Pressure Measurement - Revision history

Sfws: /* Locating the Barometer */ break into 2 paragraphs for easier reading

2022-02-19T07:35:19Z

Locating the Barometer: break into 2 paragraphs for easier reading

← Older revision		Revision as of 07:35, 19 February 2022
Line 2:		Line 2:

	==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 an 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.		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==

Sfws: /* A note of caution */ correct bracket placement

2022-02-19T07:32:32Z

A note of caution: correct bracket placement

← Older revision		Revision as of 07:32, 19 February 2022
Line 28:		Line 28:
	** 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.		** 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:		* 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.		*# 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		*# 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.		* 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.

Sfws: /* Datum */ added reference to recent WX Forum discussion

2022-02-19T07:28:15Z

Datum: added reference to recent WX Forum discussion

← Older revision		Revision as of 07:28, 19 February 2022
Line 22:		Line 22:
	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===
⚫	~~''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.~~
			''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==

Sfws: /* Setting the Datum */ Reflects previous change to previous section

2018-02-16T12:49:50Z

Setting the Datum: Reflects previous change to previous section

← Older revision		Revision as of 12:49, 16 February 2018
Line 25:		Line 25:

	==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.
⚫	For Fine Offset 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 ~~need to~~ adjust so that is displaying the current sea-level pressure for your location.

	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. It's a ~~good~~ ~~idea~~ to ~~choose~~ an ~~airport~~ ~~site~~ ~~because~~ ~~you~~ ~~can~~ be ~~sure~~ ~~that~~ it ~~has~~ ~~been~~ ~~accurately~~ ~~reduced~~ to ~~MSL~~ ~~pressure~~.		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.		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, so you can be 0.5 hPa out if you use that, whereas most personal weather stations give pressure with 0.1 hPa precision.		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?==

Sfws: /* Datum */ Note added re METAR

2018-02-16T12:40:53Z

Datum: Note added re METAR

← Older revision		Revision as of 12:40, 16 February 2018
Line 20:		Line 20:

	==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 - 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==		==Setting the Datum==

Steve: /* Setting the Datum */

2013-09-15T07:04:51Z

Setting the Datum

← Older revision		Revision as of 07:04, 15 September 2013
Line 24:		Line 24:
	==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.
	For Fine Offset see [[FAQ#Cumulus_doesn.E2.80.99t_show_the_same_pressure_as_my_Fine_Offset_console]]. On a Fine Offset station, the pressure ~~measure~~ 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 need to adjust so that is displaying the current sea-level pressure for your location.		For Fine Offset 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 need to adjust so that is displaying the current sea-level pressure for your location.

	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. It's a good idea to choose an airport site because you can be sure that it has been accurately reduced to MSL pressure.		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. It's a good idea to choose an airport site because you can be sure that it has been accurately reduced to MSL pressure.

Steve: /* Setting the Datum */

2013-09-15T07:04:24Z

Setting the Datum

← Older revision		Revision as of 07:04, 15 September 2013
Line 24:		Line 24:
	==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.
	For Fine Offset see [[FAQ#Cumulus_doesn.E2.80.99t_show_the_same_pressure_as_my_Fine_Offset_console]].		For Fine Offset see [[FAQ#Cumulus_doesn.E2.80.99t_show_the_same_pressure_as_my_Fine_Offset_console]]. On a Fine Offset station, the pressure measure 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 need to adjust so that is displaying the current sea-level pressure for your location.

	The best way to get MSL 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. It's a good idea to choose an airport site because you can be sure that it has been accurately reduced to MSL pressure.		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. It's a good idea to choose an airport site because you can be sure that it has been accurately reduced to MSL pressure.

	Alternatively, look at a isobar chart at a site like [http://www.meteorologica.info/UKisobars.htm weather superstore], and interpolate for your location.		Alternatively, look at a isobar chart at a site like [http://www.meteorologica.info/UKisobars.htm weather superstore], and interpolate for your location.

	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, so you can be 0.5 hPa out if you use that, whereas most personal weather stations give pressure with 0.1 hPa precision.		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, so you can be 0.5 hPa out if you use that, whereas most personal weather stations give pressure with 0.1 hPa precision.

	==How Accurate are PWS Barometers?==		==How Accurate are PWS Barometers?==

Sfws: Resequence

2013-09-07T09:14:19Z

Resequence

← Older revision		Revision as of 09:14, 7 September 2013
Line 1:		Line 1:
	Atmospheric pressure (also known as barometric pressure) is the pressure exerted by the weight of the air above the Earth's surface. It is measured using a [http://en.wikipedia.org/wiki/Barometer barometer]. Atmospheric pressure and its rate of change are important parameters for weather and the ocean.		Atmospheric pressure (also known as barometric pressure) is the pressure exerted by the weight of the air above the Earth's surface. It is measured using a [http://en.wikipedia.org/wiki/Barometer barometer]. Atmospheric pressure and its rate of change are important parameters for weather and the ocean.

		⚫	==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 an 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==
	The units used for pressure are hPa (hecto Pascals, although not a System International unit because of its prefix~~, this is used by meterorologists in much of the world~~).		The units most often used by meterorologists in much of the world for atmospheric pressure are hPa (hecto Pascals, although not a System International unit because of its prefix).

