Rain falls from clouds in the form of drizzle or drops, but it does not enter a cloud in those forms. Instead, the rain that is released from clouds originates as gaseous water vapor from the earth's surface. Components must be added to this gaseous water vapor before a cloud can be formed and rain created and released. These components are referred to as condensation nuclei. In addition, the air must cool before water vapor can change shape into water droplets.
When gaseous water vapors first enter the air atmosphere from the earth, they begin to cool down as the warm air surrounding them cools. This cooling process causes the water vapors to move slower, resulting in their inability to hold a vapor form any longer. These vapors react by clustering together - a process we know as condensation. Clustered water vapors become water droplets through this process.
As the water vapor loses its form, it must adhere to something to continue to exist in the air. Vapors that adhere to condensation nuclei create a cloud. The condensation nuclei needed for this process includes one or more of the following: soluble salts, organic, combustion or soil particles -- such as clay -- as well as ice. Clouds cannot be formed without the presence of water droplets and one of the condensation nuclei types. Therefore, when rain is first in the clouds, it is made up of water droplets and a soluble salt or other condensation particle.
While water droplets combined with condensation nuclei produce a cloud, it is the velocity speed of the water droplets, combined with condensation nuclei, within the cloud that determines whether rain will fall from the cloud or not. Inside the cloud are vertical motions, known as updrafts, taking place. When raindrop velocity speed becomes greater than the updraft force, that is when raindrops will fall from the cloud. Interestingly, clouds dissipate in 30 minutes or less, despite the fact that they look like they live longer.