In article http://www.straightdope.com/classics/a980313a.html there is a long discussion about the weight of clouds or lack of it.
A very good reason why clouds do, as a practical matter, have weight is the round fluffy top ones.
These type of clouds are formed when warm moist air becomes buoyant and rises, rapidly. In the rising process it gains momentum. Eventually it starts to condense and become less buoyant than the surrounding air and slows down.
Does it stop there? No. The soggy stuff keeps on rising under its own momentum as well as from the pressure of the air underneath. The tremendous amount of energy contained in the weight of the uprising cloud forms the nice round tops.
If a cloud had no weight - as a practical matter - it would stop dead the instant it reached neutral buoyancy. All clouds would be flat topped.
Ergo. Clouds have weight.