Suggest that when terraforming, liquid water increases thermal stability i.e. hydrographic extent modifies thermal min max extremes and range, decreasing range.
I noticed this because I have this problem with a small planet I am terraforming that whan I add more water vapour the min-max range increases instead of decreases as it would in reality since we know the liquid oceans on Earth act as a thermal buffer, adding warmth to or absorbing heat from the atmosphere.
The in-game result of adding water threatens to make the planetoid uninhabitable without infra even when it has 20 hydro and breathable atmo as the more hydro I add the greater the min max temperature range gets taking it outside the human comfort zone and incurring a temperature factor penalty that I cannot resolve. (I tried adding every non toxic gas and they all do the same, the temperature range increases when really it should decrease as for water.)
If you look at the two screenshots, these are successive in the process of adding water vapour to Proxima Centauri II. You can see the Hydrographic Extent increases from 5.51 to 6.56.
Min Max thermal range changes from (-11.136 <> 54.498, which is a range of 65.634) to (-9.256 <> 56.849 range 66.105). So the range has increased not decreased and the temperature factor has gone from zero to 0.123 though it fluctuates quite a bit.
I recognise there are limits to the model and in this example I have been unlucky and will have to choose my terraforming candidates more carefully in future but since the terraforming has been showing some interesting touches like the ice sheet albedo and the condensing of water vapour simulating a water cycle, I thought I would suggest it in case it appeals to Steve i.e. that liquid water could dampen (hehe) temperature fluctuations.
I added the current save in case it helps to take a look at this particular example.