John, we're talking about two different things- you're speaking of diurnal variation and I'm speaking of climatic (larger scales of time and space) of feedbacks, and how a warmer climate and water vapor and clouds will interact if the world were to get warmer- I know very well that morning and night change; when talking about this in the context of global warming we're referring to how water vapor in the atmosphere or how cloud feedbacks will effect global mean temperature on decadal scales. To put this in perspective, abstract from Willett et al (2007):

Quote:
Letter
Nature 449, 710-712 (11 October 2007) | doi:10.1038/nature06207; Received 29 March 2007; Accepted 30 August 2007


Attribution of observed surface humidity changes to human influence
Water vapour is the most important contributor to the natural greenhouse effect, and the amount of water vapour in the atmosphere is expected to increase under conditions of greenhouse-gas-induced warming, leading to a significant feedback on anthropogenic climate change. Theoretical and modelling studies predict that relative humidity will remain approximately constant at the global scale as the climate warms, leading to an increase in specific humidity1. Although significant increases in surface specific humidity have been identified in several regions, and on the global scale in non-homogenized data, it has not been shown whether these changes are due to natural or human influences on climate. Here we use a new quality-controlled and homogenized gridded observational data set of surface humidity, with output from a coupled climate model, to identify and explore the causes of changes in surface specific humidity over the late twentieth century. We identify a significant global-scale increase in surface specific humidity that is attributable mainly to human influence. Specific humidity is found to have increased in response to rising temperatures, with relative humidity remaining approximately constant. These changes may have important implications, because atmospheric humidity is a key variable in determining the geographical distribution11, 12, 13 and maximum intensity14 of precipitation, the potential maximum intensity of tropical cyclones15, and human heat stress16, and has important effects on the biosphere17 and surface hydrology17, 18.


and what I've already went over:

we have data that show that overall low clouds, which control the albedo more than any other kind, get thinner when it gets warmer. There are many different cloud types and effects, but they won't have much impact.

Some papers on specific feedback questions that can be found at http://pubs.giss.nasa.gov/ include

Bony, S., R. Colman, V.M. Kattsov, R.P. Allan, C.S. Bretherton, J.-L. Dufresne, A. Hall, S. Hallegatte, M.M. Holland, W. Ingram, D.A. Randall, D.J. Soden, G. Tselioudis, and M.J. Webb, 2006: How well do we understand and evaluate climate change feedback processes? J. Climate, 19,3445-3482, doi:10.1175/JCLI3819.1.

Tselioudis, G., and W.B. Rossow, 2006: Climate feedback implied by observed radiation and precipitation changes with midlatitude storm strength and frequency. Geophys. Res. Lett., 33, L02704, doi:10.1029/2005GL024513.

Tselioudis, G., and W.B. Rossow, 1994: Global, multiyear variations of optical thickness with temperature in low and cirrus clouds. Geophys.Res. Lett., 21, 2211-2214, doi:10.1029/94GL02004.

Naud, C., A.D. Del Genio, and M. Bauer, 2006: Observational constraints on cloud thermodynamic phase in midlatitude storms. J. Climate, 19, 5273-5288, doi:10.1175/JCLI3919.1.

Del Genio, A.D., W. Kovari, M.-S. Yao, and J. Jonas, 2005: Cumulus microphysics and climate sensitivity. J. Climate, 18, 2376-2387, doi:10.1175/JCLI3413.1.

Del Genio, A.D., 2002: The dust settles on water vapor feedback. Science, 296, 665-666, doi:10.1126/science.1071400.

Del Genio, A.D., and A.B. Wolf, 2000: The temperature dependence of the liquid water path of low clouds in the southern Great Plains. J.Climate, 13, 3465-3486, doi:10.1175/1520-0442(2000)

Point simply being that clouds will have little impact at offseeting anything we get from other feedbacks which will be net positive

Canuck, one of the links I provided you (http://www.realclimate.org/index.php/archives/2006/07/runaway-tipping-points-of-no-return/) and the Pieerehumbert book goes over your question. In short, we cannot get a runaway greenhouse on Earth-- Chris