Topic: Barometer Accuracy

[nincehelser]

Bringing this back to Acurite equipment, I find it funny so many people have complained that Acurite displays don't show altimeter.

I don't think anyone has complained that their displays don't show SLP.

I wonder why that is?   

[Jim_S]

Bringing this back to Acurite equipment, I find it funny so many people have complained that Acurite displays don't show altimeter.

I don't think anyone has complained that their displays don't show SLP.

I wonder why that is?   

My complaint (really more of a feature request) is that I can't quickly toggle between station pressure and adjusted pressure or better have the option of displaying both in MyAcurite.

[SnowHiker]

Bringing this back to Acurite equipment, I find it funny so many people have complained that Acurite displays don't show altimeter.

I don't think anyone has complained that their displays don't show SLP.

I wonder why that is?   

Does it show QNH by any chance?

 

[Jim_S]

However, the majority of stations out there do not calculate SLP.  It's not in common use with the general population.  Altimeter is the CWOP pressure standard because it is the simplest pressure reduction format that most CWOP stations can reliably deliver.

I agree. Right now I have my Access set up to for adjusted pressure and have entered my altitude (as accurately as I have been able to determine from topo maps and various altimeters).

Which pressure reduction method isn't even stated in the wunderground upload protocol.  It's been ambiguous from the get-go, and some stations even send station/absolute pressure instead.

Yep. I checked a half dozen or so stations near me and two of them were reporting station pressure. Let's face it the average user isn't the typical weather geek that frequents this forum.

Also, many use "SLP" or "MSL" as just meaning the pressure has been reduced to sea level, without any regard to the method.  I've found the only way to know what someone really means is to ask them about the details, such as if temperature and/or latitude is considered in the reduction.

Exactly. The NWS uses SLP and altimeter to make it clear which method they're using in each case but both are methods of adjusting station pressure to sea level pressure so the values can be compared at different locations.

[openvista]

Mean sea-level pressure is a pressure value obtained by the theoretical reduction or increase of barometric pressure to sea-level. The calculation corrects for the altitude difference from the pressure reading at the station elevation, to what it would otherwise be at sea-level. Land elevation affects the pressure reading at the surface. For example, a station on a hill may read a lower pressure than it would read if the station was at the same point and there was no hill. High pressure and low pressure systems are based on mean sea-level pressure to keep them comparable at any geographic location. Thus, surface pressure is different that mean sea-level pressure as it has not yet been corrected for the difference in altitude from sea-level.

I don't see any mention of temperature in the above quote. I'll grant that it's a flawed definition of MSLP.

Examining their requirements document (https://feedback.weather.com/customer/en/portal/articles/2924682-pws-upload-protocol?b_id=17298), doesn't clarify matters either as it just says supply "barometric pressure [in] inches".

So, in light of their working definition, and the fact that most weather stations seem to send altimeter, I would send altimeter.

In the end, people are sending whatever their console or software supports or defaults to. So even if WU had a hard requirement for altimeter, it wouldn't really change anything. They've got over 300k stations online. That's a monster that cannot be tamed.

[CW2274]

Bringing this back to Acurite equipment, I find it funny so many people have complained that Acurite displays don't show altimeter.

I don't think anyone has complained that their displays don't show SLP.

I wonder why that is?   

Does it show QNH by any chance?

 

For those unaware, QNH is nothing more than the altimeter setting given in millibars (basically for the Euros).

[SnowHiker]

For those unaware, QNH is nothing more than the altimeter setting given in millibars (basically for the Euros).

Or in hectopascals.

 

[openvista]

Or in inches of mercury because QNH has nothing to do with units. It's an acronym, most commonly used in aviation, that equates to altimeter. The two terms are used interchangeably.

See: https://www.skybrary.aero/index.php/Altimeter_Pressure_Settings

[CW2274]

Or in inches of mercury because QNH has nothing to do with units.

