Topic: Barometric pressure calibration - the role of temperature

[gszlag]

The trouble with temperature - Part I

Background

Earlier this year, I purchased the Ecowitt GW1000 WiFi gateway along with a Bosch BMP390 high performance barometric sensor connected to a raspberry pi. I already had the WH32B which came with my Ambient WS-2000 weather station. I live in the mid-latitudes – about 45°N.

The WH32B has an integral barometric sensor and the GW1000 also has an integral barometric sensor so along with the BMP390 I now have three barometric sensors.

The specs of the Bosch sensor are far superior to the WH32B and GW1000, therefore I am using the Bosch barometric sensor as the reference barometer in order to calibrate the other two.

Note. Barometric pressure sensors measure actual raw station pressures at your particular location/elevation. Sea level pressure numbers are calculated pressures using an equation.

Calibrating Station Pressure – the easy way

In comparing all three barometric sensors the WH32B happened to be the least accurate - being 2.55 hPa higher than the reference sensor – almost but not quite out of spec. The GW1000 was the most accurate. At most times it was quite close to the reference barometric sensor, however when barometric pressure is rising or falling quickly or even moderately, the WH32B and GW1000 were slow to keep pace with the reference sensor. A slow 60 second refresh rate did not help matters.

Both the WH32B and GW1000 sensor appear to have “noise” fluctuating randomly up to 0.33 – 0.34 hPa. The BMP390 sensor also has noise but at full sampling rate, readings (about 1 per second), seldom change more than 0.10 hPa between any two consecutive readings.

In checking the calibration of the WS-2000 console and calibrating the barometric sensor of the new GW1000 console for the first time, I noted the keisan online sea level pressure calculator was consistently producing too low readings in winter time compared to METAR SLP readings.

To make sure the reference Bosch barometric sensor was within specs, I compared my reference station pressure with a close METAR station pressure and on a stable pressure day, they matched within 0.10 hPa.

Note: METAR station elevation 193 meters. BMP390 barometric sensor elevation 193 meters.

Now that we are satisfied that our reference barometric sensor is reasonably accurate, all that was required is to enter the current live pressure reading from the Bosch BMP390 into the WS-2000  display console as ABS and since the GW1000 ABS reading matched the BMP390, the GW1000’s ABS offset was set to zero. Super simple. Done.

Out-of-sync

I use WeeWX and had noted that its calculated relative pressure readings were much higher than the keisan hypsometric calculator and on cold days were matching the local METAR more closely. This was odd because last time when I calibrated the WS-2000 console using the keisan online calculator – I thought I was bang on and matching the local METARs. A few months later when the weather turned cold, I was way out.

Most METAR reports are generated from AWOS weather stations (Automated Weather Observation System) and unlike our Ambient Weather/Ecowitt/Fine Offset clone consumer weather stations, AWOS uses temperature corrections in its pressure reduction to sea level algorithms.

Naturally, this raises the question: Should we also be correcting our weather station barometric readings to compensate for local temperatures?

..to be continued in Part II

[gszlag]

..to be continued in Part II

The trouble with temperature - Part II

...Should we also be correcting our weather station barometric readings to compensate for local temperatures?

The oft mentioned keisan conversion to sea level calculator does not correct for temperature using a 12 hour mean — and by definition, neither does the International Standard Atmosphere (ISA) calculator.

Note: Although the keisan online calculators are simple to use, we have to pay attention to the temperature starting point used in this calculator. The ISA assumes the default temperature at mean sea level is 15℃ and temperature declines as you gain altitude at the standard lapse rate of .0065℃ per meter.  However, the keisan calculators by default uses a non-standard temperature of 15℃ at your location/elevation/altitude. This is fine if your intent is to apply a temperature correction based on local temperatures. However, when you are calculating elevation offsets or calculating SLP using the keisan calculator, we should be using standard ISA temperatures.

[EDIT: David Burch's basic thesis is to calculate SLP using observed conditions, i.e.; using mean local outside temperatures. If you use ISA temps you are calculating something akin to Altimeter/QNH]

To correct for this you need to use. a different keisan calculator. It is called the keisan temperature and pressure at destination calculator. Make sure the default temperature is 15℃ at sea level and set the altitude to 0 meters. Enter in your elevation and it will calculate both the ISA atmospheric pressure at your elevation (based on 1013.25 hPa at mean sea level) and the appropiate ISA temperature for your location. The difference between 1013.25 and the calculated atmospheric pressure can be used as the elevation offset for your location.

