In this step, we will determine the world’s average surface temperature. This quantity is strongly dependent on the world’s blackbody temperature, established in Step Nineteen. However, worlds are not perfect thermal blackbodies, so this estimate will need to be adjusted.

In particular, some of the primary star’s energy input will be reflected away from the world’s surface, making no contribution to its heat budget. This factor is related to the world’s albedo, a measure of its reflectivity. Meanwhile, a world with an atmosphere containing certain gaseous components (the so-called greenhouse gases) will tend to retain some heat, warming the surface in a phenomenon called the greenhouse effect. These two factors are critical to any estimate of a world’s average surface temperature.

Unfortunately, a world’s albedo and greenhouse effect are dependent on a swarm of factors, many of which are poorly understood or beyond the scope of this design sequence. In fact, the values of these factors can change drastically over time on a single world. We will therefore determine a world’s albedo and greenhouse effect at random, or by designer choice within a range of plausible results, and then determine some of the consequences of those random choices in later steps.

Procedure

To compute a world’s average surface temperature, determine its albedo and greenhouse effect, then use these two factors to modify the blackbody temperature.

Albedo

To determine a world’s albedo at random, begin by referring to the following table.

Worlds with Class V atmospheres are a special case. The albedo of these worlds is strongly dependent on whether their surfaces are subject to volcanic activity. Even worlds with Massive presence of water, covered with thick layers of ice, may have cryovolcanoes which constantly refresh the icy surface. Check to see whether any such world falls into any of the following cases.

• If the world has a Molten or Soft lithosphere, add 0.5 to the base albedo.
• If the world has an Early or Mature Plate lithosphere, add 0.3 to the base albedo.
• If the world has an Ancient Plate lithosphere with Mobile plate tectonics, add 0.3 to the base albedo.
• If the world has an Ancient Plate lithosphere with Fixed plate tectonics, or it has a Solid lithosphere, and its blackbody temperature is lower than 80 K, add 0.3 to the base albedo.

Finally, roll 3d6, multiply the result by 0.01, and add it to the base albedo. The final result is the world’s actual albedo.

Greenhouse Effect

The greenhouse effect for a given world is measured in kelvins. The procedure for estimating the greenhouse effect depends on its atmosphere type. Apply the appropriate case from the following.

Class I Atmosphere

To determine the greenhouse effect for a Venus-type atmosphere, compute the following:

Here, M is the atmospheric mass of the world, and G is the world’s greenhouse effect in kelvins. Round the result to the nearest integer.

Class II Atmosphere

To determine the greenhouse effect for a Titan-type atmosphere, compute the following:

Here, M is the atmospheric mass of the world, and G is the world’s greenhouse effect in kelvins. Round the result to the nearest integer.

Class III Atmosphere

To determine the greenhouse effect for an Earth-type atmosphere, compute the following:

Here, M is the atmospheric mass of the world, and G is the world’s greenhouse effect in kelvins. Feel free to adjust the result by up to 1.5 times the atmospheric mass in either direction. Round the result to the nearest integer.

Class IV Atmosphere

To determine the greenhouse effect for a Mars-type atmosphere, roll 1d6-4 (minimum 0). The result is the world’s greenhouse effect in kelvins.

Class V Atmosphere

A Class-V (Luna-type) atmosphere is far too thin to create a significant greenhouse effect. The greenhouse effect in this case is always 0 kelvins.

Average Surface Temperature

With the world’s albedo and greenhouse effect established, the average surface temperature can be computed. Evaluate the following:

Here, T is the average surface temperature in kelvins, B is the world’s blackbody temperature, A is the world’s albedo, and G is the world’s greenhouse effect in kelvins.

Examples

Arcadia IV has a blackbody temperature of 281 K, a Class III atmosphere with atmospheric mass of 0.9, and Extensive water.

Alice begins by estimating the planet’s albedo. She rolls 3d6, gets a result of 11, multiplies that by 0.01 and adds it to the base albedo of 0.22. Arcadia IV has an albedo of 0.33, and so is slightly more reflective than Earth. This is most likely due to high-altitude clouds, covering a slightly greater portion of the planet’s surface than on Earth.

Alice then estimates the planet’s greenhouse effect. She rolls 3d6, gets a result of 12, multiplies that by 3 and then 0.9, and gets a final result of 32.4. She decides not to adjust this result and rounds it off to 32 kelvins, indicating a slightly weaker greenhouse effect than that of present-day Earth (a little over 33 kelvins).

Computing the planet’s average surface temperature, Alice gets:

This result is almost identical to Earth’s average surface temperature in the present day (about 287 K).

Arcadia V has a blackbody temperature of 226 K, a Class III atmosphere with an atmospheric mass of 0.7, and only has Moderate water.

To estimate the planet’s albedo, Alice rolls 3d6, gets a result of 14, multiplies that by 0.01 and adds it to the base albedo of 0.19. By an odd coincidence, Arcadia V also has an albedo of 0.33, although this is likely due to extensive sheets of ice and snow on the surface rather than high-altitude clouds.

Alice then moves on to the planet’s greenhouse effect. She rolls 3d6, gets a result of 8, multiplies that by 3 and then 0.7, and gets a final result of 16.8. Again, she decides not to adjust this figure and rounds it up to 17 kelvins. Arcadia V has a noticeably weaker greenhouse effect than Earth.

Computing the planet’s average surface temperature, Alice gets:

Arcadia V is bitterly cold, with surface temperatures averaging 221 K (about -52° C), comparable to winter temperatures in Antarctica on Earth. The planet is a little warmer than Mars, however (average surface temperature about 210 K).