How do rain clouds defy gravity?
In: Physics
Contrary to what many people believe, clouds are not vapour, they are not gas, they are liquid. They are formed from vapour that has condensed into microscopic liquid water droplets that are attracted to one another to form different types of cloud. In really high clouds they even freeze and form solid ice crystals.
The reason they stay in the air involves a bit of math. As soon as the droplets form they start falling due to gravity. Every body that falls into a fluid (like Earth’s atmosphere) will find resistance (due to air drag) and achieve a maximum velocity called terminal velocity, V.
The terminal velocity V of a sphere with a radius R (a droplet in this case) is close to 100,000 times R squared. So if you take a droplet of radius R = 0.005 mm (an usual size for the droplets found in clouds) you get that V = 100,000xR^2 = 2.5 mm/s.
Now you have to think that in our atmosphere we have hotter air below and colder air above. This creates convection air currents, wind currents that go up. So at the same time we have droplets falling at about V = 2.5 mm/s, they are also being pushed up again by this ascending currents, so the clouds keep bouncing and floating up and down these currents most of time.
Eventually though, more vapour arrives at the cloud coming from the evaporation of oceans and lakes and rivers, and the droplets get bigger and bigger. Whey they get big enough that the terminal velocity exceeds the ascending winds velocity they fall as rain.
What the other commenters didn’t quite mention is that the air has weight. The clouds aren’t sinply floating, they are like a pillow laying on top of a bed, it’s not that there’s no gravity it’s that the bed is simply heavier and holds the pillow up. A standard cumulus cloud weighs around 1.1 million pounds but each square inch of air is about 14 pounds, so all the air between the cloud and the ground plus the entire width of the cloud would be near billions of pounds, the cloud just doesn’t sink through that since it’s lighter and most importantly *less dense*
They dont. We live in what is technically a fluid atmosphere, in that fluid physics apply. So dense stuff sinks and light stuff floats. Everything on our planet, making up our planet, and our atmosphere are trapped here by gravity. But, when water evaporates it becomes less dense than the surrounding air and rises. Over time water particles bunch together in clouds, but remain less dense than the atmosphere below them, so they keep floating. When those particles group together and become big, they become to heavy to float, fall, and become rain/snow/hail/whatever.
They don’t. They form high in the atmosphere from water vapour that’s risen due to being hotter than the surrounding air. Then when they cool down, their density rises to the point that it’s now heavier than the rest of the air, and they fall to the ground in the form of rain.
The same way the steam from a teakettle does.
Clouds are collections of teeny water droplets, condensed out of the air because the humidity right there is too much to keep all the evaporated water in the air.
The droplets are small enough that they weigh JUUUST a little less than the same volume of air, or exactly the same, so gravity doesn’t pull them down any faster than the air aroud them – they’re “neutrally buoyant”. So they float at the altitude they formed at, and so does the cloud they make up.
–Dave, fun fact: a cloud on the ground is, literally, a fog bank
Latest Answers