Now that you have your fixed elevation offset for your location, is the use of a fixed offset in your weather station going to match with your local METAR’s SLP? No, yes, maybe?

Hot air, cold air

After reading a number of different technical papers about “pressure reduction to sea level” equations and the plateau effect, my head was spinning.

I did bump across something interesting though. After wading through all the scientific literature — it was a breath of uncorrected surface pressure air.

Although there were quite a few old posts in this forum about David Burch and his book – The Barometer Handbook several years ago, I thought we might revisit the topic once again by reviewing one of Mr. Burch blog articles that may also have been mentioned previously:

“Since pressure is the weight of the atmosphere above us, and because cold air weighs more than warm air, we can see that air temperature must be a factor in this correction as well as the elevation. The simple conversions that use only elevation are based on the International Standard Atmosphere, which provides a specific formula for how pressure drops with elevation. It also assumes that the temperature of the air is also following along with that of a standard atmosphere, namely starting at 15º C and dropping at a rate of 0.0065º C per meter.

This turns out to be a reasonable approximation, provided the air temperature at the elevation at which you measure the pressure is about what it would be in the standard atmosphere for that elevation. Thus if I am at 300 m elevation (984 ft) the correction to my station pressure reading (QFE) to get to QNH would be + 35.5 mb, based on elevation alone.  But this correction assumes the outside air temperature where I am is 15º - 0.0065º x 300 = 13.1º C (55.5º F).  Needless to say, all locations in the world that are at 984 ft elevation are not at 55.5º F at all times.”

Note: The complete article where the excerpt was taken, can be found at: https://davidburchnavigation.blogspot.com/2013/10/air-temperature-dependence-of-sea-level.html.

The author, David Burch* makes a good argument that in addition to correcting for elevation you must additionally correct for temperature using a 12 hour mean of local outside temperatures.

 *David Burch - author of the The Barometer Handbook

None of this is new news. We might recall that AWOS weather stations correct for temperature and if you use WeeWX software, it also corrects for temperature (and humidity too) but I never gave much thought to the question: how much of a difference does it make? In my case, under certain weather conditions, it was too significant to ignore.

In his blog article, Mr. Burch gives us an example of how temperature correction works:

“If the barometer is set to report SLP using an elevation of 200 m (656 ft) and it reads 1022.8 mb when the average outside air temperature was 30º C (86º F), then the pressure should be corrected by -1.3 mb to read 1021.5 for comparison to weather map reports for the same time and location.

Likewise in the winter, when the same pressure was measured in outside air temperature averaging 32º F, then the correction would be +1.2 mb for a QFF of 1024.0.”

NOTE: Some pressure reduction equations are expressed in aviation terms of QFE, QNH and QFF.  QFE = station pressure, QNH = altimeter (station pressure corrected for elevation) and QFF = sea level pressure (station pressure corrected for elevation and temperature).  QFF would be equivalent to METAR SLP.

Following Mr. Burch’s example and using his table from the blog article we can see that if my weather station was averaging -10℃ (14°F)  over a 12 hour period during a winter cold snap, at 193 m elevation I would need to add +2.0 hPa temperature correction in order to match local METARs. Similarly, during a heat wave in summer, I would need to subtract a certain amount to correct for high local temperatures.

Mr. Burch provides a handy sea level conversion calculator that incorporates both elevation and mean 12 hour temperature corrections at https://www.starpath.com/barometers/baro_cal.php. In addition to calculating SLP it will calculate the total correction amount (elevation correction + temperature correction) that is required to be added to station pressure. Make sure you click on the information icons for clear instructions about how the calculator works.

Even with the absolute luck on being able to compare my station’s surface pressure with a METAR’s surface pressure and on top of that and through more good fortune, I have an accurate barometric sensor (the GW1000), plus I have a high performance barometric sensor to calibrate to. Still though, attempting to compare with a variable, temperature compensating METAR SLP is more difficult than I thought it would be.

