[Space] Weather on planets & moons

[David Johnston2]

Originally Posted by Brett

[I'm trying to work out how to do a "near enough is good enough" estimate of three aspects of the weather on worlds generated using the world generation sequence in GURPS Space 4th edition. I want to be able to spot weather effects severe enough to challenge habitability.

1. Daily temperature variation
   My idea here is that it ought to be proportional to the day length and the average temperature of the planet, and inversely related to the Greenhouse factor (which depends on the thickness and composition of the atmosphere). But it ought to be capped for planets that rotate slowly: no greater than the difference between dayside and nightside temperature for such a planet in such an orbit if it were tide-locked (and without atmospheric collapse).

1. Well first let's have a look at diurnal temperature variation on Earth. Wikipedia tells us that that the degree to which it occurs depends on elevation and proximity to water. Thus the Snake River Plain which is high and dry has something like a 45 degree (celsius) variation in summer, while Washington DC has a 8 degree shift. (Note that the coldest time is about an hour after sunrise.) That gives us a rough range. After that we can posit that, assuming the same greenhouse effect, atmospheric density, and albedo (which in fact will _never_ be the case, Earth not even being consistent with itself over timeframes of more than a few centuries), that the change in variation between day and night will be directly proportional to the change in day length. Thus if you have a 36 hour day it would not be unreasonable for an otherwise Earthlike planet to have a change of 50 to 70 degrees between day and night, in an inland desert or semi-desert region. Lower atmospheric density and greenhouse effect will increase the variation, while higher density and greenhouse will do the opposite.
   
   
   

   Latitudinal temperature variation
   How much warmer are the tropics than the average? How much cooler are the poles than the average? I reckon that the effectiveness with which the atmosphere transports heat from equator to poles is proportional to the temperature difference driving that transport, and that therefore the temperature difference ought to be proportional to the square root of (average temperature * diameter / thickness of atmosphere). But things will be different on a tide-locked planet or one rotating so slowly that heat is drawn to the terminator rather than the poles, i.e. distributed by atmospheric circulation faster than by planetary rotation.

   Hydrosphere would be a massive player in heat circulation.
   
   

   Windiness
   I reckon that this is proportional to the square root of (daily temperature variation squared plus twice the difference between equator and poles) times the relative strength of the Coriolis Effect (which is the diameter of the world divided by its rotational period).

Comments?

I think actually trying to put numbers to something like that rather than going vague guidelines is a doomed idea.

[su_liam]

Originally Posted by David Johnston2

I think actually trying to put numbers to something like that rather than going vague guidelines is a doomed idea.

Probably. Good exercise, though.

[martinl]

Originally Posted by Brett

Yes. But mapping every planet and doing a full circulation model is a task for a climatologist with a supercomputer.

Yet without something along those lines you are sharply limited. I am not saying you must, or even that you should. Just that you seem to want more detail than can be realistically generated by the model you have.

(I'm not saying you shouldn't go ahead and swag it either.)

[Agemegos]

Originally Posted by martinl

Yet without something along those lines you are sharply limited.

I don't mind being sharply limited. After all, I put up with being sharply limited as the the range of greenhouse factors for garden worlds. I have limited intentions.

I am concerned with the effect of gross weatheriness on habitability. As it stands GURPS Space blithely assigns a Habitability of 8 (full habitability) to worlds with days of 400+ or even 4,000+ hours, on the basis of their average temperatures, without accounting at all from what a bound to be extreme dialy temperature variations and howling winds. That's not satisfactory. In his classic Habitable Planets for Man Stephen H Dole supposed (for convenience) an abrupt cut-off at a 96-hour day, on account of the daily temperature variation and the effect of the long night on the growth of crops. Dole counts planets with days less than 96 habitable if otherwise suitable, and planets with days longer than 96 hours uninhabitable. Even if I only sort weatheriness into rough categories like those GURPS Space uses for volcanism and tectonics I can do better than that.

Originally Posted by David Johnston2

Hydrosphere would be a massive player in heat circulation.

On the annual timescale, yes. Does anyone happen to know the relative contribution of winds and ocean currents in the latitudiinal transport of heat on Earth?

I think actually trying to put numbers to something like that rather than going vague guidelines is a doomed idea.

Quixotry has run in my family since 1265.

[martinl]

Originally Posted by Brett

Quixotry has run in my family since 1265.

This "wind mill" is a few OOMs larger than is traditional.

[David Johnston2]

Originally Posted by Brett

I don't mind being sharply limited. After all, I put up with being sharply limited as the the range of greenhouse factors for garden worlds. I have limited intentions.

