If you've ever watched a weather forecast on the News, then you may have noticed the big blue "H" representing High Pressure...or the big red "L" representing Low Pressure. However, what exactly does that mean?
To start, air pressure represents the weight of the air above a given area or level. At sea "level," the air pressure is higher than if you were to go up in elevation because the further up you go...there are less air molecules above you to push on you (less pressure).
As far as pressure over a given "area"... High pressure is the dominant player if the air molecules in the upper-levels of the atmosphere pile up over that area (upper level convergence). The converging of the upper air increases the weight (or push) of the air over that area...leading to a Higher pressure at the surface. The reverse is the case for Low pressure: Upper level air is moving out and away from a given area (diverging)...so the air weighs less over that area due to less air molecules...leading to Lower Pressure at the surface.
So, how does this effect you: Well, if you see High Pressure on a weather map over your area, then you can expect (typically) nicer weather. Yes, the upper air is converging, but that air has to go somewhere...so, it sinks down to the surface and rushes out & away from the core of the High. Sinking air warms up, and it leads to a more stable atmosphere.
If you see Low Pressure on a weather map over your area, then you can expect (typically) less ideal weather. Yes, the upper air is diverging, but as that air is moving up & away...it has to be replaced. So, air is rushing into (or converging) at the core of the surface Low. Converging / rising air... can lead to clouds and rain.(Image is courtesy of cimss.ssec.wisc.edu)
Meteorologist Deitra Tate