[wxqc] Sea Level Pressure
Clay Jackson
clayj at nwlink.com
Fri Sep 9 01:10:58 EDT 2005
Well, I'm glad my old WMII doesn't have THIS problem - I've sure had
enough others :-)
Anyway, it seems to me that there are enough CWOP/APRSWXNET folks
now, and more coming "out of the woodwork", that it might be possible
for us as a group to start sending "requirements" to vendors (which
would seem to me to be fairly simple) and perhaps even set up a
"certification" process whereby a vendor could then claim "CWOP
Certified" if their data collection and transmission algorithms could
be configured according to those standards.
I might even volunteer to help collect and publish some of this, if
someone at NOAA/MADIS can tell us the specifics on what they're
looking for.
Clay Jackson
N7QNM
On Sep 7, 2005, at 7:13 PM, Dave Helms wrote:
> 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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