Page 65 - AFMAN 15-111, 12 MAR 2019

AFMAN15-111 12 MARCH 2019 65

Chapter 13

PRESSURE

13.1. Introduction. This chapter describes the observing and reporting standards for pressure
measurements. All pressure values are initially derived from the atmospheric pressure as measured
by a barometer and may be reported in either inches of mercury (iHg) or hectopascals (hPa).

13.2. Altimeter (APHPHPHPH). The altimeter group always starts with an A (the international
indicator for altimeter in inches of mercury). The altimeter is encoded as a four-digit group
immediately following the A using the tens, units, tenths, and hundredths of inches of mercury.
The decimal point is not encoded.
13.3. Pressure Measurement Algorithms. The FBWOS sensors measure the barometric
pressure and then based on that value, compute the pressure parameters (e.g., station pressure,
altimeter setting, and sea-level pressure). Computations are made each minute. In addition to the
pressure parameters, the FBWOS also generates pressure change and pressure tendency remarks
for possible inclusion in the observations. FBWOS pressure sensors are capable of measuring
pressure from 17.5 to 32.5 iHg (600 to 1,100 hPa).
13.4. Standards and Reporting. At a minimum, all observations will include an altimeter
setting (ALSTG) in the body of the observation; additional pressure computations are recorded
and reported in accordance with this manual and local directives. (T-1). All pressure values will
be rounded down to the nearest reportable value. (T-1). In the United States and at US military
organizations overseas, pressure data is expressed as iHg for station pressure and ALSTGs and as
hPa for sea-level pressure (SLP); however, MAJCOMs may direct the use of hPa for reporting
purposes. Note: The common international unit of measure is hPa for all pressure data.
13.4.1. All pressure-related values are considered estimated any time the readings are suspect
in the opinion of weather personnel or when pressure is obtained through any method other
than the primary sensor. Exception: The AN/TMQ-53 may be used as a back-up without
estimating pressure values if it has been set-up and properly maintained IAW FCMS4-1994
and the T.O., and the values are representative and consistent with other reports in the area.

13.4.2. The primary measurements of atmospheric pressure used in observations are the
station pressure and the altimeter setting.

13.4.2.1. Station pressure is defined as the atmospheric pressure computed for the level of
the station elevation. This initial pressure calculation is used to derive all other pressure
reports. Station pressure is not disseminated but is recorded at 3-hourly intervals on the
observation form. Note: Station pressure may be referred to by the acronym “QFE” by the
international community.
13.4.2.2. The altimeter setting is a computed pressure value that an aircraft altimeter scale
is set to so it indicates the altitude above MSL of an aircraft on the ground at the location
for which the value was determined. Note: The letter “A” is used to identify an ALSTG in
observations at CONUS locations and OCONUS US military locations. The letter “Q” is
used by the international community; additionally, an ALSTG may be referred to by the
acronym “QNH.”