Look up at a cloud and it seems like nothing at all, just a soft white puff drifting by. But an average cloud weighs close to a million pounds, which is about the same as two fully loaded Boeing 747 planes hanging in the sky. Storm clouds are even heavier, sometimes running into millions of tonnes, more than the weight of the Empire State Building several times over. So how does something that heavy just float there without dropping on our heads. It is not really about how much a cloud weighs in total. It is about where that weight goes, and how the tiny bits of water inside a cloud actually behave once they are up in the air.
How much does a cloud actually weigh
A cloud is not one lump of water sitting in the sky. It is made of millions of very small water droplets or ice bits scattered across a huge stretch of air. To figure out how heavy a cloud is, scientists look at what is called liquid water content, basically how much water is packed into each cubic metre of that cloud, and then work out the total across the whole cloud. A normal cumulus cloud, the kind you see floating around on a clear day, can easily cover a full cubic kilometre of sky. Even a small amount of water per cubic metre adds up over that much space, often reaching close to 500,000 kilograms. Darker rain clouds carry a lot more water for their size, which is why they end up weighing so much more than the light fluffy ones.
Why the water droplets barely fall
The main reason a cloud stays up has to do with just how small each droplet inside it really is. As NASA explains, a cloud droplet usually measures around ten microns across, nowhere close to the size of an actual raindrop. Something that small barely feels gravity at all. Every object that falls through air eventually hits what is called terminal velocity, the point where the air pushing against it cancels out gravity pulling it down, so it stops picking up speed. For a droplet this tiny, that speed comes out to a tiny fraction of a centimetre per second. At that pace it basically looks like it is just sitting still in mid air.
Rising air does most of the work
Whatever little bit of falling does happen gets cancelled out by something else going on inside the cloud, warm air rising upward from below. Clouds tend to form where air is already moving up, sometimes gently in the case of ordinary rain clouds, and sometimes with real force, like the strong updrafts inside a thunderstorm that can push upward at several metres per second. This rising air works like a slow moving elevator, constantly pushing the droplets back up faster than gravity ever manages to pull them down. That is really why a cloud can sit there for hours without breaking apart the second it forms.
Moist air weighs less than you think
There is one more piece to this puzzle, and it is about the air itself rather than the water in it. As the United States Geological Survey points out, a cloud can weigh as much as a jumbo jet, yet because that weight is spread across so much space, it barely counts for anything density wise. Ordinary dry air is mostly nitrogen and oxygen, and both of these are heavier than a single molecule of water vapour. So when air gets more humid and fills up with water vapour, it actually turns a little lighter than the drier air sitting below it. That lets a cloud rest on top of the denser air underneath, almost the way oil sits on top of water without mixing in.
Why clouds eventually turn to rain
Nothing stays up there forever though, and the same physics that keeps a cloud floating also explains why it eventually lets go of its water. Inside the cloud, droplets keep bumping into each other and sticking together, growing bit by bit. Once a droplet has grown from a few microns to about a millimetre wide, its volume has gone up by nearly a million times. At that size it falls a lot faster, jumping from barely a centimetre per second to several metres per second, and that is finally enough to fall as rain instead of hanging around in the cloud. That slow build up, from countless tiny floating droplets to a smaller number of much heavier drops, is what eventually turns a quiet drifting cloud into an actual rain shower.