What relationship exisits between the temperature of the air and its capacity to hold water vapor?

Warm air holds more moisture than cold air, when warm moist air hits cold air, It can't hold it and drops as rain.
Warm moist air entering your Air Handler, being cooled(cold air can't hold as much moisture as warm air), it drops, not as rain, but as the condensation water.
There is also DEw Point to look into. When they are close to each other, fog will be formed. As the temperature increases, the dew will dissipate.