Just a quick note that I thought might be of interest to you. NASA have made the most detailed measurements yet of water vapour in the lowest ten miles of the atmosphere, using a satellite with specific sensors for this gas, which is strongly absorbent of radiated heat from the surface of the earth. It is thought that the heat-trapping ability of water vapour could increase the effect of global warming by carbon dioxide by as much as twice that it its absence.
Andrew Dessler from Texas A & M University has employed data gathered from the Atmospheric Infrared Sounder (AIRS) on the NASA Aqua satellite, measured over the period 2003 - 2008. The devise is the first with the ability to differentiate between different amounts of water as are present at different altitudes.
Through a combination of the satellite data and global average surface temperature readings, information has been garnered to determine how exactly water vapour influences and changes with temperature. It appears that a warming of the planet by 1 degree C will cause an elevation in humidity, and trap heat with an additional 2 Watts per square metre, which is in line with the predictions of climate models. Thus, the feedback effect of water vapour on global warming is both large and positive.
"Water Vapour Warming,"By Olive Hefferman. http://www.nature.com/climate/2008/0812/full/climate.2008.129.html