I may be rehashing an old issue, and if so, sorry about that. Having said that, you can have negative water vapor levels during terraforming?
Please note - I am not complaining below. I'm going through my understanding of how terraforming works. It only sounds like complaining
I have discovered that terraforming is much more complex than it used to be. Just taking into account orbital eccentricity is vital, and something I completely discounted when I started this campaign. In addition to eccentricity, water percentage and water vapor issues are probably the biggest noticeable change. Needing to pump water vapor into the atmosphere, then have it precipitate out to create a hydrographic percentage, makes the entire process more complicated and time consuming, as the precipitation takes time, and the water vapor alters the atmosphere's greenhouse effect, if I'm getting this right. That means that you can't finalize the planet's temp while there is water vapor in the atmosphere, because the precipitation will alter the atmosphere's greenhouse effects, altering the temp.
I noticed the negative atmospheric water vapor percentage recently while trying to remove the last bit of water vapor from an atmosphere where I didn't need it any more because the hydro percentage had been raised to the zero cost level. I forgot, as usual, and let it go a bit too long before I realized I hadn't gotten a warning in the event log noting that all of the atmosphere component had been removed, as I would have if I was removing something like chlorine or fluorine. When I checked, I had -.0156 atmospheres of water vapor. I guess that means I can reduce a planet's hydro percentage by inducing negative levels of atmospheric water vapor?
Something else I have noticed is that the precipitation process seems to cause massive temperature swings. I haven't documented this, but I will next time I encounter this. In this case I was terraforming a planet that originally had temps in the 280 degree range. I first pumped a bunch of Frigusium to reduce the temp, then some water vapor to get the precipitation process going, then some oxygen. I got the temp down under a 100 before putting the water vapor in. Over time, as the water vapor precipitated out, I noticed radical temperature swings, sometimes back up to over 200 degrees. Very strange. I had other things going on that I was paying attention to, so as I said, I didn't document this. I'm sure its all down to the interaction of the numerous variables Steve has set up for atmospheric effects.
Kurt