True. The difference is we here say altimeter, which is issued in inHg. Over there, they say QNH, which is issued in mb/hPa.

[galfert]

I already knew the answer to this question. But I sent Acurite Support this question for documentation purposes.

Email I sent Acurite Support:

Mar 2, 3:47 AM CST

Hello,
I'd like to know if the Acurite Access device calculates Sea Level Pressure or Altimeter pressure? Which one was it designed to send to myAcurite.com after knowing your elevation?

Thank you,
George

Response back from Acurite Support:

From: Sheri (AcuRite Support) <support@chaney-inst.com>
Sent: Sunday, March 3, 2019 10:34 AM
Subject: My AcuRite Help Inquiry
    
Sheri (AcuRite)

Mar 3, 9:34 AM CST

Dear George,

Thank you for contacting AcuRite. We will be happy to assist you.

The AcuRite Access calculates Sea Level Pressure.

Have a great day,
Sheri
AcuRite Customer Support
www.AcuRite.com

[Jim_S]

I already knew the answer to this question. But I sent Acurite Support this question for documentation purposes.

Email I sent Acurite Support:

Mar 2, 3:47 AM CST

Hello,
I'd like to know if the Acurite Access device calculates Sea Level Pressure or Altimeter pressure? Which one was it designed to send to myAcurite.com after knowing your elevation?

Thank you,
George

Response back from Acurite Support:

From: Sheri (AcuRite Support) <support@chaney-inst.com>
Sent: Sunday, March 3, 2019 10:34 AM
Subject: My AcuRite Help Inquiry
    
Sheri (AcuRite)

Mar 3, 9:34 AM CST

Dear George,

Thank you for contacting AcuRite. We will be happy to assist you.

The AcuRite Access calculates Sea Level Pressure.

Have a great day,
Sheri
AcuRite Customer Support
www.AcuRite.com

Here is what the weather service says about pressure (the bolding is mine):

ALTIMETER SETTING: This is the pressure reading most commonly heard in radio and television broadcasts. It is not the true barometric pressure at a station. Instead it is the pressure "reduced" to mean sea level using the temperature profile of the "standard" atmosphere, which is representative of average conditions over the United States at 40 degrees north latitude. The altimeter setting is the pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude (above mean sea level) of the aircraft on the ground at the location for which the pressure value was determined. The altimeter setting is an attempt to remove elevation effects from pressure readings using "standard" conditions.

MEAN SEA LEVEL PRESSURE: This is the pressure reading most commonly used by meteorologists to track weather systems at the surface. Like altimeter setting, it is a "reduced" pressure which uses observed conditions rather than "standard" conditions to remove the effects of elevation from pressure readings. This reduction estimates the pressure that would exist at sea level at a point directly below the station using a temperature profile based on temperatures that actually exist at the station. In practice the temperature used in the reduction is a mean temperature for the preceding twelve hours. Mean sea level pressure should be used with caution at high elevations as temperatures can have a very profound effect on the reduced pressures, sometimes giving rise to fictitious pressure patterns and anomalous mean sea level pressure values.

I would still be willing to bet 1000 quatloos that Acurite is actually using altimeter pressure or some other simplified algorithm of their own and not using the strict NWS formula for MSLP. Both are methods of adjusting station pressure to sea level. So telling you "Sea level pressure" is accurate in either case.

[galfert]

My question to Acurite was very clear with no room for ambiguity because I presented two possibilities. I got a response back that was very clear.

[CW2274]

I would still be willing to bet 1000 quatloos that they are actually using altimeter pressure or some other simplified algorithm of their own and not using the strict NWS formula for MSLP.

1000 quatloos...

