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Viewing Single Post From: The giant space ship example
Chris Ho-Stuart
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Dec 19 2011, 07:20 AM

Earth and Venus are as though there was no sunlight and is heated from surface. Correct?

Correct. It is important to recognize that this spreadsheet is not intended to represent the planets accurately, but to represent your space ship example. It does not deal with sunlight at all. In Venus in our solar system, there is plenty of solar heating within the atmosphere, which alters the temperature profiles. The physics of how the greenhouse effect works is unchanged; but there is more going on than only a greenhouse effect. Your star ship problem is a useful simplification in which other complications can be ignored.

You should think of the Venus example in the spreadsheet as being a bit like Venus used as a starship. I should add this to the readme sheet!


And it seem you have greenhouse effect gases, but how those value changed?
Never actually understood got you meant grey gases, maybe this is my problem

I am looking at a gas which absorbs all frequencies equally well. That's what "gray" means. For the starship, the atmosphere can even absorb shortwave without complicating things at all, because there is no incoming radiation to worry about.

In fact, solar absorption within a real planet's atmosphere does not make much difference as long as the solar energy input to the atmosphere is not enough to prevent convection. In that case, you still have a temperature profile determined by the adiabatic lapse rate, and you can calculate the flux of radiation from the surface up through the troposphere just as I have done in the spreadsheets; although with a real gas you need to consider different frequencies separately. I will get to that in version 3. That's where you should see the difference between grey gas and real gas more clearly.

Above the troposphere things look very different. Here the equilibrium lapse rate is determined by radiation only, without any convection. For example solar absorption in Earth's stratosphere causes temperature to increase with altitude. The effect of the stratosphere on radiation up from the surface, however, is negligible, because this part of the atmosphere is so thin. The "effective radiating altitude" on Earth is well below the stratosphere.


Can I see for example what 50% humidity does to affect dwarf temperature.
Or these greenhouse effect sort of like place values and work in progress?

I'm not planning to consider humidity at all. The thing about humidity is that it changes the adiabatic lapse rate, due to the effects of condensation in rising air. To consider that, you need to go to a more advanced model, such as MODTRAN. You can play with MODTRAN at this link.

What you CAN do with the spreadsheet is alter the lapse rate, and see what impact that has on things. A weaker lapse rate will give a weaker greenhouse effect.

What that means is humidity plays a double role. It increases absorption (which strengthens the greenhouse effect) and it reduces the lapse rate (which weakens the greenhouse effect). As it turns out, the strengthening effect is greater than the weakening effect; but there's no quick and easy law to show it has to be that way. It's a matter of detailed observation, or else of detailed calculations -- both of which confirm that the net effect of greater humidity is a stronger greenhouse effect.

Try this. In the spreadsheet, enter "Earth" as the example. This will use an absorptivity co-efficient or 171.95, which was simply the value chosen to give about the right value for emission to space.

Now enter "DALR" in the lapse rate cell (row 18). This uses a lapse rate of about 9.76, rather than the 6.5 used by default.

You can see the consequences in the %change column.

The lapse rate has increased by just over 50%. The emission at the top of the atmosphere (row 35) has deceased by nearly 5%. That is, there is a larger difference between the surface temperature and the temperature of emission to space. The greenhouse effect is now stronger; up 9.2% from 33.85 degrees to 37 degrees.

I've just noticed that row 20 should be retitled from "Atmosphere total height" to "Total height for all-troposphere case", or better, just left out.


edit: In readme you mention darker orange having values to do with greenhouse-
I see orange on definition page and i guess orange on integration page are
slightly darker- is that orange on integration meant by darker orange?

Oops. That's out of date. There is no darker orange now in version 2. I'll update the readme sheet appropriately. Thanks!

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