In summer here in the mid latitudes, calibration is a bit easier as morning temperatures average around the ISA temperatures of 15º - 0.0065º x 193 m = 13.75º C and since summer temperatures are more consistent from day to day, 12 hour mean temperature corrections tend to be smaller than in winter. Be careful though. On a particularly hot summer day as temperatures skyrocket by mid-morning, your METAR SLP may be already temperature compensating downwards and your WH32B and GW1000  REL pressures may be reading too high in comparison.

Here is another way to see if your METAR station is correcting for temperature. For example, in winter, with super dense frigid air, the calculated difference between METAR SLP minus METAR Station Pressure minus elevation offset (a spreadsheet works nicely here) ranged up to a 2.5 hPa difference to account for cold weather temperature effects alone. That is a significant amount.

Depending where you live, in winter your Relative Pressure might be too low and in summer, your Relative Pressure might be too high.

So no, most METARs do not use a fixed offset to calculate temperature corrected SLP.  Depending on temperature, the total offset amount can change with every hourly METAR report.

...to be continued in Part III

[gszlag]

[..to be continued in Part III

The trouble with temperature - Part III

To calibrate or not to calibrate

How do we fix this if indeed, we even want to fix this?  We could endlessly re-calibrate whenever Relative Pressure readings on the WS-2000 or GW1000 console drift too far from METAR SLP. Or we could re-calibrate based on monthly, quarterly or seasonal average temperatures. Or if you are a WeeWX user, just let WeeWX do the post-processing, temperature corrected SLP calculations automatically. Or just live with a fixed offset year around and leave it as is.

Do we really need to worry about compensating for temperature? I think it all boils down to your particular location and climate. For example, if you live close to sea level < 30m then the answer is no – temperature doesn’t matter. At sea level, with the WMO’s full blessing – a simple constant added to surface pressure can be used year round.

 If however, you live in a continental type of climate where annual temps can vary 60℃ to 70℃ or more or if you live at higher elevations then you definitely want to factor in the effects of temperature.

Ultimately, we have to live with a fixed offset system because that is what we got with our weather station. However, when we are setting our barometric readings or comparing readings with a METAR, we should consider using temperature offsets/corrections when outside temperatures diverge significantly from ISA standard temperatures.

Unless you lucked out with an accurate barometric sensor like my Ecowitt GW1000, the sensors in the WH32B and GW1000 are older generation sensors that could be more accurate and precise. And when you think about it, an accurate station pressure reading “drives” all the other measurements and calculations. The GW1000 is better in this respect than because it has an additional offset (ABS offset) so you can do a “hardware” calibration for the GW1000 if necessary. In the GW1000, the elevation correction/offset is separated out as a REL offset in the GW1000 calibration screen. This makes calibration easier but where do we put the variable temperature corrections? The answer is you can’t – at least not directly.

Looking at the spec sheet for the Ambient Weather WS-2000 we can assume that our weather station barometric sensors out-of-the-box can have significantly large errors (± 2.77 hPa) and to calibrate requires us to “move the needle” up or down to match with a local METAR. It is a method suggested by all the instruction manuals however although that method works to a certain point, — it is a bit of a compromise. Unless you live next door to the airport, if possible and ideally, we should be “calibrating” to our own surface pressure at our own hyper-local climate - not to another station’s variable offset SLP.

Bottom line is that we are trying to calibrate our fixed offset personal weather stations with a variable offset METAR station.*

* Important note: International weather enthusiasts may be scratching their heads wondering; “What METAR SLP values?” “We don’t have those.” See country specific notes at the end of this article.

If you calibrate with a local METAR, your ideal window to calibrate/set your barometer just became a bit smaller. You would have to wait until your local METAR is not compensating for temperature and is applying a fixed offset (correcting for elevation only) in order to compare your relative pressure with the METAR SLP.

If you don’t want to wait for what seems to be that ever elusive ideal temperature window to calibrate, you could use the star path calculator to input your local 12 hour mean temperature and compare the result with your local METAR. It should be much closer. If it is not, perhaps your 12 hour mean temp is different from the METAR’s mean temps. Try again at another time or on another day.

ABS/REL system vs ABS offset/REL offset system vs something else?

Ambient Weather personal weather stations like the one I have, use a ABS/REL/fixed offset system to calibrate the  barometer. For my new Ecowitt weather station assembled from components (a GW1002 equivalent) or other Fine Offset manufactured “clones” you will have, depending on your model, the same ABS/REL/fixed offset system or the ABS offset/REL offset system. Ecowitt’s barometer calibration instructions alternatively suggests that a more precise method is to calibrate side-by-side with an accurate reference barometer. This is the approach I much prefer and the one I use.