If they is the NWS, they are merely putting out what the public is used to seeing via airport data, in almost all cases, not some spin on it of their own. I'll say this again....Mets use MSLP for forecasting and their everyday jobs, which is displayed in millibars (remember, no one knows what that is). What we average Joe's are fed everyday is the altimeter simply because it's given in inHg. As long as I can remember, this has been the case, and see no reason that that will change. There is absolutely nothing that prevents one from using MSLP, should one choose to do so, equipment capable or not. Figure it out. I used the altimeter with my VP2 console for years, wasn't perfect but I got the gist.

[SnowHiker]

 Mean sea level pressure should be used with caution at high elevations as temperatures can have a very profound effect on the reduced pressures, sometimes giving rise to fictitious pressure patterns and anomalous mean sea level pressure values.

I would still be willing to bet 1000 quatloos that Acurite is actually using altimeter pressure or some other simplified algorithm of their own and not using the strict NWS formula for MSLP. Both are methods of adjusting station pressure to sea level. So telling you "Sea level pressure" is accurate in either case.

I do know from using a Davis Vantage Pro at high elevation (over 7000 ft), that the algorithm they use for their "barometer" readings that take the average temperature into account are so erratic as to be completely meaningless, at least to me. 

So my guess would be that if Accurite pressure readings make sense at high altitudes then they are closer to altimeter than to MSL.

[Jim_S]

1000 quatloos...
If they is the NWS, they are merely putting out what the public is used to seeing via airport data, in almost all cases, not some spin on it of their own. I'll say this again....Mets use MSLP for forecasting and their everyday jobs, which is displayed in millibars (remember, no one knows what that is). What we average Joe's are fed everyday is the altimeter simply because it's given in inHg. As long as I can remember, this has been the case, and see no reason that that will change. There is absolutely nothing that prevents one from using MSLP, should one choose to do so, equipment capable or not. Figure it out. I used the altimeter with my VP2 console for years, wasn't perfect but I got the gist.

(sigh) Sorry, I thought it was obvious, "they" is Acurite (I fixed my post). The NWS defines and makes if very clear what the difference is. Acurite simply says "Sea Level Pressure" but doesn't explain how they (Acurite) calculates it. I find it very hard to believe that their (Acurite's) formula goes to the trouble of averaging 12 hours of temperature. I also see no benefit in them (Acurite) calculating something different than what the NWS provides the public.

Back to my original question on barometer accuracy. I did find this: https://www.acurite.com/learn/accuracy

Barometric Pressure    Tolerance
inHg or hpa    +/- 0.15 inHg (+/- 5 mbar) after Learning Mode has completed

[Jim_S]

I do know from using a Davis Vantage Pro at high elevation (over 7000 ft), that the algorithm they use for their "barometer" readings that take the average temperature into account are so erratic as to be completely meaningless, at least to me. 

Well the NWS does warn that using SLP can be a problem at high elevations.

Mean sea level pressure should be used with caution at high elevations as temperatures can have a very profound effect on the reduced pressures, sometimes giving rise to fictitious pressure patterns and anomalous mean sea level pressure values.

[SnowHiker]

Well the NWS does warn that using SLP can be a problem at high elevations.

Mean sea level pressure should be used with caution at high elevations as temperatures can have a very profound effect on the reduced pressures, sometimes giving rise to fictitious pressure patterns and anomalous mean sea level pressure values.

Yes, that's exactly what I was verifying and why I quoted that text also. 

[CW2274]

1000 quatloos...
If they is the NWS, they are merely putting out what the public is used to seeing via airport data, in almost all cases, not some spin on it of their own. I'll say this again....Mets use MSLP for forecasting and their everyday jobs, which is displayed in millibars (remember, no one knows what that is). What we average Joe's are fed everyday is the altimeter simply because it's given in inHg. As long as I can remember, this has been the case, and see no reason that that will change. There is absolutely nothing that prevents one from using MSLP, should one choose to do so, equipment capable or not. Figure it out. I used the altimeter with my VP2 console for years, wasn't perfect but I got the gist.

