Air pressure changes can be seen on the mesoscale and microscale as well as the synoptic scale. The air pressure decreases by either decreasing the mass above an area or causing a mass of air to rise. For example, a midget will weight less than a football lineman because the midget has less mass. If someone is pushing down on the midget's shoulders, he/she will weight more. This is synonymous with high pressure and sinking air. If someone began to lift the midget from the scale, he/she would weight less. This is synonymous with low pressure and rising air.
You will run into the terms mesohigh and mesolow. In a thunderstorm or thunderstorm complex, the mesohigh is associated with the downdraft while the mesolow is associated with the updraft. A downdraft contains dense rain cooled air that is accelerating toward the surface. The higher density and the fact the air is sinking cause the surface pressure to rise in this region of a thunderstorm. An updraft contains lower density warm and humid air that is rising. The updraft becomes a region of relative low pressure. Air enters the updraft region of a storm and exits in the downdraft region. Some air exits in the thunderstorm's anvil.
The upper levels are impacted by the surface mesolow and mesohigh. A thunderstorm complex can create a shortwave in the upper atmosphere (i.e. 500-mb) over time. The dense air associated with the downdraft increases the surface pressure but will often times cause height falls in the upper levels. This same effect can be seen on the synoptic scale. Underneath a shallow arctic air mass which moves into the U.S. is high pressure at the surface BUT there is a longwave trough in the upper levels. The cold air at the surface causes the thickness of the atmosphere to decrease because cold air is denser. This causes height falls in the upper levels. Pressure changes can be used to analyze frontal positions, outflow boundaries, and micro and macro bursts.