How do clouds hold so much water but then, as if someone opened a valve, it just starts falling out? Why doesn’t it constantly just trickle out?


How do clouds hold so much water but then, as if someone opened a valve, it just starts falling out? Why doesn’t it constantly just trickle out?

In: Other

You are working from an incorrect assumption that this is the case. Clouds are constantly forming and “unforming” as the water evaporates and condenses. Within and under any given cloud, there will be pretty much constant formation of water droplets and falling of rain.

The reason that you think that a tap is just turned on and it starts raining, is because you only notice it when it falls on you. Your position is only an extremely small area underneath any given cloud – so where you are, it is either raining or not. Whereas underneath the entire surface area of the cloud, it is much more likely to be raining at some given point.

There are also some independent variables that you haven’t taken into account , like atmospheric temperature, pressure, wind, height of the cloud, etc. For example, a cloud could be “raining” but very high up. By the time the rain reaches ground level it may have evaporated again so you don’t feel it.

Tl;dr it’s a lot more complicated than you seem to think

Clouds and rain seem pretty complicated, but it’s basically just all about temperature. All air on Earth has some moisture in it, and warmer air can hold more moisture in it than colder air can.

Once the amount of moisture in the air exceeds what the air can hold, it starts collecting together into visible water (e.g. clouds or fog). At that point the water droplets are very small, so they can sort of float in the air. If the moisture level goes up (or temperature goes down), the tiny suspended droplets can get even bigger, and which point they no longer float and fall to the ground as rain.

So how does this work in practice? Start with a place with pretty high temperatures and humidity. At ground level, you can’t see the moisture, but you can definitely feel it in the air. As the moist air rises, it cools down because the higher up you go, the colder it generally gets. The air still has the same amount of moisture in it, but the colder temperature means the moisture tends to stick together in tiny droplets that form clouds. If the temperature stays the same, the cloud can just hang out floating in the air. But if the temperature drops further (because a cold front moved in, for example), the moisture really starts condensing out of the air and falls on you as rain.

Edit: This is extremely easy to demonstrate if you bring a cold drink outside on a hot, humid day. Visible water droplets will suddenly “appear” on the outside of the glass, until they are large enough to roll down and get your table all wet. The water rolling down the glass was always present in the air, but it wasn’t until the cold drink made the surrounding air cold enough that the water condensed out of it to turn into little drops of water.

Clouds aren’t floating up there, at all. The cloud is water vapor, which is finely dispersed liquid, not the gas phase.

The reason clouds stay up there is a large amount of rising air and water vapor creating an updraft. It rains when the water forms droplets large enough that this updraft can’t support them anymore.