Accumulation of mist forms a cloud. Water vapour is always present in air, but during summer it is more due to higher temperature. Only a slight fall in temperature, condenses the air, which is full of moisture.
So, when saturated warm air rises to higher altitude with lower temperature, condensation occurs leading to the formation of a cloud.
The water droplets in a cloud are pulled by gravity and rainfall occurs. When the warmer layer of air causes water vapour to evaporate clouds do not produce rain. But if the cloud meets the moist air, water droplets will gradually saturate, leading to rainfall.
Below Clouds info from Wikipedia (Read More):-
A cloud is a visible mass of droplets or frozen crystals suspended in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae. Clouds are studied in the nephology or cloud physics branch of meteorology.
http://en.wikipedia.org/wiki/Clouds