			Alternatives units are
	Alternatives units are mb (millibars, this is the equivalent used for public communications, but its scientific definition has varied from 101,325 Pa to (from 1982) 100,000 Pa), mmHg (millimetres of mercury, strictly a function of temperature and gravitational force as well as atmospheric pressure, abandoned in 1929), torr (now defined as exactly 1/760 of a standard atmosphere, almost equivalent to mmHg, but mmHg varies by temperature and gravitational strength), kPa (kiloPascals is a SI unit, used in Canada), inHg (inches of mercury, used in USA and by some UK instruments, but strictly a function of temperature and gravitational force as well as atmospheric pressure) are alternatives.
			*mb (millibars, this is the equivalent used for public communications, but its scientific definition has varied from 101,325 Pa to (from 1982) 100,000 Pa),
			*mmHg (millimetres of mercury, strictly a function of temperature and gravitational force as well as atmospheric pressure, abandoned in 1929),
			*torr (now defined as exactly 1/760 of a standard atmosphere, almost equivalent to mmHg, but mmHg varies by temperature and gravitational strength),
			*kPa (kiloPascals is a SI unit, used in Canada),
			*inHg (inches of mercury, used in USA and by some UK instruments, but strictly a function of temperature and gravitational force as well as atmospheric pressure).

	Cumulus [[cumulus.ini\|supports]]:		Cumulus [[cumulus.ini\|supports]]:
Line 23:		Line 31:

	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, so you can be 0.5 hPa out if you use that, whereas most personal weather stations give pressure with 0.1 hPa precision.		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, so you can be 0.5 hPa out if you use that, whereas most personal weather stations give pressure with 0.1 hPa precision.

	== Calibration ==
⚫	If you find that your sensor is inaccurate, it may be possible to apply a calibration ~~within~~ ~~cumulus~~ (configuration menu).

⚫	==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 an 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.

	==How Accurate are PWS Barometers?==		==How Accurate are PWS Barometers?==
	The figure below shows a comparison between the pressure measured by a Fine Offset PWS and a nearby AWS. You can see that the comparison is excellent.		The figure below shows a comparison between the pressure measured by a Fine Offset PWS and a nearby AWS. You can see that the comparison is excellent.

		⚫	If you find that your sensor is inaccurate, it may be possible to apply a calibration (accessed from configuration menu) within cumulus.
	[[file:KyleVsBrockworthPress.png\|500 px]]		[[file:KyleVsBrockworthPress.png\|500 px]]

Sfws: /* Setting the Datum */ Remove description of setting sea level pressure in cumulus, replace with setting in weather station

2013-09-07T09:07:59Z

Setting the Datum: Remove description of setting sea level pressure in cumulus, replace with setting in weather station

← Older revision		Revision as of 09:07, 7 September 2013
Line 15:		Line 15:

	==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.
	In Cumulus, the pressure datum is set by going to Configuration/Calibration and entering a number into the Pressure slot in Offsets.
			For Fine Offset see [[FAQ#Cumulus_doesn.E2.80.99t_show_the_same_pressure_as_my_Fine_Offset_console]].

	The best way to get MSL 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. It's a good idea to choose an airport site because you can be sure that it has been accurately reduced to MSL pressure.		The best way to get MSL 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. It's a good idea to choose an airport site because you can be sure that it has been accurately reduced to MSL pressure.

			Alternatively, look at a isobar chart at a site like [http://www.meteorologica.info/UKisobars.htm weather superstore], and interpolate for your location.
	On the other hand, most airport sites only give pressure to the nearest hPa, so you can be 0.5 hPa out if you use that, whereas most personal weather stations give pressure with 0.1 hPa precision.

	The best time to set your datum is when the pressure is ~~steady;~~ in ~~other~~ ~~words~~ ~~when~~ ~~Trend~~ ~~shown~~ at ~~the~~ ~~bottom~~ ~~left~~ in the ~~Cumulus~~ ~~display~~ is ~~zero~~ hPa/h.		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, so you can be 0.5 hPa out if you use that, whereas most personal weather stations give pressure with 0.1 hPa precision.

			== Calibration ==
	To figure out what to put into the Cumulus box, use the following equation, where <math>P_c</math> is the pressure on your console and <math>P_s</math> is the pressure from a nearby station:
			If you find that your sensor is inaccurate, it may be possible to apply a calibration within cumulus (configuration menu).
	<math>Correction = P_s - P_c</math>

	==Locating the Barometer==		==Locating the Barometer==

Sfws: /* Units */ added dates

2013-06-14T09:32:24Z

Units: added dates

← Older revision		Revision as of 09:32, 14 June 2013
Line 2:		Line 2:

	==Units==		==Units==
	The units used for pressure are hPa (hecto Pascals, although not a System International unit, this is used by meterorologists in much of the world) or mb (millibars, this is the equivalent used for public communications), but mmHg (millimetres of mercury, not used now), kPa (kiloPascals, used in Canada), inHg (inches of mercury, used in USA and by some UK instruments) are alternatives. ~~Cumulus [[cumulus.ini\|supports]]:~~		The units used for pressure are hPa (hecto Pascals, although not a System International unit because of its prefix, this is used by meterorologists in much of the world).

			Alternatives units are mb (millibars, this is the equivalent used for public communications, but its scientific definition has varied from 101,325 Pa to (from 1982) 100,000 Pa), mmHg (millimetres of mercury, strictly a function of temperature and gravitational force as well as atmospheric pressure, abandoned in 1929), torr (now defined as exactly 1/760 of a standard atmosphere, almost equivalent to mmHg, but mmHg varies by temperature and gravitational strength), kPa (kiloPascals is a SI unit, used in Canada), inHg (inches of mercury, used in USA and by some UK instruments, but strictly a function of temperature and gravitational force as well as atmospheric pressure) are alternatives.

			Cumulus [[cumulus.ini\|supports]]:
	*0 = mb (millibars)		*0 = mb (millibars)
	*1 = hPa (hectoPascals)		*1 = hPa (hectoPascals)