In reply to phizz4:
I spent several years paragliding, trying to understand what made it rain (and why it didn't). I came to the conclusion that you need both (a) 100% humidity over a range of altitudes and (b) a catalyst to trigger the accretion of individual H2O molecules into raindrops. I think latent heat of vapourisation also affects the process. As raindrops start to form, the molecules give up their latent heat - this heats up teh air slightly, reducing the relative humidity a tiny bit and slowing the process down.
It's always seemed to me that the more water there is in the atmosphere, the more scope there is in the weather forecast being wrong (i.e. cloudy & humid, instead of raining).
You could probably do a PhD in why it rains.