Here it is captured on film:

It's certainly not that speck of a plane. It's that cloud.

Take an average cumulus or cumulonimbus cloud like those that show up most summer afternoons. Say that cloud was about a half-mile wide, deep, and high, looking like a soft cotton ball—but looks can be deceiving. Essentially it's a big bag of water floating through the sky, riding on air currents, and that water can weigh 500 tons or more!

    Consider the amount of water on the ground from a good rainstorm, the streams in gutters and the stuff pouring out of your downspouts. Scoop up a gallon of that water from a puddle and weigh it. Heavy, isn't it? At a little over eight pounds it's just a tiny fraction of the cloud above, still flying along. If it ever did cut loose from the sky and fall, it would be a major disaster for everything (and everyone) on the ground!

    So, why doesn't a cloud fall to the ground? Simply because its water droplets are dispersed across such a large area. If you boiled enough water to fill your house with steam, you'd have about the same concentration of water vapor as in a cloud.

Something else that helps keep a cloud from falling is the wind. A cloud forms because the air is rising, and the water condenses out as the air rises and cools. The same updraft that forms the cloud also helps keep water vapor suspended, at least until droplets begin combining to form raindrops. In that case pieces of the cloud would be falling in the form of a rain shower. However, even huge thunderstorms drop but a tiny part of their weight as rain (or hail or snow). Most of it evaporates back into the air and forms more cloud.

    While it is oversimplification of a complex subject—this article would become a thesis if we got into temperature, altitude, humidity, mass ratios, density, condensation, and particle physics—when suspended droplets, continually in motion within the cloud, merge as drops and become heavier than the supporting air or its wind currents, gravity takes over and we thankfully have showers from those fluffy monsters in the sky instead of normal disasters.