eli5: How do (presumably very heavy) clouds stay suspended in the sky? More specifically how are they floating AND flat on the bottom?

Question

And what causes them to suddenly drop all of that water in the form of rain.

Judging by the amount of rain they must weigh a ton.

In: 337

Answer 1

Clouds aren’t any heavier than air. If you take air at the surface and cool it down, you start forming water droplets. No mass is added or removed – the moist warm air and cold air with suspended water droplets have the same weight. It’s basically a really misty day, but high up in the sky.

As air rises, it expands and cools, meaning that it can hold less water (it’s relative humidity increases). The ‘flat bottom’ is basically the point where the temperature is cold enough for the air to hit 100% humidity and invisible water vapor start condensing into visible droplets. It’s the equivalent of the dew point here on the ground.

You don’t immediately get rain because if the droplet falls, it hits warmer air and evaporate – never making it to the ground. If there is wind, that can also keep the water droplets in the air.

There are multiple things that can cause rain. The simplest and most relevant to your question is that the clouds moved to an area with colder air. The cold air hold less water, causing more droplets to form. If the air below is also cold, there’s minimal evaporation and the water hit the ground.