Condensation occurs then air cools and the air ins't able to hold the same amount of water as before, because cold air holds less water vapor than warm air. 100% relative humidity in warm air is much more 'water' in the same amount of air than 100% relative humidity of cold air. With decreasing temperature the air's ability to hold the water vapor decreases and all water in excess will condensate to water out of the air, it will condensate.
|curves of relative humidity at certain temperatures|
Absolute humidity on the volume quantity of water in a particular volume of air. Measured in Grams per Cubic Meter, Pounds per Cubic Foot or any other mass unit and any volume units can be used.
Relative humidity is defined as the ratio of the partial pressure of water vapor to the saturated vapor pressure of water at a certain temperature. It is expressed as a percentage.
The danger for electronic equipment is the moment then the surrounding air is cooling down to the point there the relative humidity goes up to 100%. At this moment condensation starts and water drops will develop. The relative humidity cannot increase more than 100%, the air isn't able to hold the same amount of water anymore, the water in excess must go somewhere ..