Chapter 2— PRESSURE 



for observations of station pressure. The micro- 

 barograph will be used only for "tendency." 

 The mercurial barometer is then used mainly 

 for comparison of the readings from the aneroid 

 barometer. Refer to Federal Meteorological Hand- 

 book No. 1 (FMH-1) for instrument priority 

 in determining station pressure. 



To properly obtain a correct reading from the 

 aneroid barometer the observer should follow 

 the next three steps: 



1. Reduce the effect of friction by tapping 

 the face of the instrument lightly with your 

 finger. 



2. Read the scale at the pointer, straight 

 on, not left or right, to avoid the angle of 

 parallax which will cause high or low reading, 

 to the nearest 0.005 inch or 0.1 millibar, es- 

 timating any values that fall between these 

 graduations. 



3. Apply all posted corrections to the in- 

 strument that have been predetermined from 

 instructions in FMH-1, chapter A12, Operations 

 of Equipment. 



If your station has an AN/GMQ-29() installed, 

 all that is required to obtain the pressure reading 

 is to observe the digital readings on the display 

 module of the unit. The pressure readings are 

 derived from the BAROMETRIC PRESSURE SEN- 

 SOR ML-642/GMQ-29, which responds to ab- 

 solute pressure by means of a pressure sen- 

 sitive capacitor. Pressure changes cause the 

 capacitor to change value and to, in turn, vary 

 frequency of a precision oscillator. A discrimina- 

 tor converts the pressure-related frequency into 

 a direct-current voltage, on a scale of 0.00 

 Volts Direct Current (VDC) to +10VDC for an 

 associated pressure range of 800.0 to 1100.0 

 millibars. 



4. Then raise the vernier slide until its 

 lower edge is above the mercury column, after 

 which lower the vernier slide gradually until 

 the bottom edge is tangent to the meniscus of 

 the mercury. With proper adjustment, you see 

 two triangular white sections on either side of 

 the point of contact. 



5. Lower the mercury about 1/4 inch from 

 the ivory point; do not change the setting of the 

 vernier. 



6. Read the attached scale on the left of the 

 mercury column. Note the value of the division 

 which is immediately below the zero mark of the 

 vernier. It is graduated to the nearest five- 

 hundredths of an inch. Now read upward and find 

 the first mark on the vernier scale that coin- 

 cides with a mark on the fixed scale and read 

 the vernier to the nearest two-thousandths of an 

 inch. Estimate the final thousandths, if any. 

 The observed reading is accurate to the nearest 

 thousandth of an inch. Add this vernier reading to 

 the figure noted on the fixed barometer scale. 

 The result is the observed barometric reading. 

 (See fig. 2-1.) 



7. Determine the total correction (instrument 

 error, gravity, and temperature) and add it 

 algebraically to the observed barometric reading 

 to obtain the station pressure. 



Weather offices should be equipped with all of 

 the necessary corrections; if not, they can be 

 found in the Smithsonian Meteorological Tables. 

 A copy of these tables is in every weather office. 



For a more detailed description of determining 

 station pressure from mercurial barometers, 

 refer to the Manual of Barometry FMH-8 Vol. 1 

 (NA50-10-510). 



Sea Level Pressure 



To obtain station pressure from the mercurial 

 barometer, the following general procedures 

 are normally used: 



1. Read the attached thermometer to the 

 nearest 0.5°. 



2. Turn the thumbscrew at the bottom of 

 the barometer to raise the mercury level until 

 its surface just touches the tip of the ivory point 

 (i.e., until the tip coincides with its image in 

 the mercury). If a dimple forms on the surface, 

 the cistern has been raised too far. 



3. Lightly tap the metal portion of the 

 cistern and the metal casing near the top of 

 the mercury column. 



Sea level pressure is obtained by several 

 methods, depending on the elevation of the station. 

 It is computed, recorded, and transmitted for each 

 hourly, 3-hourly, and 6-hourly observation. 

 Stations with low elevations above sea level (or 

 below sea level) use a constant reduction factor. 

 Stations for which a constant reduction factor 

 has not been established use Meteorological 

 Pressure Reduction Computer CP-402/UM and 

 a table of pressure reduction ratio, "r", which 

 is described later in this chapter. 



CONSTANT ADDITIVE CORRECTION.— Most 

 naval shore activities and all ships can reduce 

 station pressure to sea level pressure by a 



17 



