A cloud's mass depends on what you count

For an idealized cumulus cloud of about 1 km³, “roughly a million tonnes” can describe the mass of the air in that volume together with its water. The liquid water itself is on the order of hundreds of tonnes—for example, about 500 tonnes in a common estimate. This is not an average for all clouds: type, size, altitude, and water content change the estimate substantially. [1–3]