This chapter presents procedures and practices to be followed for the measuring, recording, and reporting of pressure. Atmospheric pressure is the force exerted by the atmosphere at a given point. Section 1, Definitions, defines pressure related terminology, Section 2, Observing, Determining and Reporting Procedures (NA LAWRS), covers the procedures for observing, determining and reporting pressure at sites other than LAWRS, and Section 3, Observing, Determining and Reporting Procedures at LAWRS, covers procedures at LAWRS. Practices and procedures for the operation of pressure measuring instruments and related equipment are presented in Chapter 17, Operation of Equipment.

Altimeter setting defines the pressure value to which an aircraft altimeter scale is set so that the altimeter indicates the altitude above mean sea level of an aircraft on the ground at the location for which the value was determined. Altimeter setting shall be reported in the body of all reports (METAR and SPECI). Other pressure data (including sea level pressure) shall be reported in the remarks section only.

14-3.   ATMOSPHERIC PRESSURE

Atmospheric pressure is the pressure exerted by the atmosphere at a given point. The various pressure parameters shall be determined from the barometric pressure after appropriate corrections are applied. The method used shall depend on the type of sensor and the available computational aids. These aids may be systems that result in a direct readout of the desired parameter, pressure reduction calculators, or tables. Designated stations may use constants to convert measured pressure to the desired pressure parameter.

14-4.   BAROMETRIC PRESSURE

The atmospheric pressure measured by a barometer is barometric pressure. In this chapter, the term "barometric pressure" refers to the actual pressure sensor value. The sensor value may be an altimeter setting, station pressure, or simply a direct pressure value without applied corrections depending on the type of sensor.

14-5.   FIELD ELEVATION, H_a

Field elevation, H_a, is the elevation of the highest point on any of the runways of the airport.

14-6.   PRESSURE ALTITUDE, PA

Pressure altitude is the altitude, in the standard atmosphere, at which a given pressure will be observed. It is the indicated altitude of a pressure altimeter at an altitude setting of 29.92 inches (1013.2 hPa) of mercury and is therefore the indicated altitude above or below the 29.92 inches constant-pressure surface.

Pressure change is the net difference between the barometric pressure at the beginning and end of a specified interval of time, usually the 3-hour period preceding an observation. If the pressure is rising or falling at a rate of at least 0.06 inch per hour and the pressure change totals 0.02 inch or more at the time of the observation, a pressure change remark shall be reported.

14-8.   PRESSURE CHARACTERISTIC (NA LAWRS)

Pressure characteristic is the pattern of the pressure change, as would have been indicated by a barograph trace, during a specified period of time, usually the 3-hour period preceding an observation.

14-9.   PRESSURE FALLING RAPIDLY (NA LAWRS)

Pressure falling rapidly occurs when station pressure falls at the rate of 0.06 inch (2.03 hPa) or more per hour which totals 0.02 inch (0.68 hPa) or more at time of observation.

14-10.   PRESSURE RISING RAPIDLY (NA LAWRS)

Pressure rising rapidly occurs when station pressure rises at the rate of 0.06 inch (2.03 hPa) or more per hour which totals 0.02 inch (0.68 hPa) or more at time of observation.

14-11.   PRESSURE TENDENCY (NA LAWRS)

Pressure tendency is the pressure characteristic and amount of pressure change during a specified period of time, usually the 3-hour period preceding an observation. The pressure tendency includes two parts: the characteristic (an indication of how the pressure has been changing over the past three hours) and the amount of the pressure change in the past three hours. The characteristic shall be based on the observed or recorded (barogram trace) changes in pressure over the past three hours. The amount of pressure change is the absolute value of the change in station pressure or altimeter setting in the past three hours converted to tenths of hectopascals.

14-12.   SEA LEVEL PRESSURE

Sea level pressure is a pressure value obtained by the theoretical reduction of barometric pressure to sea level. Where the earth's surface is above sea level, it is assumed that the atmosphere extends to sea level below the station and that the properties of the hypothetical atmosphere are related to conditions observed at the station. Sea level pressure shall be computed at designated stations by adjusting the station pressure to compensate for the difference between the station elevation and sea level. This adjustment shall be based on the station elevation and the 12-hour mean temperature at the station. The 12-hour mean temperature shall be the average of the present ambient temperature and the ambient temperature 12 hours ago. Stations within + 50 feet of sea level may be authorized to use a constant value to adjust station pressure to sea level pressure. Otherwise, stations shall use reduction ratios to calculate sea level pressure. When sea level pressure is missing at stations that would normally report sea level pressure, the remark SLPNO shall be added in the remarks section. (See paragraph 15-41, Sea-Level Pressure.)

14-13.   STANDARD ATMOSPHERE

Standard atmosphere is a hypothetical vertical distribution of the atmospheric temperature, pressure, and density, which by international agreement is considered to be representative of the atmosphere for pressure-altimeter calibrations and other purposes.

14-14.   STATION ELEVATION, H_p

Station elevation, H_p, is the officially designated height above sea level to which station pressure pertains. There may be occasions when the station elevation differs from the field elevation.

14-15.   STATION PRESSURE

Station pressure is the atmospheric pressure at the assigned station elevation (H_p). Station pressure shall be determined by adjusting the corrected barometric pressure to compensate for the difference between the height of the barometer and the designated station elevation.

14-16.   DENSITY ALTITUDE, DA

Density altitude, DA, is the pressure altitude corrected for virtual temperature deviations from the standard atmosphere.

14-17.   BAROMETER ELEVATION, HZ

Barometer elevation (HZ) is the height of the pressure instrument(s) above mean sea level surveyed accurately to within one foot. At LAWRS, this height is posted on or immediately adjacent to the instrument(s).

14-18.   POSTED PRESSURE CORRECTION

At LAWRS, posted pressure correction is the value added to the reading obtained from the station's altimeter setting indicator (ASI), or digital altimeter setting indicator (DASI), to correct it to a comparison standard.

14-19. RESERVED