Final answer:
The standard atmospheric pressure for the purpose of setting aircraft altimeters within the standard pressure region is historically defined as the pressure to support a column of mercury 760 mm high, which is approximately 101,325 Pa (1 atm) at sea level.
Step-by-step explanation:
The standard pressure region for aircraft operation refers to the atmospheric condition where the altimeter must be set to a specific pressure value, which is the standard atmospheric pressure. This standard pressure is historically defined as the pressure required to support a column of mercury exactly 760 mm tall, where the atmospheric pressure at sea level is 101,325 Pa (1 atm). However, the IUPAC has changed the definition of standard pressure from 1 atm to 1 bar (100 kPa) in 1982, but many literature resources still use the previous definition. For purposes of setting aircraft altimeters within standard pressure regions, pilots use the historical standard of 760 mm of mercury (29.92 in.) at sea level, which is what aircraft altimeters are calibrated to when flying in a standard pressure region.