We're only going to look at the weight of the actual cloud particles, and realize different clouds have varying densities of cloud particles (and, thus, different weights). Let's use your basic "everyday" cloudthe cumulus cloud with a volume of about 1 cubic kilometer (km) (0.62 miles) located about 2 km (1.2 miles) above the ground. In other words, it is a cube about 1 km (0.24 cubic miiles) on each side. One measurement (
http://snowball.millersville.edu/~adecaria/ESCI340/esci340_cp_lesson01_cloud_properties.pdf) of cumulus-cloud density is about 0.5 gram per cubic meter. A 1 km3 cloud contains 1 billion cubic meters.
Doing the math: 1,000,000,000 x 0.5 = 500,000,000 grams of water droplets in our cloud. That is about 500,000 kilograms or 1.1 million pounds (about 551 tons). But, that "heavy" cloud is floating over your head because the air below it is even heavier the lesser density of the cloud allows it to float on the dryer and more-dense air.