FOUR/1 Base Temperature

STEP ONE: Calculate base temperature from 4.1.1. If you are not generating a terrestrial world or chunk you are finished, otherwise continue to FOUR/2.

Table 4.1.1 Base Temperature

T (in Kelvin) =   255
(D / L0.5)0.5

... where: D is the orbital distance (in AU)
L is the star's current luminosity

(Subtract 273 from T to convert from Kelvin to Celsius.)


The temperature here is based upon rather earth-like planetary albedo and do not include the greenhouse effect (which raises Earth's temperature by about 33 degrees). This figure (not adjusted for albedo and greenhouse effect) is a decent enough approximate to decide what atmosphere and hydrosphere a world would have. Greenhouse can only warm the world, but the albedo may cool it of or warm it. Earth's base temperature is about 255°K.


The temperature also represent solar infall, of course. For stars hotter than the sun, the peak frequency will be shifted towards the ultraviolet, while for coolers star it shift toward the infrared.


Wien's law gives us 3,000,000/T as the peak frequency, where T is the temperature and the result is in nanometers. A G2-star peaks in 520nm (visible light:green), a FO-star at the border towards UV light while a M0-star peaks in the near infrared. Stars emit some radiation at all wavelengths, however.


All stars vary slightly in luminosity. For normal main-sequence stars of G and K types, variability is usually only 0.1-0.3%, and not enough to do any huge impact on climate. More variable main sequence stars may vary by up to 1%. Instable stars, such as subgiants and giants can vary much more. Small red flare stars (see chapter ONE) can also increase briefly in luminosity. For worlds without a thick atmosphere these flares may be a hazard and raise temperature significantly. Another possible source of exceptional stellar variability could be a close companion or even a large gas giant in very close orbit, whose magnetic fields interact to provide extreme flare activity for brief periods. This would require both a gas giant with a significant magnetic field and a close orbit (within 0.5 AU, preferably even closer).


If a planet has a significantly eccentric orbit its base temperature will vary greatly during a year. This variation may in turn affect what the atmosphere looks like and the greenhouse effect. For instance, a world may be cold enough to deposit carbon dioxide as ice during "winter" and then turn it to gas during "summer", thus increasing the already big seasonal differences. On the other hand, increased cloud cover may increase albedo and thus regulate the temperature.


Many gas giants have internal heat which raise the temperature by up to 20 degrees. Not all have it, though, particularly some small gas giants may be without internal heat source.


This and the following four sections are a simplified way of generating temperature. On real planets there are complex heat exchange system between various types of terrain, between clouds and ground, reflection and absorbtion etc.