SCIENCE FILE: An exploration of issues and trends affecting science, medicine and the environment.

Q: How much does a cloud weigh?

A: According to physicists at San Francisco’s Exploratorium, the water in an average cloud weighs about 500,000 pounds, roughly the same as a 747 jumbo jet. In a typical cloud, there are about 2.8 billion droplets per cubic foot. The water does not plunge to the Earth like a meteor, however, because friction between the droplets and the air around them is greater than the force of gravity. It is only when the droplets become larger than 0.04 inches in diameter that they overcome friction and fall as rain.