by David Murphy
A high pressure system generally marks an area of dry, tranquil conditions on a weather map.
Also referred to as an anticyclone or simply a high, it marks a large area of sinking air. As air sinks, it exerts more and more pressure on the surface beneath it (which is what gives the high its name---high for the higher amount of barometric pressure measured beneath it).
The reason highs produce dry, often sunny conditions is that air warms as it sinks. Since warm air can store far more gas-state water than cold air, the sinking air around a high basically eats-up the liquid-state water that make up clouds, turning it into invisible water vapor and clearing out the sky.
The air within high pressure centers expands and spreads over large areas, making most highs very large. It's not uncommon for a high pressure system to cover many U.S. states at one time. The largest highs are capable of covering roughly half the nation. The air within a high moves slowly and pragmatically.
By the way, you may have questions about the name anticyclone, the technical term referring to a high. The word cyclone describes the direction of rotation that's found within any storm system (which is always counterclockwise in the Northern Hemisphere). In other words, everything from a low to a tornado, to a hurricane can technically be called a cyclone. Anticyclone merely labels a high as having winds that are opposite of storm systems---which makes sense, since highs produce exactly the opposite sort of weather.