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MYSTERY
CLOUDS: HOW CLOUDS FORM
Saturation
There is a limit to how much water vapor air can possess. This limit depends on the pressure and temperature of the air. Just like cooling down water causes it to freeze, cooling down air can make it condense, meaning that the water vapor turns to liquid water (or in some cases ice crystals). So colder air can hold less water vapor. Increasing the pressure will do the same thing. When the air holds as much water vapor as it possibly can for its temperature and pressure, it is saturated. In saturated air, the rate of evaporation is equal to the rate of condensation. Sometimes condensation doesn't occur as fast as needed to balance the evaporation, and the air becomes supersaturated. One common reason for this is a lack of condensation nuclei. If condensation nuclei are available, the water vapor will then condense onto them efficiently.
Nucleation
It is possible for clouds to form in the atmosphere directly from water vapor condensation, without the help of anything else. But this usually requires extremely cold temperatures. A more common way for clouds to form occurs when a small particle serves as a "nucleus" or central core around which ice can grow from either liquid or gaseous water. This process is called nucleation.
Links:
http://www.vivoscuola.it/us/rsigpp3202/umidita/lezioni/form.htm
http://www.weatherquestions.com/How_do_clouds_form.htm
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