In reality, its much more complicated than just coming into existence. In fact, it involves a lot of factors.
When air rises in the atmosphere, it condenses. When this happens, the water vapor also condenses. This is called density. When the density of this air and water vapor mixture reaches a certain point, visible water droplets form in the air as a result of the condensation and creates clouds. Temperature is also a factor, but when that is thrown into consideration, it becomes pretty complicated. In general, this explanation provides an accurate representation of the cloud-forming process.