[wxqc] Sea Level Pressure
Dave Helms
dshelms at comcast.net
Fri Sep 9 07:06:47 EDT 2005
Hi Clay,
We do have a requirements document, its the CWOP Guide we wrote last
spring. We documented measurement and processing requirements in great
detail in the guide; this section of the guide was not written for the
weather station owners, but for vendors supporting the effort.
Dave
Clay Jackson wrote:
> 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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