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Pressure Measurement: Difference between revisions
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==Locating the Barometer== | ==Locating the Barometer== | ||
For most personal weather stations, the barometer is in the console, usually attached to the printed circuit board. These barometers are very cheap devices that simply record the load on a diaphragm. Unlike the barometer in | For most personal weather stations, the barometer is in the console, usually attached to the printed circuit board. These barometers are very cheap devices that simply record the load on a diaphragm. | ||
Unlike the barometer in a professional automatic weather station, they have no temperature compensation. Therefore, for best pressure readings, you should locate the console in a place where the temperature is reasonably constant, not in direct sunlight or near a heater. | |||
==Units== | ==Units== | ||
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==Datum== | ==Datum== | ||
Pressure decreases exponentially with altitude, so for consistency it is reduced to the common datum of mean sea level (MSL). | 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 true ''airport field elevation'' (i.e. 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== | ||
All weather stations allow you to adjust the pressure for sea level. You should not make the adjustment within Cumulus. | All weather stations allow you to adjust the pressure for sea level. You should not make the adjustment within Cumulus. | ||
The best way to get MSL (sea level) pressure at your location is to go to a site like [http://www.wunderground.com/ Weather Underground] and enter your location. It will find a weather station nearby and give you the current MSL pressure. ''Be aware of my note above, the station you choose might not correct to sea level pressure'' unless it an airport that correctly follows ICAO or WMO standards. | |||
Alternatively, look at a isobar chart at a site like [http://www.meteorologica.info/UKisobars.htm weather superstore], and interpolate for your location. You can be sure this uses sea level values. | |||
The best time to set your datum is when the pressure is not changing either with time or between nearby locations. Note most sites only give pressure to the nearest hPa, and airport METAR (except when reported in tenths) always rounds down, so you can be 0.9 hPa out if you use that, whereas most personal weather stations give pressure with 0.1 hPa precision. | |||
For Fine Offset clones, see [[FAQ#Cumulus_doesn.E2.80.99t_show_the_same_pressure_as_my_Fine_Offset_console]]. On a Fine Offset station, the pressure measured at the station is called Absolute Pressure, and the pressure corrected to sea level is called Relative Pressure, and the console displays the characters "rel" when it is displaying Relative Pressure. It is therefore the Relative Pressure reading which you will adjust so that is displaying the current sea-level pressure for your location. | |||
==How Accurate are PWS Barometers?== | ==How Accurate are PWS Barometers?== | ||