ELI5: If a thundercloud contains over 1 million tons of water before it falls, how does this sheer amount of weight remain suspended in the air, seemingly defying gravity?

Read the Story

Show Top Comments

At the scale of cloud droplets viscosity is a force vastly superior to gravity. Gravity is applied to mass, viscosity* is applied to surface area, and smaller things have more surface than they have mass. Imagine you drop a stone into water – it will sink to the bottom right away. Now if you grind this stone into sand and let this sand fall into water it wouldn’t sink right away, despite being the same mass. It will take its time, and if you stir this sand just a little, it will make a swirling sand cloud in the water which can persist for a few minutes – precisely because sand particles have much more surface area than the original stone while having the same mass. The same thing happens with water droplets in the cloud. Very small water droplets just float on the upward air currents (note that thunderclouds form when there are strong upward currents to begin with). When this droplets become bigger by joining each other (reducing their overall surface area) they start falling to the ground making rain. Edit: *viscous friction actually.

nmxt

Imagine a plastic bag in a huge desert. If it’s windy, it floats in the air. But that’s obvious because a plastic bag is light enough… Now… imagine a LOT of plastic bags. Like a million tons of it. Thats a lot… If you were to crumple them into a big ball, no wind would move that. It weights a million ton. However… if you DON’T crumple them together, and they can flow individually in the desert, they just float individually in the wind. A cloud is (superficially) similar to that… This is an OVERsimplification of the science, but I think it is simple enough for a 5 year old 😀

Guwrovsky

Floating has nothing to do with weight. It’s all about density. 1 million tons of water vaper, which is less dense then air, will float. A single drop of water, which is more dense then air, will fall.

defalt86

Imagine a small cat. (Trust me this is going to be relevant). That is about the size that creature needs to be and of the weight that things like viscosity of air starts to actually matter to it, more than gravity (on our planet). Imagine swatting some ants off a table and you see them fall on the ground barely even noticed the fall. You see them walk on surfaces without a care about what is up or down. This is because at that size forces like viscosity and surface tension become more relevant to their experience. For example fairy fly experiences flying through air like swimming in oil or such. Now when a cloud forms up in the sky, the droplets of water in there aren’t heavy enough to really “feel gravity” against the mass of air that they are suspended in. Don’t take me wrong they are still subjected to it the same way every thing including the mass of air is. It is incorrect to consider cloud as one object, it isn’t. Each droplet should be considered as individual. The thundercloud is not million tons of water in the air. It is million tons of water in form of small droplets. Because water like to stick to it each other once the droplets combine they become big enough that air can’t support their weight and gravity takes over, and they fall as rain.

SinisterCheese

Air has weight as well, and if you have two boxes of equal size and fill one with air, and the other with water vapor, the box with the water is going to weigh less than the air. This is because water vapor has a lower density than air. There is “more” air than water vapor in any given volume. And because of this water vapor rises, its sort of like if you had a balloon under water, the balloon has a lower density than the water, so it floats. Water vapor has lower density than liquid water, and lower than air. So water vapor “floats”.

BlessedTacoDevourer