In the U.S., air pressure
at the surface is reported in inches of mercury while air pressure aloft is
reported in millibars. However, scientists generally use pressures in millibars.
Millibars are a direct measure of pressure, like pounds per square inch, but
in the metric system. Inches of mercury measure how high the pressure pushes
the mercury in a barometer. To convert between inches of mercury and millibars,
one millibar is equal to 0.03 inches of mercury.
The direct reading from
a barometer is the station pressure. Two things affect this barometer reading,
the high and low pressure caused by weather systems and the change in pressure
caused by the station's elevation. No matter what the weather systems are doing,
air pressure decreases with height. That is why it is important to examine a
uniform pressure field by reducing it to sea level.
Sea-level pressure is defined
as: "A pressure value obtained by the theoretical reduction of barometric
pressure to sea level." Another kind of barometric reading is the altimeter
setting, which aircraft use. It's defined as: "The pressure value to which
an aircraft altimeter scale is set so that it will indicate the altitude above
mean sea level of an aircraft on the ground at that location." A rule of
thumb for the altimeter correction is that the pressure drops about 1 inch of
mercury for each 1000 foot altitude gain. And if using millibars, the correction
is 1 millibar for each 8 meters of altitude gain. These rules of thumb work
pretty well for elevations or altitudes of less than a two or three thousand