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?
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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.
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