|MadSci Network: Earth Sciences|
The molecules that make up the atmosphere have mass and are held close
to the earth by earth's gravitational force. The exertion of gravity on
this mass gives them weight. The weight of this mass creates pressure
below. You can find discussions on atmospheric pressure at a number of
sites on the web. Two I'd recommend:
The University of Illinois
Try the links in the MadSci Library for more information on Earth Sciences.