One of the things I need is a way to get a feel for the weather on a world. Here are some loose approximations I use to help with that -- and note that I'm continually revising these rules to be easier or more precise, in turns, based on my mood.

0.

1.

Density: Unless otherwise convinced, assume a density of 1. Metallic cores are denser (1.3 to 2).

Albedo ("A") depends on continents, hydrographics, and trade codes. I typically begin with A = [1 - Hydrographics/10], minimum 0.1. I then modify it based on trade codes: for each of Ag, In, Ri, multiply A by 0.9. For each of De, Ic, Na, Ni, Po, multiply A by 1.1. If the [world size + atmosphere] is 9 or less, multiply Albedo by 1.1. Then modify by Flux%. Maximum Albedo is 0.9, and minimum Albedo is 0.1.

Greenhouse Effect ("G") depends on atmosphere, hydrographics, and trade codes, but is otherwise a random value between 0.0 and 2... or even higher. I usually set G = [Atmosphere + Hydrographics]/10, then modify it further, based on trade codes: for each of Ag, In, multiply G by 1.1. For each of De, Ic, multiply G by 0.9. Then modify by Flux%.

Atmospheric Pressure ("Pressure") lessens

Planetary Eccentricity ("Ecc") varies the world orbit, which in turn affects temperatures and weather. I typically assign eccentricity equal to

Axial Tilt ("Tilt") creates a

2.

I label this "HZ Orbit", in AU.

Next, determine and record the mainworld's orbit. This might or might not be the same as the HZ Orbit -- note that most mainworlds in the Traveller Universe tend to be close to the HZ Orbit, regardless of its suitability for human life. If the world has a breathable standard atmosphere, or is a Ga or Ag world, it's likely in the HZ Orbit. If it's a vacuum world, it might be anywhere. You get the idea.

I label this "World Orbit", in AU.

Finally, determine the mainworld Temperature Mod. It's used for calculating world surface temperatures.

I label this "Temperature Mod".

"Giants" includes I, I, III, and even IV. To determine the HZ Orbit for a giant, roll 2D. The result is the HZ Orbit number. If the result is less than 7, then set the result to 8. Values greater than 12 are by referee fiat.

3.

Day Length Modification. If Density is other than earthlike, multiply it in as well.

Year Modification. I modify the year length due to the sub-magnitude of the star, but at the moment I haven't got a clever-but-simple rule for handling that.

4.

Now I'm ready to figure out details that pertain to the weather.

If you are using Planetary Eccentricity, you will have two additional Blackbody Temperatures -- one for the extreme summer season (multiply by [1 + Eccentricity]), and one for the extreme winter season (multiply by [1 - Eccentricity]).

If you're using Pressure, you'll have to wait, because I don't have a properly bastardized rule for that yet.

Axial Tilt was already calculated directly in degrees Celsius, and represents latitudinal temperature variation. The greater the value, the further down from the poles (in degrees of latitude) this variation is felt.

0.

**Flux**. For any given value, I may introduce Flux,*for example as a percentage change*. So for example, if a value is 360, I may instead calculate 360 + Flux% (resulting in 342 to 378). If a value is 0.4, I may instead calculate 0.4 + Flux% (resulting in 0.38 to 0.42). Some values might vary by Flux x 10% -- orbit numbers, for example.1.

**World Data**. Determine more facts about the world.Density: Unless otherwise convinced, assume a density of 1. Metallic cores are denser (1.3 to 2).

Albedo ("A") depends on continents, hydrographics, and trade codes. I typically begin with A = [1 - Hydrographics/10], minimum 0.1. I then modify it based on trade codes: for each of Ag, In, Ri, multiply A by 0.9. For each of De, Ic, Na, Ni, Po, multiply A by 1.1. If the [world size + atmosphere] is 9 or less, multiply Albedo by 1.1. Then modify by Flux%. Maximum Albedo is 0.9, and minimum Albedo is 0.1.

Greenhouse Effect ("G") depends on atmosphere, hydrographics, and trade codes, but is otherwise a random value between 0.0 and 2... or even higher. I usually set G = [Atmosphere + Hydrographics]/10, then modify it further, based on trade codes: for each of Ag, In, multiply G by 1.1. For each of De, Ic, multiply G by 0.9. Then modify by Flux%.

