A medium-sized cumulus cloud weighs about the same as 80 elephants.
A typical cumulus cloud (the most common cloud) is about 1 cubic kilometer in volume and 2 km above ground. Air density is estimated to be at about 1.007 kg/m3. The clouds (water droplets) are less dense and come close to 1.003 kg/m3 (which is also why they float). There are estimated to be 1,000,000 cubic meters of droplets in an average cumulus cloud. The weight can be calculated using this equation (density by volume by a factor):
1.003 kg/m3x 1,000 x 1 km3 = 1,003,000,000 kg
Which is over a billion kilograms of droplets or close to 2.2 billion pounds.
Assuming a blue whale is close to 160 (160,000 kg) tones in weight, a cumulus cloud weighs as much as 6,268.75 blue whales!