Other than wishing for better/faster barometric sensors, wish list items to improve accuracy and make calibrating/setting barometers easier might include or require the following information:

1. Elevation [above sea level]: required at initial setup to calculate elevation offset.
2. Elevation offset: automatically calculated by the weather console at initial set up.
3. Station Pressure = raw station pressure ± sensor calibration offset.
4. Station Pressure calibration offset [offset to correct for sensor error].
5. Temperature correction offset: 12 hour temperature rolling mean calculated automatically.
6. Altimeter/QNH: station pressure corrected for elevation.
7. SLP/QFF: station pressure corrected for elevation and temperature.

Final conclusion?

If you do not use weather software like WeeWX to do the temperature corrections for you, you will have to calibrate your barometer and calculate REL (Relative Pressure) based on a simple fixed offset. Whether that fixed offset contains a temperature correction or not – that is entirely up to you. Unless you live at or close to sea level, just keep in mind the effects of temperature extremes when you are calibrating or comparing your fixed offset REL values with METAR SLP values.

When you are checking your calibration and notice that your barometric readings are “out”, definitely don’t automatically assume you need to re-calibrate each time this happens. Instead, use the starpath calculator (or another temperature offset calculator) to see if a temperature correction is currently being applied to the METAR station you are comparing to.

If you are using WeeWX you may have noticed that its barometer readings can vary significantly during the day compared to your display console’s REL reading or the GW1000 REL reading.  That’s because WeeWX is applying temperature corrections. When the 12 hour mean temperature happen to cancel themselves out we are more or less measuring SLP that is corrected for elevation with no temperature correction and the WeeWX barometer reading should match your weather station’s REL reading very closely.

Notes:

In order to help distinguish between the terms used in this post, I have used the term METAR SLP rather than just SLP as both METAR SLP and Altimeter/QNH are elevation corrected to SLP. METAR SLP is additionally temperature corrected using 12 hr mean temperatures. Altimeter is not. Although the term SLP can be used generically to mean Altimeter/QNH/QFF/METAR SLP, my preference is to refer to SLP as station pressure that is both elevation corrected and temperature corrected to mean sea level.

Plateau effect: More study required. In the U.S. a plateau correction is applied to the SLP for all stations with elevations higher than 305 m ASL. The plateau correction increases SLP pressure at these stations during summer and decreases SLP at these stations during winter.

Barometer and barometric pressure. Sometimes barometric pressure is used to denote sea level pressure (SLP) but there are many references to air pressure/barometric pressure/atmospheric pressure =  station pressure or surface pressure. To further complicate things, “barometer” could refer to the instrument itself or to SLP. In the UK, QNH is referred to as SL (sea level pressure?)

For meteorological purposes, I might suggest the following simplified terminology instead:

Pressure/Station pressure = calibrated station pressure at your elevation/altitude AMSL(above mean sea level).

Altimeter/QNH = station pressure that is elevation corrected to mean sea level pressure.

SLP = station pressure that is elevation and temperature corrected to mean sea level pressure.

In Canada where I live, most METAR stations report both Altimeter and METAR SLP at least every hour on the hour. More often in bad weather. Some non-airport stations only report SLP. By convention and for aviation purposes, Altimeter is reported in inHg. For public dissemination on the Environment Canada weather and climate web sites, EC reports SLP in kPa.

Some country specific notes for calibrating :

In the U.S. Altimeter is dominant and METAR SLP is not as common.
In Mexico, in checking a number of METARS, I could only find Altimeter.
In the UK, METAR QNH is reported by some weather services as SL.
In Japan it looks like they report the Q code (QNH) plus Altimeter.

The rest of the world seems to use mostly QNH in METAR reports;  i.e. Q1013.

METAR QNH is reported in 1.0 hPa increments, i.e. Q1013, Q1014, Q1015, etc. versus METAR Altimeter increments of 0.01 inHg; i.e. A2992 , A2993, A2994, etc. versus METAR SLP increments of 0.1mb; i.e SLP133, SLP134, SLP135, etc.

