Air saturated with water can become over-saturated without adding water when it is cooled down. When the temperature of the air is reduced, the water holding capacity of air is also reduced.
The air at higher temperatures at hold more water as compared to the one at the lower temperature. This is due to the fact that water vapors are present in warmer air. Cooling down the saturated air leads to condensation of water vapor and makes it over-saturated.
By cooling the air, which reduces its capacity to hold moisture as water vapor sometimes to the point that the water vapor it holds is sufficient for saturation. This atmospheric cooling can be brought about in a number of ways, as by the arrival of a cooler air mass or by the movement of an air mass up a mountain side. If the cooling continues beyond the point of saturation, and provided there are sufficient condensation nuclei in the air around which tiny cloud or fog droplets can form, the excess moisture will condense out of the air as cloud or fog droplets or various forms of precipitation at the Earth’s surface.
