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Viewing Single Post From: The giant space ship example
Chris Ho-Stuart
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Dec 10 2011, 04:48 AM
So, you think the earth heat capacity adds nothing and only greenhouse gases will make up the 33 C.

The original reason why I started that post [and got distracted] was the idea was to have dwarf at 10 C.
So atmosphere plus greenhouse gases equals 10 C.
So should subtract however much warmer greenhouse gases are going to add.
So instead having it at 10 C it should be at 10 minus 33 or -23 C.
Yes, I think Earth's heat capacity makes no difference to equilibrium temperatures. Heat capacity only changes the rate at which it heats up or cools down. We have several posts in the thread so far talking about this difference between finding a temperature at which things are stable (equilibrium); and finding the rate of cooling or warming when conditions are unstable (when the temperature is not at equilibrium).

There are a couple of qualifications I should add to that!

Heat capacity does have an impact on the temperature variation between day and night; more heat capacity means temperatures get smoothed out. The effect on mean temperature depends on how you calculate it. It makes no difference (to the first order) for the "effective" temperature (obtained by averaging energy flows and then getting a temperature for the mean flow). But smoothing will tend to raise the cooler temperatures by more than it lowers the high temperatures; this is because of the 4th power relation between temperature and thermal energy flow.

The other qualification is that heat capacity in the atmosphere, as well as working directly on the amount of heat able to be stored, also has a second effect because the lapse rate depends on heat capacity; and lapse rate has a big impact on the efficacy of the greenhouse effect. Perhaps counter-intuitively... more heat capacity means LESS lapse rate, and LESS additional equilibrium temperature because the greenhouse effect is less effective! This only matters if there are greenhouse gases in the atmosphere, of course, but the effect actually depends on the heat capacity of all molecules in the atmosphere, not only the greenhouse gases.

If that sounds a bit counter-intuitive, at least we'll be able to run calculations and look at consequences for different values once I put up the spreadsheet.

About that... the spreadsheet for a grey atmosphere is done, but not debugged. I'm doing all this from scratch, and at present the spreadsheet has errors in it somewhere. I want to get rid of as many bugs as possible before posting it; then I'll actively invite bug reports and test results for all comers.

As for your reasons for putting the dwarf planet at 10C... you cannot presume a 33K greenhouse effect unless dwarf is a MUCH closer match to Earth. That would mean moist adiabiats, wide surface variation, diurnal cycles, many greenhouse gases all with different absorption spectra, and so on, and so on... way beyond what I can calculate for myself with a simple spreadsheet, or anything else for that matter!

So I'll be able to calculate a greenhouse effect in the simple case of Dwarf; but it's not going to be the same number as the 33K we get for Earth.

Cheers -- Chris
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The giant space ship example · Physical theory for climate