Because the METAR Q code (QNH) increments by a large amount of 1.0 hPa at a time, it might be better to use surface pressure as the calibration start point as surface pressure readings resolves down to 0.01 hPa rather than 1.0 hPa. Use NOAA madis surface observation maps to get accurate surface pressure (P) for your METAR location and from there you could calculate QFF or a more accurate QNH but what would you compare QFF to if international METAR reports only QNH? You would have to calibrate your barometer to QNH. QNH however, is not corrected using local temperatures which might be an issue for calibrating high altitude weather stations.

Discussion: In Canada and the U.S. we are used to calibrating our barometers to METAR SLP. If the weather station to be calibrated is in the same pressure zone as the local METAR we see on occasion that Altimeter = METAR SLP.  Think about that for a moment…

Resources:

Calculate QNH from QFE or calculate QFE from QNH: http://www.hochwarth.com/misc/AviationCalculator.html

Calculate QFE, QNH and QFF (uses current temp to calculate QFF):
https://www.metpod.co.uk/calculators/pressure/

Elevation Offset calculator (uses Imperial units):
https://www.starpath.com/calc/Weather%20Data%20Calculators/baroheight.html

SLP calculator(uses 12 hour mean temperatures): https://www.starpath.com/barometers/baro_cal.php

Barani SLP calculator (uses current temperature and latitude):
http://www.wind101.net/sea-level-pressure-advanced/sea-level-pressure-advanced.html

Analogue Weather SLP Excel spreadsheet (uses current temperature and relative humidity):
https://www.analogweather.com/uploads/7/7/7/5/77750690/sea_level_pressure_speadsheet_rev_2.xls

Keisan Temperature and pressure at destination calculator:
https://keisan.casio.com/exec/system/1224562962

Bosch BMP390 high performance barometric sensor specs:
https://www.bosch-sensortec.com/media/boschsensortec/downloads/product_flyer/bst-bmp390-fl000.pdf

—

[Jim_S]

Thanks for posting. I enjoyed reading the information. Now you have me thinking about building my own reference barometer using the Bosch BMP390.

[gszlag]

Thanks for posting. I enjoyed reading the information. Now you have me thinking about building my own reference barometer using the Bosch BMP390.

Glad you enjoyed it.

At the time I bought the BMP390, I couldn't find a python script for it on github..Might be one now however at the time I ended up doing a small mod on a BMP388 python script to make it work. If you go with the  BMP390, just let me know and I can send it to you.

[Jim_S]

I was thinking of building something like this based on arduino and the BMP390 instead of the BMP180 chip (which is discontinued). I don't know how compatible or interchangeable they are so maybe this isn't a good plan.

[gszlag]

I was thinking of building something like this based on arduino and the BMP390 instead of the BMP180 chip (which is discontinued). I don't know how compatible or interchangeable they are so maybe this isn't a good plan.

That would be a really cool project..hmm..I've seen that display layout before..vaguely familiar..
I believe the BMP390 works with Arduino and the raspberry pi.
.See: https://learn.adafruit.com/adafruit-bmp388-bmp390-bmp3xx/arduino

[Jim_S]

That would be a really cool project..hmm..I've seen that display layout before..vaguely familiar..
I believe the BMP390 works with Arduino and the raspberry pi.
.See: https://learn.adafruit.com/adafruit-bmp388-bmp390-bmp3xx/arduino

Thanks for the information. I've done some projects like this in the past but it's been a while. So who knows if I can put this together.

[gszlag]

That would be a really cool project..hmm..I've seen that display layout before..vaguely familiar..
I believe the BMP390 works with Arduino and the raspberry pi.
.See: https://learn.adafruit.com/adafruit-bmp388-bmp390-bmp3xx/arduino

Thanks for the information. I've done some projects like this in the past but it's been a while. So who knows if I can put this together.

..finally found what your project lreminded me of: https://digitalbarograph.com/barograph_indepth.asp

The AQUATECH DBX2 uses a Bosch sensor which could very well be the BMP390.
I suspect the AQUATECH might cost a bit more than the Arduino project.

[Jim_S]

..finally found what your project reminded me of: https://digitalbarograph.com/barograph_indepth.asp

The AQUATECH DBX2 uses a Bosch sensor which could very well be the BMP390.
I suspect the AQUATECH might cost a bit more than the Arduino project.

LOL, the AQUATECH starts at $500. All I'm really looking for is a pressure sensor that accurately displays station pressure. I'll do the corrections myself using a spreadsheet or one of the online sites you mention.