Atmospheric Pressure ("Pressure") lessens

**day-night**temperature variations. I typically set Pressure = [Atmosphere / 8] + Flux%.Planetary Eccentricity ("Ecc") varies the world orbit, which in turn affects temperatures and weather. I typically assign eccentricity equal to

**[Flux x Flux]%**.Axial Tilt ("Tilt") creates a

**latitudinal**temperature variation, starting from the poles. More importantly, the size of the axial tilt directly affects atmospheric turbulence and weather patterns. I typically assign tilt equal to [5 x 2D] + [Flux x 2] degrees.2.

**Orbital Data**. Now's the time to gather orbital data. We need three pieces: first, the Habitable Zone (HZ) Orbit. Happily, the star's class maps relatively cleanly to the habitable zone's orbit. So, figure out the HZ Orbit, based on the primary, using my handy dandy table.*Note: HZ Orbit is a proxy for stellar mass (divided by a large constant), and therefore becomes useful when calculating world surface temperature*.I label this "HZ Orbit", in AU.

Next, determine and record the mainworld's orbit. This might or might not be the same as the HZ Orbit -- note that most mainworlds in the Traveller Universe tend to be close to the HZ Orbit, regardless of its suitability for human life. If the world has a breathable standard atmosphere, or is a Ga or Ag world, it's likely in the HZ Orbit. If it's a vacuum world, it might be anywhere. You get the idea.

I label this "World Orbit", in AU.

Finally, determine the mainworld Temperature Mod. It's used for calculating world surface temperatures.

*Note: "Temperature Mod" is a value used to replace formulas requring Luminosity, with the approximate value being 333 / sqrt( Orbit ), where Orbit is a proxy for stellar mass (divided by a large constant), as above.*I label this "Temperature Mod".

*Rob's Handy Dandy Multipurpose Star Table*
Code:

```
Orbit AU Year Temperature Mod Habitable Zone Star
1 0.4 58d 500 M V
2 0.7 178d 400 K V
3 1.0 365d 330 G V
4 1.6 935d 250 F V
5 2.8 7y 200 A V
6 5.2 27y 160
7 10 100y 100 Some Giants
8 20 400y 70 *Most Giants*
9 40 1600y 50 Some Giants
10 77 5900y 40 Few Giants
11 154 24ky 30 Few Giants
12 308 95ky 20 Few Giants
13 615 378ky 15 Few Giants
14 1230 1.5my 10 Few Giants
15 2500 6my 5 Few Giants
```

"Giants" includes I, I, III, and even IV. To determine the HZ Orbit for a giant, roll 2D. The result is the HZ Orbit number. If the result is less than 7, then set the result to 8. Values greater than 12 are by referee fiat.

3.

**Day and Year Length**. Now we can figure these out.**Year length**(years) = (World Orbit)^2. Default year lengths are pre-computed in my handy dandy table.**Day length**(hours) = 2d6 x 4 - 3 + HZ Orbit / World Orbit.Day Length Modification. If Density is other than earthlike, multiply it in as well.

Year Modification. I modify the year length due to the sub-magnitude of the star, but at the moment I haven't got a clever-but-simple rule for handling that.

4.

**Temperatures**Now I'm ready to figure out details that pertain to the weather.

**Blackbody Temp**(K) = Tbb = (HZ Orbit) x (1-A) x Temperature Mod.**Average Temp**= Tavg = Tbb x G - 273, in Celsius. See Note 5.**Hadley Latitude**= sqrt( Tbb x World Size ) x 2.6. See Note 6.**Note 5**. This number, of course, indicates what the tropics are like -- hot or cold. If cold, then we know we have a Tundric world on our hands. It super hot, then we may have a Hellworld, or worse.**Note 6**. The Hadley Cell lessens the day-night temperature variation, and also tells us the width of the temperature bands.. The result is in degrees, and typically should be less than 90. This cell also has a random element and additional considerations, but this is one of the important values. The larger the number, the wider the tropics extend outward, and so on. A Hadley Cell of 90 degrees means the average temperature is the average for both day and night, and all over the world, including the poles.If you are using Planetary Eccentricity, you will have two additional Blackbody Temperatures -- one for the extreme summer season (multiply by [1 + Eccentricity]), and one for the extreme winter season (multiply by [1 - Eccentricity]).

If you're using Pressure, you'll have to wait, because I don't have a properly bastardized rule for that yet.

Axial Tilt was already calculated directly in degrees Celsius, and represents latitudinal temperature variation. The greater the value, the further down from the poles (in degrees of latitude) this variation is felt.

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