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!

http://greenearthfacts.com/weather/how- ... oud-weigh/

What you are looking for is the weight of a cloud? Well scientist Peggy LeMone says that a little cumulus cloud weighs about the same as 100 elephants! A thunderstorm cloud is like 200,000 elephants!Perhaps it could be better answered by calculating the weight of one inch of rain per acre. Then estimate the surface area covered by a thunderstorm producing 2.5" of rain, per hour, over an area 30 miles in diameter!! It is an amazing answer!!!

http://wiki.answers.com/Q/How_much_does_a_cloud_weigh