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The above chart shows
upper-level isobars (lines along which, at a constant height, the
barometric pressure is equal). The 
scale on the left gives the barometric pressure along each plotted
isobar. The height above sea level at which the upper-
level
pressure is 500 mb, about half of its sea level value, is near 5100
meters (16732 feet). Hence assume this chart represents
the isobars at a time at which these values apply at 5100 meters
above sea level. At an elevation of 5100 m, the pressure drops
by 4 mb about every 6 meters. Hence the scale on the right
represents the height at which the 500-mb pressure occurs
above or below the isobar which has the given pressure at the 500-mb
level. One can just as well depict the pressure situation with
lines of constant height along which particular pressures occur as
using isobars at a constant height.
(The National Weather Service customarily uses the
lines-of-constant-height convention. Heights are generally
indicated in decameters. On the above chart the decameter
numbers would be 509.4, 510, 510.6 and 511.2 dm.)
