Condensation from the Atmosphere

Have you ever noticed the beads of water that form on a cold beverage container in warm humid weather?  Or been annoyed by the fog that obscures your bathroom mirror when you are ready to shave or apply makeup?  Or seen white clouds billowing out of the tailpipe of a car on a cold day? These are all examples of water vapor in the air condensing into a liquid.  In the first two cases, the liquid condensation had formed on a cool surface, but condensation also occurs any time that you can see clouds, fog, or steam, as in the third case.  Water vapor is colorless and transparent, so when you can see a cloud, it is visible because of light reflecting off of tiny droplets of liquid.  In other words, condensation has already occurred by the time that you can see anything.
The study of the properties and behavior of moist air is called psychrometrics and one of the main tools used in that field is the psychrometric chart.  A cartoon of a psychrometric chart is shown below, with a few of the properties that are contained in a real chart labeled on the cartoon.

 
The horizontal axis represents the temperature of the air, and lines of constant temperature appear to be vertical.   The vertical axis represents the mass ratio of water vapor to dry air.  This is termed the “humidity ratio”.  The lines curving up and to the right are lines of constant relative humidity. 
Assume that air is at a certain condition, say, the temperature and humidity ratio marked by the green dot on this cartoon: 

Now, if that air were cooled without changing the moisture content (i.e. the humidity ratio) the condition marker would move horizontally as shown by the blue arrow.  If the air cooled enough, it would eventually reach the line marked 100% relative humidity.  At that point, liquid water starts to condense out forming liquid droplets—either a film or small droplets on a solid surface, or fog or clouds out in the open air.
This figure shows the temperature at which condensation will begin as a function of humidity ratio. 

 It is really just the 100% relative humidity line from the psychrometric chart seen from a little different perspective.
This figure shows the amount of heat (per unit mass of dry air) that must be removed in order to reach the 100% relative humidity line as a function of the initial temperature and the humidity ratio.