[wxqc] Sea Level Pressure

Wed Sep 7 22:13:52 EDT 2005

Hi Mike,

Yep, lots of potential pressure derivations combinations to consider.  
Going after the pressure problem has been on the honey do list, but a 
little lower than other items like finishing the CWOP Guide and quick 
start brief, developing the NWS CWOP "Road Show" slides for the WCMs, 
developing the snow instructions, revalidating the dew point/RH 
performance thresholds with Philip, enhancing the CWOP Software Support 
Page, yadi, yadi, ya.

Ultimatedly, we will have to document exactly how the larger weather 
station and software vendors sample raw data and generate the parameters 
(e.g. what specific algorithms) in order to understand how to use CWOP 
information more effectively. 

Dave
CW0351

Mike Barth wrote:

> 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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