Water or water vapour as it is more scientifically called gets into the air by evaporation. Plants also lose water through their leaves and stems, and this follows the same path into the sky.
Before rain can fall the water vapour must first cool so that it can condense. This means that it turns to water droplets. When you have a bath some of the steam that rises from the hot water forms a thin film of water on any of the cold surfaces in the bathroom such as the mirror or the window. It is little or more complicated than just cooling, however. Water vapour needs to have something to form on so that it can condense into droplets. In the air this is usually the tiny particles of dust which are ever present. This means, of course, that our industries can affect the rainfall in an area because of the smoke and other substances which usually accompany them.
The clouds which develop from these water droplets are of many different kinds, and not all of them will give rise to rain.
If you have spent a holiday in an area where there are high hills or mountains, it is likely that it will have been cloudy or have rained a lot. This is because the air which flows across the hills is forced rapidly upwardly them, causing it to cools as it expands. And if the air is very moist as it is, for example, when it reaches the lake district of England, the water quickly condenses so that this area will usually have a very high rainfall.
We have seen how the clouds occur, but this does not explain why this water should suddenly fall as rain, or even snow or hail. The main difference between a cloud droplet and a raindrop seems to be size-a cloud droplet is very tiny and floats in the air and a raindrop is larger and heavy enough to fall as it is pulled by gravity. It is not easy to see why the droplets should grow to grow to rain drop size but one reason may be that there are always a few larger droplets present which may fall and bump into other droplets to grow still larger, and so on.