What is the effect of pressure on dewpoint?

Varying the pressure of a gas changes the dewpoint temperature of the gas.  When the gas pressure is increased, the dewpoint temperature also rises.   When the gas pressure is decreased, the dewpoint temperature falls. 

Consider the following example:  

Pressure: 50 psia, Temperature: 70°F, Dewpoint: -40°F

If the pressure of this gas were lowered to 20 psia, the dewpoint would drop to -54°F. 

If the pressure of this gas were increased to 80 psia, the dewpoint would rise to -32°F

The term “pressure dewpoint” is sometimes used when measuring the dewpoint temperature of a gas at pressures higher than atmospheric pressure.  Technically speaking while “pressure dewpoint” is the correct terminology, most often people simply refer to it as “dewpoint” regardless of operating pressure.