1. What percent of the atmosphere is water vapour? 2. The lapse rate and latent heat of evaporation
There are many sources that say it's on average 2 percent.
I had long assumed this was correct, it turns it out that it's complete rubbish, and it's actually about one-tenth of that.
Here is a good overview, which says:
... if all of the water in the atmosphere rained down at once, it would only cover the globe to a depth of 2.5 centimeters, about 1 inch.
This number seems to be widely accepted and seems very plausible, although some say as much as