(sigh) Sorry, I thought it was obvious, "they" is Acurite

https://www.youtube.com/watch?v=OjYoNL4g5Vg

[openvista]

I'd be very surprised if entry-level support for Acurite even understands the difference between "sea level pressure" and "altimeter". I'd also bet Acurite uses the term "sea level pressure" somewhere in their documentation even if they're referring to altimeter. So the support rep probably found that reference and simply parroted it.

I have a $600 reference barometer whose documentation is filled with references to "sea level pressure". Guess what? They are referring to altimeter. This is common among weather station & barometer manufacturers. Altimeter is SO MUCH EASIER and cheaper to program.

I really wish we could adopt the acronyms QFE (station pressure), QNH (alt) and QFF (MSLP) in the weather community. So much clearer.

[nincehelser]

My question to Acurite was very clear with no room for ambiguity because I presented two possibilities. I got a response back that was very clear.

What yo did is manipulate the question to get the answer you wanted. 

Acurie in NO way caclulsates SLP, they certainly don't take 12-hour temperature data into the fact.

This is a case of asking someone who actually knows rather than someonwho has never used the harware in their llife.

[freddie]

basically for the Euros

No, it's the rest of the world that uses millibars/hectopascals for QNH.  The USA and Canada are the only ones who use inches of mercury for pressure units of measurement.

[CW2274]

basically for the Euros

No, it's the rest of the world that uses millibars/hectopascals.  The USA and Canada are the only ones who use inches of mercury for pressure units of measurement.

You are correct, I over simplified.

[Jim_S]

I'd be very surprised if entry-level support for Acurite even understands the difference between "sea level pressure" and "altimeter". I'd also bet Acurite uses the term "sea level pressure" somewhere in their documentation even if they're referring to altimeter. So the support rep probably found that reference and simply parroted it.

I have a $600 reference barometer whose documentation is filled with references to "sea level pressure". Guess what? They are referring to altimeter. This is common among weather station & barometer manufacturers. Altimeter is SO MUCH EASIER and cheaper to program.

I really wish we could adopt the acronyms QFE (station pressure), QNH (alt) and QFF (MSLP) in the weather community. So much clearer.

My thoughts exactly. Thank you for posting!

[Jim_S]

I do know from using a Davis Vantage Pro at high elevation (over 7000 ft), that the algorithm they use for their "barometer" readings that take the average temperature into account are so erratic as to be completely meaningless, at least to me. 

Davis, at least, publishes (in detail) the algorithm they use for Sea Level Pressure (and other derived variables). This is what I was hoping for from Acurite. Here is a Link: https://www.davisinstruments.com/product_documents/weather/app_notes/AN_28-derived-weather-variables.pdf

But even they don't calculate SLP the same way the NWS/NOAA does. Here is what they say:

This is a simplified version of the official U.S. version in place now. The accepted method is to
use lookup tables of ratio reduction values keyed to station temperature. These are based on
station climatology. These values are unavailable for every possible location where a Davis
user may have a station, thus this approach is not suitable.

So, if you were to ask Davis if they use Altimeter or SLP, just like Acurite, they would say SLP (and at Davis they might even know what you're talking about) but they don't mean the same thing as the NWS does when the say "Sea Level Pressure".

[Jim_S]

... and some stations even send station/absolute pressure instead.

It turns out this isn't just a problem with clueless PWS owners. I have several weather apps on my iPhone and today I was checking "Yahoo Weather" which I feel has the nicest, most intuitive, interface but it's reporting station pressure (27.7...seriously) not adjusted pressure.   

I have my Access set up for "adjusted pressure" based on my best guess estimate of my elevation (using topo maps and various altimeters). So far it's been tracking the NWS barometer readings very closely so I'm happy. If I decide I need to fine tune it further I can do that by adjusting the elevation which could be off by +/- 5-10' or so.

Again I'd like to thank everyone that contributed to the discussion. I may not have always agreed but I appreciated the input.