[..to be continued in Part III
The trouble with temperature - Part IIITo 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.htmlCalculate 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.htmlSLP calculator(uses 12 hour mean temperatures):
https://www.starpath.com/barometers/baro_cal.phpBarani SLP calculator (uses current temperature and latitude):
http://www.wind101.net/sea-level-pressure-advanced/sea-level-pressure-advanced.htmlAnalogue 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.xlsKeisan Temperature and pressure at destination calculator:
https://keisan.casio.com/exec/system/1224562962Bosch BMP390 high performance barometric sensor specs:
https://www.bosch-sensortec.com/media/boschsensortec/downloads/product_flyer/bst-bmp390-fl000.pdf—