ANSWER
550 tons (500 metric tons). Or 100 elephants, according to Peggy LeMone of the National Center for Atmospheric Research. (Ms. LeMone was not the first to work out the weight of a cloud, but may be the first to state it in an equivalent number of elephants.)
Here's how the math works out: Cumulus clouds come in a variety of sizes, but for our purposes assume a medium sized cloud, something on the order of one cubic kilometer. That's 1,000,000,000 cubic meters. And a medium sized cumulus cloud typically has 0.5 grams of water per cubic meter. Thus 1,000,000,00 (m3) x 0.5 (g/m3) = 500,000,000 grams. Or 500,000 kilograms. Or 500 metric tons. (Or about 550 tons - well over a million pounds.) And since an elephant weighs roughly six tons, one could say that a cloud weighs roughly the same as 100 elephants.
Peggy LeMone also gave elephant-equivalent weights for other atmospheric phenomena: A large storm cloud? 200,000 elephants. A hurricane? 40 million elephants.
For more information about Peggy LeMone and elephant-based meteorology: http://abcnews.go.com/sections/wnt/SciTech/clouds_krulwich030902.html
WHO GOT IT RIGHT: Arne Haaning, Robert Walker, Jack Judge, Robin Campbell, Marika Thiessen, Yael Reuben, Scott Flaxman, Kathi Reynolds, Kristy Morgan, JP Weigt, Marc Quinlivan, and Bob Milligan.