[Platokidd]

Okay, this thread makes my head hurt.

In short, correct me if I'm wrong here. I'm really just a novice at all this stuff.

Neither the gw1000's or the the hp2551 will correct pressure with temperature change. Could be why we/ I use 15c for temp for calibrating,.  Thus, most of the year pressure readings will be off a few points.
 
I played around with https://www.starpath.com/calc/Weather%20Data%20Calculators/baroheight.html and can see how temps can effect the reading. A pws the would correct pressure to temp would be nice.

Maybe I need to have a winter and summer offset??

[mcrossley]

A pws the would correct pressure to temp would be nice.

AFAIK the Davis VP2 console applies both temperature and humidity corrections in its sea level reduction method.

[gszlag]

Okay, this thread makes my head hurt.

In short, correct me if I'm wrong here. I'm really just a novice at all this stuff.

Neither the gw1000's or the the hp2551 will correct pressure with temperature change. Could be why we/ I use 15c for temp for calibrating,.  Thus, most of the year pressure readings will be off a few points.
 
I played around with https://www.starpath.com/calc/Weather%20Data%20Calculators/baroheight.html and can see how temps can effect the reading. A pws the would correct pressure to temp would be nice.

Maybe I need to have a winter and summer offset??

Yeah, a bit of a tome..had to break it up (exceeded character limit!)

True - Fine Offset hardware does not calculate temperature corrections in its consoles. They use fixed offsets only.

I think you have the gist of it however you should not be off a few points for most of the year - probably the most would be in the coldest days of winter. Summer time corrections are less than winter time because warmer air is less dense. Review David Burch's example in the post where he compares a summer correction with a winter correction - the summer correction is less.

Personally, I just leave the fixed offset year around and leave REL as is because  WeeWX does the temp corrections.

If you are not using WeeWX, then optionally ,if you wish, you can re- calibrate seasonally or use some other time frame and use the starpath calculator to compare its results with your REL and your METAR.

There are quite a few SLP online calculators of which the popular keisan calculator is just one. From what I can see there is likely no reason why you can't put the 12 hour average temp into these calculators to do a temperature compensation.

I have to plead a mea culpa here. I too originally thought that 15C was the sea level temperature in the keisan calculator. It is not. Yes, it is the standard temperature at sea level when you are using the International Standard Atmosphere (ISA )but if you are using the keisan calculator you might consider putting in either the current temperature of your station or the 12 hour average temperature of your station and compare those results with the starpath, Barani calculator etc and your local METAR.

If you don't have a reference barometer to calibrate your weather station barometric sensor(s) then you have to compare with a METAR when you and the METAR are in the same pressure zone. If your METAR is temperature correcting and your station is not..therein lies the problem..the trouble with temperature.

Still though, the David Burch (starpath calculator) is the only online calculator I could find that explicitly uses  12 hour average temperatures. Everything else uses current temperature.

[gszlag]

A pws the would correct pressure to temp would be nice.

AFAIK the Davis VP2 console applies both temperature and humidity corrections in its sea level reduction method.

I am not familiar with the Davis machines but remember seeing some old posts about the Davis console's ability with SLP calculations. Perhaps some Davis owners can also comment whether the Davis Vue console has this capabilty. I thought I saw something about some VP2 owners using a Vue console?

In any case, a display console that has temperature correction abilities built-in would be great.

Basically in this article, I was obliquely implying that Fine Offset (if they enter the prosumer marketplace), should have that capability. Replacing the ABS/REL system with Station Pressure/SLP /Elevation Offset/Altimeter etc ( a total firmware revamp) would be on the wish list as well.

Pay more for a more capable/premium pws? Absolutely.

[CW2274]

A pws the would correct pressure to temp would be nice.

AFAIK the Davis VP2 console applies both temperature and humidity corrections in its sea level reduction method.

I am not familiar with the Davis machines but remember seeing some old posts about the Davis console's ability with SLP calculations. Perhaps some Davis owners can also comment whether the Davis Vue console has this capabilty. I thought I saw something about some VP2 owners using a Vue console?

I'm one. The Vue console has the ability to display the altimeter setting whereas the VP2 console does not. With either Davis console, all that's done is enter your console's elevation MSL, select what pressure reduction you want (altimeter for me with the Vue), and hit enter. Done. Further tweaking can be done if you desire, but elevation alone works just fine for me and I'm at 2400' MSL.

