Units of Measurement
There are two basic units used to measure and report the atmospheric pressure: inches
of mercury and millibars.
Atmospheric pressure is measured using either a mercurial or aneroid barometer. Air
pressure pressing against a mercurial barometer causes mercury to rise in an
evacuated glass tube. Air pressure at sea level causes the mercury to rise in the glass
tube, on the average, 29.92 inches of mercury (standard sea-level pressure). Mercury is
used because it is such a dense, heavy liquid. The same pressure would cause water to
rise approximately 400 inches of mercury in the same tube. An aneroid barometer uses
a thin metal strip in an evacuated case to measure pressure.
In the United States, we report the barometric pressure in inches (for example, 29.92
inches), and this is the unit of measurement that you will be most concerned with.
However, a pilot will occasionally ask for the altimeter in millibars which is the scientific
unit of measurement. Normally you will have to contact the weather reporting service for
your station to obtain this reading. Table 1-2 gives a comparison of inches to millibars.