Celestia Forums

Hi, I'm wondreing how Celestia computes the temperature of planets. Are the only parameters albedo and orbital data? For Venus, the temperature is way too low, (it has a substantial greenhouse effect) so I thought it might be a wrong value for the albedo. But in order to get the right temperature, I had to assign a negative albedo to Venus, which makes little or no sense if albedo is the fraction of reflected radiation from the planet. Can anyone help me out?

ArneB wrote:Hi, I'm wondreing how Celestia computes the temperature of planets. Are the only parameters albedo and orbital data?

Yes, the temperature is calculated from the temperature and radius of the parent star, the planet's current distance from the star, and its albedo. As I recall, Celestia uses:

T = Ts*((1-A)/4)^0.25*(Rs/D)^0.5

Where Ts is the star temperature, Rs is the star radius, D is the planet's distance from the star, and A is the planet's albedo. It's a good estimate of the equilibrium temperature for a planet that is either rapidly rotating or that has good heat transport from daylight to night side, and it's pretty accurate for the temperature of Venus' visible surface (ie the clouds).

A future version of Celestia will hopefully include a parameter called GreenhouseFactor (or something similar) which allows the user to introduce a temperature increment to allow for the greenhouse effect of the atmosphere - planets with a declare GreenhouseFactor would then display both "Temperature" and "Surface Temperature" on-screen.

Grant

Thanks! Hope that can be done in the future. It's amazing how many facts that can be derived from orbital data, mass and size.

granthutchison wrote:

ArneB wrote:A future version of Celestia will hopefully include a parameter called GreenhouseFactor (or something similar) which allows the user to introduce a temperature increment to allow for the greenhouse effect of the atmosphere - planets with a declare GreenhouseFactor would then display both "Temperature" and "Surface Temperature" on-screen.

Is there actually a way to figure this out?

The only formula I've found was in another world design system at:
http://www.users.zetnet.co.uk/trisen/so ... four5.html

but I have no idea if that's right or not.

granthutchison wrote:But is it an oversimplification? I dunno.

No, actually, I do know. It makes no sense. I can't believe he includes Atmosphere, Weather and Climate in his reference list, since that's a pretty good source for information that immediately undermines the equation.

1) Greenhouse gases at very low concentrations produce a linear radiative forcing effect; at higher concentrations the radiative forcing varies with the log of the concentration - in Earth's atmosphere, water vapour and CO2 are in the log zone; methane, nitrogen oxides and sulphur dioxide are in the linear zone. So it makes no sense to mix their effects together without knowing their relative abundances.
2) Molecule-for-molecule, the radiative forcing effect varies considerably, even among the biogenic molecules he lists - methane is 21 times more effective as a greenhouse gas than carbon dioxide, and the nitrogen oxides and ammonia are 200 times as effective. So adding partial pressures of these gases without weighting their contributions is also unrealistic.

So it's pretty clear, after a bit of reflection and some data-hunting, that this formula falls into the '...constructed just to make "decent" results as I see it' category mentioned in the introduction to this website.
But gad, don't you just want to smack people who can't be bothered to give references on the page or clearly mark contrived formulae like that one?

Grant