[gszlag]

A pws the would correct pressure to temp would be nice.

AFAIK the Davis VP2 console applies both temperature and humidity corrections in its sea level reduction method.

I am not familiar with the Davis machines but remember seeing some old posts about the Davis console's ability with SLP calculations. Perhaps some Davis owners can also comment whether the Davis Vue console has this capabilty. I thought I saw something about some VP2 owners using a Vue console?

I'm one. The Vue console has the ability to display the altimeter setting whereas the VP2 console does not. With either Davis console, all that's done is enter your console's elevation MSL, select what pressure reduction you want (altimeter for me with the Vue), and hit enter. Done. Further tweaking can be done if you desire, but elevation alone works just fine for me and I'm at 2400' MSL.

Nice. From what I found in researching the role of temperature is that most of the world uses strictly QNH (altimeter) in their METAR reports. A display console should have the ability to calculate both. If not, we have to resign ourself to software - an extra step with bonus features but yeah, I agree that built-in capabilities would be real nice. But FO is not quite there yet.

[CW2274]

A pws the would correct pressure to temp would be nice.

AFAIK the Davis VP2 console applies both temperature and humidity corrections in its sea level reduction method.

I am not familiar with the Davis machines but remember seeing some old posts about the Davis console's ability with SLP calculations. Perhaps some Davis owners can also comment whether the Davis Vue console has this capabilty. I thought I saw something about some VP2 owners using a Vue console?

I'm one. The Vue console has the ability to display the altimeter setting whereas the VP2 console does not. With either Davis console, all that's done is enter your console's elevation MSL, select what pressure reduction you want (altimeter for me with the Vue), and hit enter. Done. Further tweaking can be done if you desire, but elevation alone works just fine for me and I'm at 2400' MSL.

Nice. From what I found in researching the role of temperature is that most of the world uses strictly QNH (altimeter) in their METAR reports. A display console should have the ability to calculate both. If not, we have to resign ourself to software - an extra step with bonus features but yeah, I agree that built-in capabilities would be real nice. But FO is not quite there yet.

I said long ago that if I had your equipment the first thing I'd want on my wish list is the capability to enter ones elevation. Really doesn't seem too hard ya'd think. Reading all the hoops and unbelievable time consuming process you folks face is mind-boggling, and frankly, frustrating to me...can't imagine for you guys.

[davidefa]

in this thread there is a reference to a davis application note AN28
The original link does not work for me this should be a copy
Find it very interesting

P.S.
This should be the official working link

[gszlag]

in this thread there is a reference to a davis application note AN28
The original link does not work for me this should be a copy
Find it very interesting

P.S.
This should be the official working link

Interesting look at some of the old Davis posts -definitely some "hot" discussions back then. Looks like confusion about barometers is universal.

Love that technical document from Davis. Plain language, easy to read -definitely adheres to the objective of "making the complicated -simple(r)".

Thanks for sending it.

From reading the doc it looks like Davis firmware emulates a simplified version of the classic AWOS presure reduction ratios with lookup tables. David Burch also made the comment a decade ago that the ratios for any particular METAR station are not publicly available - so you have to calculate something similar.

Also interesting to read that if you set the Davis elevation to zero (station pressure) you can export to third-party software to do the SLP calcs.

I am tempted to say there are a dozen or more ways to calculate SLP plus each country has there own version of pressure reduction formulas. The UK ignores humidity for instance. One advantage of post-processing weather software like WeeWX is that if you don't like the SLP calculation - let's say you hate humidity - you can modify the script and get rid of it.

In your part of the world METAR only reports QNH. Would overseas (outside NA) Davis owners (or any pws owner) generally set their consoles to Altimeter/QNH and calibrate/compare to METAR QNH?

[CW2274]

in this thread there is a reference to a davis application note AN28
The original link does not work for me this should be a copy
Find it very interesting

P.S.
This should be the official working link

(or any pws owner) generally set their consoles to Altimeter/QNH and calibrate/compare to METAR QNH?

