Clouds, which are filled with tons upon tons of water droplets and even ice crystals, seem to float so effortlessly in the sky. Why is this the case, and what keeps them from plummeting to the ground? How much do these water-filled clouds actually weigh?
Let's start by answering whether or not air itself, the clear, blue sky surrounding a cloud, has weight. Atmospheric, or barometric, pressure is described as the force per unit area exerted by the weight of the atmosherere. Therefore, air does have weight, and it is very frequently measured by meteorologists using an instrument called a barometer.
Now that we determined that clear air has weight, it is simple to say that air also has a density, which is its weight for a chosen volume, such as a cubic kilometer. Since clouds are made up of particles, they also must have a weight and density. So, what exactly are they?
We can in fact put a numerical weight on a typical, fair-weather cumulus cloud, without actually setting it on a scale. The first thing that needs to be done is to determine the density of the overall cloud, again made up of tiny water droplets. Scientists have discovered that an estimate of a cumulus cloud's density is about 0.5 grams of water per cubic meter. Then, we need to determine how big the cloud actually is (or its volume). Researchers also calculated that the average cumulus cloud is 1 cubic kilometer, or 1 billion cubic meters, in volume. Multiplying the volume by the density, we discover that our cumulus cloud contains 500,000 kilograms, or 1.1 million pounds, of water droplets. That's the equivalent weight of five adult blue whales (the largest animal on the planet)!
The average cumulus, or fair-weather, cloud (which is photographed here) has been determined by researchers to weigh 1.1 million pounds. Though, not every cloud is created equal. A cloud with less water droplets, such as a high, wispy cirrus cloud, weighs less than a cumulus cloud. Meanwhile, a cloud with more water droplets, such as a thunder/cumulonimbus cloud, weighs more.
So, now that we know the average cumulus cloud weighs 1.1 million pounds, how does this much weight stay in the air and not crash down on our heads? It is actually because the cloud weighs less than that of the clear, dry air surrounding it! Atmospheric/barometric pressure, or the weight of the dry air particles, is almost 1,000 times heavier than the weight of all the water particules in that singular cloud! So, the "lighter" water droplets/cloud is less dense than the surrounding dry air in a given volume, allowing the cloud to rise and safely float above our heads.
A secondary explanation for why clouds float despite their heavy weight, is because of the size of the water droplets contained within them and the physical space over which they are spread. Most clouds contain water droplets and/or ice crystals that are rather small, and are spread out over a huge space (miles, even) within the clouds. Because of this combination, the affect of gravity on these water droplets, and therefore their fall speed, is negligible, allowing them to suspend in the air and the cloud to float in the sky.
Though, various processes can allow water droplets to become larger, heavier, and more dense than the surrounding dry air. If this occurs, cloud droplets form into precipitation and now have a meaningful fall speed, sinking below the dry air particles and dropping out of the sky, landing on Earth as some form of precipitation. So, maybe some clouds do crash down onto Earth afterall.
Scientific American: https://www.scientificamerican.com/article/why-do-clouds-float-when/