RIDGES OF HIGH PRESSURE AND TROUGHS OF LOW PRESSURE
A ridge of high pressure is an area of relatively higher heights. What we mean when we say “heights” is the distance in decameters from the surface of the earth a pressure level is at. So when 500 millibars is at 580 decameters, that pressure is found five thousand, eight hundred meters above the earth’s surface, which is over three and half miles up! This is also a region of sinking air. With high pressure causing sinking air, it leads to warmer and drier conditions at the surface.
Contrast this to a trough of low pressure, which is an area of relatively lower heights. With this in mind low pressure causes rising air and leads to cooler and wetter conditions at the surface.