This is exactly why I bought my Vue console. In the States, all pressure readings that are disseminated to the public is the altimeter from ASOS/AWOS's because it's what used as the "official" observation for a given area. Being that ASOS/AWOS's are located at airports and is critical to aviation safety, I tend to trust their pressure readings over my neighbors. Since all METAR strings here also have SLP in millibars, you can use it as well, it's just not as readily available as the altimeter, which is issued as inHg. So, for me, I do straight up comparisons with my four neighboring ASOS's. Apples to apples.

[Jim_S]

in this thread there is a reference to a davis application note AN28
The original link does not work for me this should be a copy
Find it very interesting

P.S.
This should be the official working link

Thanks for posting. That's great information!

[gszlag]

Perhaps some Davis owners can also comment whether the Davis Vue console has this capabilty.

Thanks to all the Davis owners for replying to my query.

So now we know that Fine Offset's competition can and does correct SLP for elevation, temperature and humidity.

We also know that there is very different METAR reporting depending what country you live in.

Here in Canada, unlike the States, Altimeter is never used for public weather forecasts - only METAR SLP is used so that is our "official" standard. Elsewhere in the world,  METAR SLP appears to be very rare and QNH (oddly in 1.0 mbar increments) appears to be the "official" standard.

Therefore going back to my original post, I asked two questions:

1. How does one calibrate a barometric sensor, say in Europe, when METAR SLP is absent?

2. For an interesting point for discussion; In those juridictions where METAR reports SLP, I've observed that Altimeter sometimes equals SLP at different temperatures and different pressures. Can anyone explain why? I might have some theories, but I am interested in everyone's comments.

[gszlag]

1. How does one calibrate a barometric sensor, say in Europe, when METAR SLP is absent?

2. For an interesting point for discussion; In those juridictions where METAR reports SLP, I've observed that Altimeter sometimes equals SLP at different temperatures and different pressures. Can anyone explain why? I might have some theories, but I am interested in everyone's comments.

Got an answer to Question #1. I guess I should have checked the old posts first.

Thanks to a lot of posts from primarily UK weather enthusiasts,  QNH is indeed the preferred reference used to calibrate barometric sensors.

Question #2. More old posts reading. Definitely I am not the first to observe METAR SLP = Altimeter/QNH on occasion. But I could not find any clear explanations about how this occurs - perhaps METAR SLP 12 hr temperature mean happens to calculate out to to the same ISA temp at a particular elevation despite the fact the two formulas are different.

The ISA (International Standard Atmosphere) temp at my elevation is 13.75C.  As David Burch ponted out - if your station temperature is not 13.75C year round, you must correct for temperature and add or subtract from QNH to get a more accurate QFF.

If you haven't already, I highly recommend you read David Burch's entire article here: https://davidburchnavigation.blogspot.com/2013/10/air-temperature-dependence-of-sea-level.html

Pay special attention to his graphs. Note the blue line vs the red line where the data points illustrated are differences between current temperature SLP, 12 hour mean temperature SLP calculation vs actual METAR SLP readings.

I should explain further about these graphs because they are important.: If you use the current outside temperature to calculate MSLP/SLP you should get "OK" type of results. Dr. Burch says to get far better MSLP/SLP calculations you must use 12 hour mean outside temperatures at your location.
--
[Edited Oct 8, 2021 - grammar edits]
[Edited Oct. 29, 2021. added last para re: importance of 12 hr mean temps]

[Jim_S]

1. How does one calibrate a barometric sensor, say in Europe, when METAR SLP is absent?

I don't have a Davis station and I live in the U.S. and have access to SLP and altimeter but I've always calibrated to altimeter. With, as you say, a "fixed offset" station using SLP just adds extra variables that the station can't allow for.

[gszlag]

1. How does one calibrate a barometric sensor, say in Europe, when METAR SLP is absent?

I don't have a Davis station and I live in the U.S. and have access to SLP and altimeter but I've always calibrated to altimeter. With, as you say, a "fixed offset" station using SLP just adds extra variables that the station can't allow for.

Sorry, I have been away and just got back.

That is correct. Fine Offset weather stations use fixed offsets.

The whole precept about Dr. David Burch's blog article is about applying temperature corrections to Altimeter (QNH). Altimeter ignores measured/observed temperatures completely - something a meteorologist might find difficult to ignore. Even pilots have to temperature correct their aneroid altimeters in flight should temperatures diverge too far from standard ISA temperatures.

Dr. David Burch's recommendation is to use Altimeter if that is what's available but if SLP is available - use it instead.