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Pressure Measurement: Difference between revisions
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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. | 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=== | |||
''Warning - if you are looking at an external site and using the pressure there to compare with your personal weather station, check whether the pressure reported there is corrected to sea level.'' | |||
* The Saratoga template 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). | |||
* The general point is that internationally SLP (or QNH) is reported in the standard METAR Pressure Group. Within USA, and in METAR from overseas military airports operated by USA, QFE is reported in the standard METAR Pressure Group. | |||
** In those sites following USA guidance, the SLP pressure is reported in the Remarks Group of the METAR. There are METAR readers available that extract the SLP wherever it is within a METAR, but Saratoga does not. | |||
* A further complication is that QFE is defined in two ways: | |||
*# In USA, "Field elevation in pilot terms" is at a height equal to 10 feet above the mid point of main runway (a typical cockpit height) when reporting QFE. That reports 0.3 mBars above international standard. | |||
*# The international standard, used elsewhere in world, uses the airport (or runway) height (with no correction for height of cockpit) when reporting QFE | |||
* Finally, it should be noted that all METAR pressures in Pressure Group are rounded down to nearest mBar, so may read up to 0.9 mBar out from a reading expressed to nearest tenth of millibar. | |||
For more technical discussion see [https://www.wxforum.net/index.php?topic=43509.msg443105#msg443105 https://www.wxforum.net discussion of altimeter pressure]. | |||
==Setting the Datum== | ==Setting the Datum== | ||