[wxqc] Sea Level Pressure

[Mike Barth]

WeatherLink is apparently reporting a variable that we can refer to as
"NWS sea level pressure", since it uses the same SLP reduction algorithm
(using 12h-old temperature) as is used by ASOS's to report SLP.  See this 
explanation from Davis:

http://pond1.gladstonefamily.net:8080/pipermail/wxqc/2005-May/000182.html

The "altimeter setting" variable that we want is the same as the value
that is reported on the TV news, and is simply station pressure with
an offset calculated as a function of elevation, using the
U.S. standard atmosphere assumption.  I have a Lacrosse WS2310, using
WeatherDisplay software, and what I did was simply add the offset via
WD, and didn't add any offset on the console.

The differences between SLP and altimeter are accentuated at altitude.
Near sea level they're very close, so people who are near sea level
may still have some confidence in the QC results (maybe!).

We may have a multitude of different algorithms being used by the different 
weather station hardware and software platforms, and this may have a lot
to do with the high QC failure rate we've seen for the network as a whole.

DaveH:  Can you take an action to find a volunteer to figure out what the
different hardware/software platforms are doing, and possibly get them
all reporting altimeter?  Alternatively, we could use the station table
to track which sites have validated altimeter data and then toss all
obs that haven't been validated?  Etc.

Mike


On Wed, 7 Sep 2005, Gary Oldham wrote:

> WeatherLink reports the same data that's shown on the Vantage Pro console. 
> If the user has set his altitude on the console (or via the WeatherLink 
> software, which then makes the adjustment on the console), then that value 
> (adjusted for altitude only) is what is reported by WeatherLink.  As far as I 
> know, that's the same way it works with all the third party software as well, 
> including VWS.  VWS will allow a user to use their station pressure (meaning 
> an altitude value of "0" on the Vantage Pro console) and will do the 
> adjustment within VWS; I don't know whether this adjustment goes to altimeter 
> value or sea level. 
> Confusingly, the VWS manual makes the following statement: "An air data 
> calculator is included in Virtual Weather Station. Virtual Weather Station 
> uses the same formulas used in airplane data computers flying throughout the 
> world.The altitude and air data information provides air density, pressure 
> altitude, corrected temperature and pressure to sea-level conditions."  The 
> "air data calculator" on the Station Settings window in VWS does show both 
> the temperature and weather station-reported pressure, so perhaps VWS *is* 
> including temperature in its pressure corrections.  WeatherLink, to the best 
> of my knowledge, does not.
>
> Gary Oldham
> www.ag-weather.com
> www.weatherforum.net
>
> Mike Barth wrote:
>
>> I don't know very much about the Davis data, so I might be wrong, but the
>> impression I had was that the calculation of sea level pressure (instead of
>> the desired altimeter) is done by the Davis Weather Link software.  If this
>> is correct, then VWS might be reporting the correct quantity.  Can someone
>> who knows more about Davis and/or VWS comment on this question?
>> 
>> Mike
>> 
>> On Wed, 7 Sep 2005 rich.taft at att.net wrote:
>> 
>>> I also have a VP2 using VWS.  I've been getting 97 of 97 errors on my QC 
>>> report. Doesn't this mean that the pressure data that we VP2 users paid 
>>> big bucks to collect is also being ignored by NOAA MADIS?
>>> 
>>> I'm sure the VWS can provide both corrected and uncorrected data. If it?s 
>>> putting the wrong data into the packet then that needs to be addressed 
>>> with the Author of VWS.
>>> 
>>> I'm a little unsure of what the problem with the data is.  Are you saying 
>>> that VP2 does Temperature & Humidity compensation on the raw Pressure data 
>>> prior to sending it to the Data logger, or is it something else?
>>> 
>>> -- 
>>> Richard Taft
>>> www.taftphoto.com
>>> CW3815
>>> 
>>> -------------- Original message ----------------------
>>> From: Mike Barth <Michael.F.Barth at noaa.gov>
>>> 
>>>> John,
>>>> 
>>>> Sorry I didn't reply sooner...
>>>> 
>>>> I'm afraid the QC processing doesn't have the ability to take into 
>>>> account
>>>> that some APRSWXNET stations are reporting SLP instead of altimeter.  I 
>>>> suggest
>>>> you contact Davis about fixing this -- the APRSWXNET protocol requires
>>>> altimeter.  In the meantime, you should ignore the QC results.
>>>> 
>>>> Mike
>>> 
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