I am concerned with the effect of gross weatheriness on habitability. As it stands GURPS Space blithely assigns a Habitability of 8 (full habitability) to worlds with days of 400+ or even 4,000+ hours, on the basis of their average temperatures, without accounting at all from what a bound to be extreme dialy temperature variations and howling winds. .

Well yes, but if they expect extreme daily temperature variations and howling winds, countermeasures can be taken if only in the form of the major population clusters being in coastal areas that can make use of the oceanic heat sink with low profile architecture, possibly subterreanian architecture. Vulcanism is less predictable and harder to protect against.

[IrishRover]

The density of the atmosphere is going to be VERY important for many reasons. A denser atmoshpre holds heat better, ovrall, so the variations might be less sever if thre's more air. It might also be able to hold more water--resulting in bigger downpours where it does cool off.

Also, a 75 MPH (Hurricane force) wind is no big deal here on earth--we get that sort of wind force a few times each year, with rain mixed in. But--if there's a lot more air, it has a lot more force for a given windspeed.

Oh--denser atmospheres means that it's easier to generate aerodynamic lift, too--and harder if there's a lot less air.

[Agemegos]

Originally Posted by David Johnston2

Well yes, but if they expect extreme daily temperature variations and howling winds, countermeasures can be taken if only in the form of the major population clusters being in coastal areas that can make use of the oceanic heat sink with low profile architecture, possibly subterreanian architecture.

Yes, but that makes only a fraction of the land surface available for habitation and for growing crops. Which means diminished carrying capacity. Which means that the Habitability rating ought to be diminished. Which is what I am working towards.

Vulcanism is less predictable and harder to protect against.

I don't think so. Volcanoes tend to stay put for quite a long time, and they only challenge habitability for a tiny fraction of Earth's surface or for a very tiny fraction of the time. Ten hours of hurricane-force cold dry winds twice every 200 hours, everywhere would be a much more serious challenge than ten times Earth's volcanism.

Originally Posted by martinl

This "wind mill" is a few OOMs larger than is traditional.

I don't think you can judge that without knowing what sort of quixotries my family got up to. A ballpark estimate of the violence of weather on a planet does not seem all that dangerous.

I am not looking for the kind of sensitivity that climatologists need to work out whether Greenhouse Effect warming will be 4.7 K or 3.3 K. I only want to know whether the daily temperature variation is large enough to preclude agriculture or not. This is analogous to a good-enough-for-gaming estimate of how ST, HP, and spedd ought to scale with SM, not a complete biomechanical model of human gait and throwing/striking actions.

Anyway. The two of you reckon that I am on a fool's errand, that I can't produce estimates that at better than nothing without unreasonable effort. Got it. Message received and understood. I'm going to try anyway.

[Trachmyr]

I for one would be more than happy with a Scale for Windiness akin to the Tectonics & Volcanism listed in Space. And a random roll table (with modifiers for Hydrodynamics, Diameter, Length of day, Atmospheric Mass, Axial Tilt) to give Day/Night, Seasonal and Pole/Equator temperature coefficients would be fine by me.

Much like the Boxed text in SPACE "Turning Up the Heat", you can give modifier ranges for things like mountains, terrain and other features that aren't expressed in SPACE. Giving the GM the power to modify scores within a guideline.

And yes, the Day/Night coefficients given for Tide locked worlds seem a good place to start.

As for Habitability Score... I think you have up to +2 to play with before needing to alter the table, if you Hijack the world's resource modifier, and use only habitability for Carrying Capacity. Resource modifier can then be used as a modifier for average wealth instead.

-------

I will also ask again for a simple range of magenteosphere modifiers to Habitibility, using a setup similar to Tectonics/Volcanism, taking into account Density and Rotation speed.

[martinl]

Originally Posted by Brett

I don't think you can judge that without knowing what sort of quixotries my family got up to. A ballpark estimate of the violence of weather on a planet does not seem all that dangerous.

"Wind mill" referred to the prototypical windmill Mr. Quixote himself had a grievance with, which is indeed a few OOMs smaller than a planetary climate, which is arguably another kind of "wind mill." Now having beaten the wordplay in question nigh on to death with a rock, I will allow it to expire quietly in this ditch over here. Don't mind the whimpering.

I only want to know whether the daily temperature variation is large enough to preclude agriculture or not.

Flat Black has plants that grow PVC plumbing. Adapting crops to longer day night cycles, extremes of temperature, or high wind force seems quite doable, especially since there are vegetated areas of Earth where similar conditions prevail.

Anyway. The two of you reckon that I am on a fool's errand, that I can't produce estimates that at better than nothing without unreasonable effort. Got it. Message received and understood. I'm going to try anyway.

I feel no urge to stop you, and in fact don't think saying "what you're trying to do is hard because of X, Y, and Z" is an inherently hostile or obstructive act. I should probably know better.

Please, carry on.