Formation of rain is not so simple as it sounds. There are 5 steps involved in the formation of rain-

First is Evapotranspiration-
It is basically a process by which water is transferred from the land to the atmosphere by evaporation from soil and other surfaces and by transpiration from plants. This process we get humidity or water vapor in the air.

Second step is Upliftment and Cooling-
If the water vapor condenses on the surface of earth, we get different phenomenons like fog,dew etc. In order to get rainfall it is necessary that water vapor gets higher in the atmosphere. The uplift meant of water vapor can be achieved by different mechanism like by mountains or by the Cold fronts pushing the warm front above in mid latitude area or the intensive warming of earth surface causes the upliftment in low latitude area. As the vapor gets higher it expands and that expansion is the reason behind the decrease of temperature.

Third step is Condensation-
The cooling alone can't led to formation of water droplets, we need to add one more ingredient before we could get water droplets and that ingredient is known as hygroscopic nuclei. They are basically microscopic particle like dust particle, salt particles, smoke, soot. The process of condensation starts around them. And after the condensation we get microscopic water droplets These droplets are so small that they remain suspended in the atmosphere.

Forth step is Aggregation-
This process is the least known process. Due to some less known mechanisms involving vapor pressure and others, the water droplets starts colliding with each other. It takes approximately 8 million of water droplets to form raindrop of 5 mm diameter.

Final step is Precipitation-
after the innumerable amount of collisions, we get drops that are big enough to be pulled down by gravity.

And when there is not enough moisture left to form rain drops in the sky, the rain stops.

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