Rain clouds most commonly develop in the troposphere. The troposphere is the lowest layer of Earth's atmosphere and extends from the Earth's surface up to about 8 to 15 kilometers (5 to 9 miles) above sea level, depending on your location.
This layer is where we experience weather, including rain, thunderstorms, and clouds. The reason for this is that the troposphere contains most of the atmosphere's water vapor, which is essential for cloud formation and precipitation.
In this layer, the air becomes cooler as you go higher, which allows the moist air to rise, cool, and condense to form clouds. When these clouds get heavy enough with condensed water droplets, they release the water in the form of rain.
The layers above the troposphere, such as the stratosphere, do not usually have the right conditions for rain cloud formation because they lack sufficient moisture and their temperatures are not typically conducive to the processes needed for cloud formation.