|MadSci Network: Earth Sciences|
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:
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.
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