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Pressure Measurement: Difference between revisions
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==Datum==
Pressure decreases exponentially with altitude, so for consistency officially it is reduced to the common datum of mean sea level (MSL). You can see those figures on the isobar lines of a weather map.
''A note of caution - if you are looking at an external site and using the pressure there to compare with your personal weather station, check whether what you see there is corrected to sea level.'' A widely used template on amateur sites that reports barometric pressure taken from "METAR" issued by airports does not differentiate between altimeter pressure (at field elevation level in pilot terms - QFE) and sea level pressure (at nautical height in pilot speak - QNH), and it is normal for all airports following North America guidance to report the former in the standard METAR Pressure Group.
==Setting the Datum==
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