I know it depends on the water cycle, but I don’t know more
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The atmosphere has water molecules in it. We often refer to the amount of water in the air as humidity. We usually use *relative* humidity to talk about how much water is in the air, and that’s a percentage. A percentage of what? The amount of water that would be in the air at that temperature if it were surrounded by water.
At the water surface fast water molecules will sometimes break free from the liquid and go into the air. Similarly, some of the water molecules in the air will hit the liquid and rejoin. When there are very few water molecules in the air, more molecules escape the liquid than rejoin, causing net evaporation. When there’s a lot of water molecules in the air more water molecules rejoin than escape, causing net condensation. When the rates are balanced we call that equilibrium and 100% humidity.
The rates of escaping and rejoining change with temperature. This means that 100% humidity is actually a different amount of water at different temperatures. So if you take 100% humidity air at 70°F and cool it to 50°F your humidity would be 180%! That water really wants rejoin to become liquid.
If there’s dust or other nucleation sites in the air with all those water molecules then they might “rejoin” on the surface of the dust and grow into tiny water droplets like mist. This is what you see when you see a cloud, hundreds of thousands of tiny water droplets reflecting and refracting light till it looks white, just like the fibers in paper.
Finally, if there’s enough tiny droplets and they are still growing then they start running into each other and combining into bigger droplets. There have to be a lot of droplets for this to happen which is why rain clouds look darker, there’s more droplets so less light makes it through. When the droplets get bigger and heavier they start to fall faster and faster, finally becoming rain.
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