The 500mb chart can be a useful weather tool for the weather savvy mariner along with the more familiar surface pressure charts. The 500mb chart is a constant pressure chart which means that everywhere on the chart the air pressure is the same (500mb). This occurs in our atmosphere, on average, at a height of about 5600 meters or about 18,000ft above sea level but varies from place to place due to the density of the air column.
The heights depicted on this chart represent the level at which the air pressure reaches 500mb or about one half the normal surface pressure of about 1013mb). The lines depicted on the chart are lines of equal height and are given in “tens of meters” above sea level so that the “540 line” on the chart means that the 500mb level is located at a height of 5,400 meters above sea level. Heights increase when the air is warmer and less dense and fall when the air is colder and denser so that the distance between these height lines indicates the slope of the 500mb surface.