Re: Why is the barometric pressure lower when the air is heavily moisture laden

Miles, there are two kinds of answers to this question.  The
first is the very technical kind.  The relationship between pressure, 
density and temperature is given by:
  
P=(density)x(Constant)x(temperature).

Moist air is less dense than dry air so at a given temperature, pressures 
would be lower when density is lower.  (The difference is very very 
small...but technically true.)

A second, more practical answer has to do with wind flow associated with 
areas of lower pressure and a little geography. Winds flow 
counterclockwise and inward toward areas of low pressure (check out the USATODAY 
graphic).  As a low pressure system moves toward Virginia Beach, the 
wind flow would bring moist air from the Atlantic Ocean inland.  These 
surface lows often track from southwest to northeast, sliding up the 
Atlantic Seaboard before moving offshore.  Everywhere "ahead" of the 
surface low...warm moist air from the Atlantic creats humid, usually cloudy 
conditions. 

Further west, a favored spot for development of surface lows is just east 
of the Rocky Mountains in West Texas, Oklahoma, and Kansas.  When these 
lows develop, the wind flow brings low level moisture surging inland from 
the Gulf of Mexico, providing one of the necessary ingredients for the 
thunderstorms that develop year round in the Gulf Coast States.

When low pressure systems are located such that the air coming from 
south and east of the center is over a dry region...the air mass will 
remain dry.