As with a Low Pressure System, defining a High Pressure System is relative to the air around it. The average high pressure system is about 1013 mb but this isn’t specific.
As air rises when warm and falls when cold, we can quickly deduce that a High Pressure system must be made up of heavier, colder air. As air isn’t rising and condensing into cloud, the cold air mass usually makes for a less dynamic air mass leading to more constant, stable weather and clear skies.
In the Northern hemisphere, because of the coriolis effect, high pressure systems revolve in a clockwise direction. In the Southern Hemisphere High Pressure Systems revolve in an anticlockwise direction.
In general terms, warm, moist air rises and creates a more dynamic low pressure system. Low pressure systems are usually linked to wind, rain and ‘weather’ whereas High Pressure systems normally accompany blue skies, dry, stable conditions and sunshine.