DEW POINT:

This is the calculated temperature to which air must be 
cooled for saturation (100% relative humidity) to occur. 
As air cools, it loses its ability to hold water and if it 
cools below the dew-point then the water vapour in the 
air has to condense (change into a liquid) to form dew, 
clouds, fog or frost of it is cold enough. If the dew-point 
and the temperature are close together in the late afternoon,
when the air begins to turn colder, fog is likely during 
the night.