Hi Sipra,

The relationship of cloud cover to near-surface temperature and humidity is considered by Groisman et al. 2000 Journal of Climate

http://journals.ametsoc.org/doi/pdf/10.1175/1520-0442%282000%29013%3C1858%3ATROCCT%3E2.0.CO%3B2;

however, the relationship is not causal since it describes "the overall effect" of e.g. overcast conditions over temperature or over humidity.

It's actually a good question to think about. The quick answer is that, yes, there is a direct relationship which can be seen just looking at maps of the spatial distribution of cloud cover and water vapor. Example: http://earthobservatory.nasa.gov/GlobalMaps/view.php?d1=MYDAL2_M_SKY_WV&d2=MODAL2_M_CLD_FR

As that web page points out, however, the cloud cover is a function of temperature and water vapor (relative humidity is probably the best single predictor of cloud cover, depending on scale).

Something to keep in mind is that the vertical structure is very important in these issues. That web page, for example, is showing what boils down to the vertically integrated water vapor and vertically projected cloud cover. Note that there are places with large values for cloud cover and low values for water vapor. Subtropical stratocumulus decks, are especially obvious; these areas sit under the warm, dry air that is the sinking branch of the Hadley circulation, and basically all the water in the column is in the boundary layer which is only 1km deep. But these clouds can be 100% overcast in these shallow layers, despite the column water being fairly small.

If you are interested in looking at the global distribution and interdependency of clouds and water vapor, I would suggest using a sounder like the NASA AIRS products.

Thank you Dr. Brian. The example is very helpful. Could you please suggest relation between water vapor and high level clouds?

I don't know much about cirrus. One article you could start with would be:

http://dx.doi.org/10.1029/1999GL011194

Thank you Dr. Brian.

There must be something on the planet surface for water vapor to form cloud cover from water vapor, like surface land or ocean temperatures, vegetation, Pseudomonas hosts, mountains and the like--or at high altitudes we are conducting unintentional cloud seeding with the jet airplane exhaust across the planet.  You could call all those "vapor-to-cloud conversion factors."  

Otherwise invisible water vapor is omnipresent across the planet like you can see at http://www.ssec.wisc.edu/data/comp/wvmoll.html and only about 1-5% of the time (by area) is there a conversion factor present to form clouds from the vapor in an area.  

For example the Southern Hemisphere between South Africa, South America, New Zealand and the Antarctic has lots of water vapor in circulation, but rarely forms clouds, because the vapor-to-cloud conversion factors are not present in that area.