interpreted as moving in a straight line down a hall or searching 12 10'x10' squares.

Walking: 40 feet per round (800 yards per turn, 2.7 miles per hour). Causes 1 Fatigue point after 1 hour.

This is your standard walking rate, both indoors and outdoors. This is the rate used for overland movement, and the players can decide when they want to rest.

Hustling: 80 feet per round (1600 yards or 0.91 miles per turn). Causes 1 Fatigue point after 1 turn.

This is the standard combat encounter speed.

Running: 120 feet per round (2400 yards or 1.36 miles per turn). Causes 1 Fatigue point after 20 rounds (3 after 1 turn)

This is your running speed during a pursuit. It can only be sustained for a few minutes. 

Sprinting: 200 feet per round. Causes 1 Fatigue point after 1 round.

This the fastest a PC can go, they are bolting for their life their life here. This is also the speed used during a charge in combat.

The above speeds are for the unencumbered human and elf. Gnomes haves speeds of 30/60/90/150. Each Encumbering Item decreases speed by 10/20/30/50.

Weapon vs Armor Adjustments

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Dan Collins at Delta's D&D Hotspot recently did a statistical deep dive into the weapon vs armor table in the 1e AD&D PHB. Essentially the numbers come from the Chainmail man to man melee table, but Gygax did his math wrong. This is why the numbers in the PHB don't even remotely reflect historical reality. The numbers in Chainmail aren't exactly historically precise either, but at least they feel like they could be reasonable.

In my last effort at putting together houserules for OSE I considered using a weapon vs armor table, but since the one in AD&D is so ridiculous I tried to make my own. To do this I thought about restricting AC just to specific armors as originally intended and have dex add to hp instead of con. However, in the end I decided against it because if AC equaled actual armor types then I would have to redo the AC for every single monster, which I wasn't about to do. So AC is an abstract concept representing how difficult it is, or how much effort is required, to actually cause a significant injury. I tried assigning specific +or- values to hit for each weapon vs armor types, e.g. giving +2 to hit chain for maces to represent the fact that the chain doesn't protect against bludgeoning weapons, but the padding underneath would. I found it too fiddly though, and I ended up giving certain weapons Target Specialization(granting advantage or disadvantage) vs certain armor types, such as chain, or padded, or plate, and this bonus would apply to monsters with a similar type of 'natural armor' like dragon scales, etc. So this is what I came up with:

So spurred on by Dan's analysis and by this post with a further conversion of the chainmail numbers, I decided to do my own take. 

First, since chainmail used a 2d6 as a basis we can assume that a 7 to kill is average. So going from there I extrapolated the probabilities of getting below or above seven and converted that to a bonus or penalty, numbers above 7 result in some -%, and numbers below seven result in some positive %:

then I took the numbers needed to roll a hit in Chainmail, resulting in a kill, and converted the 2d6 number to a % bonus or penalty. For the missile weapons I took an average value of all three ranges:

From this it's simply a matter of dividing by five to convert to a d20 to hit roll as +or-1 is the same as +or-5%. I ignored the values for shields as I have my own rules for shields and their use is very situational, as opposed to armor which is always there. So this is what the Chainmail values to kill would be converted as adjustments to hit:

Some of the numbers make sense to me, like the huge penalties to bows vs plate; its not really any of the numbers that bother me, but comparing them to each other. What's the difference bewteen a horse bow and a composite bow? I'm assuming a horse bow implies a bow used by the different steppe cultures, but all those bows were composite, so why have two listings? And why is the 2-handed sword better at killing someone in armor than the lance? That is so ridiculous. and why are the polearm and halbard so drastically different? did Gary not know that a halbard is a polearm? And the Spear, I mean come on. And the drastic differences between a mace, morningstar, and flail; why would weapons that are so similar have such huge differences in effectiveness? 

So, I think we can conclude that Gary set a few of these numbers based on what he knew about history and pulled the rest out of his ass. I think I'm going to stick with advantage/disadvantage as it better represents the vicissitudes of combat, rather than try to work this table so it represents reality.