The instrument correction (if used) is
determined by the barometer calibration facility during
the required semiannual calibration.
The removal correction is the pressure
correction based on the difference in height (in feet) of
the barometer and the runway or station elevation. To
find the removal correction (inches of mercury),
multiply the difference in height in feet by 0.001 inch of
mercury per foot. The correction in hectopascals is
found by multiplying the difference in feet by 0.036
hectopascals per foot. The removal correction is added
to the barometric pressure if the barometer is higher
than the runway, and subtracted if the barometer is
lower than the runway. Once determined, the same
removal correction is always added to the indicated
barometric pressure unless the barometer is moved.
Aboard naval ships, since the station elevation is
the height of the barometer, no removal correction is
added when determining station pressure. Temperature
corrections are required only for barometers used
Station pressure is calculated to the nearest 0.005
inch, or 0.1 hPa. When requested or given in a radio
conversation, station pressure is identified with the Q-
Sea-level pressure is a theoretical pressure at the
station if the station were actually at sea level. It is
calculated on a CP-402/UM pressure reduction
computer by using station pressure and an "r" factor that
must be obtained from a table.
The "r" factor is based on station elevation and is
determined by station temperature. These "r" factors
are based on a complex series of calculations found in
the Manual of Barometry, NAVWEPS 50-1D-510.
Tables of "r" values for each station are available from
FNMOD Asheville, North Carolina.
Some Navy and Marine Corps weather stations are
authorized to use a constant additive correction to
reduce station pressure to sea-level pressure. Sea-level
pressure is always higher than the station pressure with
the exception of stations located below sea level (for
example, a station located in Death Valley, California,
at 280 feet below sea level). A constant additive
correction factor (for example, +0.017 inch) for a
particular station would be added to the station pressure
(in inches) every time a sea-level pressure is required.
Authorized shore stations are assigned a constant
additive correction factor by FNMOD Asheville.
Since the height of shipboard barometers changes,
depending on the load the ship is carrying, shipboard
corrections for sea-level pressure are found by
multiplying the height of the barometer above the water
line in feet by 0.001 inch of mercury per foot (to obtain a
correction for inches of mercury) or by 0.036
hectopascals per foot (to obtain a correction for the
millibar or hectopascal scale readings). The corrections
are then added to the station pressure.
Commonly abbreviated "SLP," sea-level pressure
is identified in radio conversations by the Q-signal
QFF. Sea-level pressure is normally calculated to the
nearest 0.1 hPa.
Altimeter setting is a simplified sea-level pressure
in inches that may be "dialed" into an aircrafts altimeter
so that the altimeter will indicate the correct elevation
above mean sea level of an airfield or flight deck when
the aircrafts wheels are on the runway or flight deck.
Commonly abbreviated ALSTG, altimeter setting is
identified in radio conversations by the Q-signal QNH.
For example, a pilot requesting altimeter setting over
the radio should say "What is QNH?" The answer
would be "QNH Three Zero Point Zero Two Inches" if
the altimeter setting were 30.02 inches.
Weather observers should not underrate the
importance of the altimeter setting. Many aircraft
accidents have been caused by faulty settings.
Altimeter settings are computed for all surface aviation
observations with the exception of single-element
specials, and must be determined with extreme care.
Altimeter setting is computed using station
pressure and a pressure reduction computer. Unlike
sea-level pressure (computed on the opposite side of
same instrument), the altimeter setting is computed
using only the station elevation and station pressure as
arguments, and the setting is read to the nearest 0.01
Altimeter settings may also be obtained from a
Digital Altimeter Setting Indicator or an Automated
Surface Observing System (ASOS), as you will see in
the chapter on equipment.
NOTE: Many years ago, altimeter settings were
calculated from the runway elevation (station elevation)
plus 10 feet, to compensate for the average height of the
altimeter instrument above the wheels of an aircraft.