Updated: Jan 10
Constant pressure charts like the 500 mb chart shown below may seem obvious to interpret. It has many features similar to what you might see on the surface analysis chart. There are H's and L's that represent highs and lows and there are lines that resemble lines of constant pressure or isobars. However, those H's and L's are not marking the position of high and low pressure centers and the lines are not lines of constant pressure or isobars. So why is this different from the surface analysis chart?
The surface analysis chart like the one shown below depicts the pressure that is recorded by barometers located at the surface. However, if we plotted the surface pressure strictly from these observations of pressure it would look a lot like a surface relief map or topographic map. That is, higher elevation terrain generally has a lower pressure. So you would simply be plotting the difference in pressure due to the height of the terrain and not due to the relative differences in the pressure.
Consequently, meteorologists adjust the observed pressure at the surface to a height we know as mean sea level. This depicts the surface pressure as if there were no terrain features higher or lower than sea level. In other words, the surface analysis chart depicts how pressure changes while holding the height constant. This is formally described as pressure depicted at a constant geopotential height of zero.