Atmospheric Pressure: The force exerted by the weight of air,
NASA
Review some general facts about air pressure, how it changes, and what happens if it changes. The page includes a list of suggested additional activities as well as a web site demonstration of air pressure changes on a hot air balloon.

High-Pressure Areas and Low-Pressure Areas

Air Masses and Fronts,
Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin
This page about weather and why there is weather. In addition, it explains some of the symbols often seen on weather maps.

Air Masses and Fronts,
Naval Meteorology and Oceanography Command Public Affairs Office
Read about cold or warm air mass source regions and consider how your weather may vary depending on the air mass source that affects your area.