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 outdoors. 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- signal QFE. SEA-LEVEL  PRESSURE 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 Altimeter setting is a simplified sea-level pressure in inches that may be "dialed" into an aircraft’s altimeter so that the altimeter will indicate the correct elevation above mean sea level of an airfield or flight deck when the aircraft’s 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 inch. 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. 1-39


 


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