[wxqc] Barometer Tweaking

Dave Helms dshelms at comcast.net
Sun Feb 5 23:37:37 EST 2006 

You might try reading the CWOP Guide section on pressure too, I borrowed 
heavily from the UIUC site. 
http://mywebpages.comcast.net/dshelms/CWOP_Guide.pdf

Sam Drinkard wrote:

> Chuck,
>
>    I don't know if this might confuse the issue even more, but here's 
> a pretty good link that does a decent job of describing atmospheric 
> pressure.  Altho it does not go into great detail about the difference 
> between station pressure & altimeter, the graphics do explain *why* 
> the pressure is different, and with a bit of thinking, you'll see how 
> the altimeter readings are obtained.
>
> http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/prs/def.rxml
>
> The whole website is great for general reading and info.
> Sam
> ------------------------------------------------------------------------
>
>
> Dave Helms wrote:
>
>> Chuck,
>>
>> Here is as simple as I can say it, the (altimeter) pressure at the 
>> airport is not the true pressure at the airport (the true pressure is 
>> the raw, uncorrected, station pressure which is not typically sent to 
>> the public).  Its the pressure you would have at the airport IF you 
>> got in an elevator and pushed "G" and descend through bedrock to sea 
>> level directly below the airport.  The altimeter pressure correction 
>> ADDs the hypothetical pressure between a station's true elevation and 
>> sea level  so  weather folks can  do an apples to apples  comparison 
>> between stations with different elevations.
>> Here is the easy way out of this deal, on an afternoon with little or 
>> no winds...
>> 1.  Set your Ultimeter console pressure units to "inHg" (or just 
>> "in") for inches of Mercury
>> 2.  Call the weather station automated report #, 209-257-1292
>> 3.  Write down the pressure the AWOS says is the current pressure 
>> (altimeter), it will be XX.XX inches (of Mercury or the atomic 
>> element code "Hg", get it!)
>> 4.  Adjust your Peet Ultimeter to the AWOS pressure using the Peet 
>> instructions for adjusting pressure
>> 5.  Watch the QCMS for a week or two to see if you hit the target
>>
>> The Peet Ultimeter instructions are terrible at telling people how to 
>> set their pressure.  Essentially, whatever reference pressure you use 
>> to set your pressure (in CWOP's case, airport ASOS altimeter), that 
>> is the "type" of pressure your station is then reporting.
>>
>> This will work, trust me.
>>
>> Dave
>> CW0351
>>
>> Charles Bland wrote:
>>
>>> Dave,
>>>
>>> I'm trying to get a mental handle on this statement.
>>>
>>> "Altimeter corrects pressure to sea level so you do not have to 
>>> account for elevation differences between your station and the 
>>> reference station. "
>>>
>>> A quick piece of background.... What started all this is the CWOP 
>>> Quality page for my station. At the time (not now; it's all 
>>> screwed-up now because of my earlier outage) the data quality site 
>>> said my barometer was off but temps and humidity were fine. Since 
>>> watching that, I've been on a quest to understand barometer 
>>> calibration and how I can my barometric data quality up.
>>>
>>> So, from the quote above, are you saying that the corrected sea 
>>> level pressure, as measured at the airport, is probably the same 
>>> corrected pressure for my house, even though I am 1810 feet higher?
>>>
>>> I took a drive to day with my barometer. I had set it to the reading 
>>> announced by the airport AWOS, 30.07". By the time I got to the 
>>> airport, the barometer was reading 2 inches higher. No surprise. I 
>>> set it again then drove home.
>>>
>>> Yup, the reading was 2 inches lower.
>>>
>>> According to the fudge-factor you gave me, I would expect to see a 
>>> 1.8" difference, which isn't far from the 2" delta that was measured.
>>>
>>> How do I pull this all together?
>>>
>>> Also, is my Ultimeter 2100 capable of displaying absolute air 
>>> pressure? It doesn't come out and say that in the manual. It does 
>>> say I can display pressure in three difference units, hPa/mbar, 
>>> inHg, or mmHg. I'm going to presume that inches and mm HG are 
>>> relative. How about hPa/mbar? Relative or absolute?
>>>
>>> Dave et al, thanks for your patience in this. It is facinating 
>>> stuff, but also a steep learning curve.
>>>
>>> Chuck
>>>
>>> I went for a nice Sunday afternoon drive today and found a 2" Hg 
>>> difference from my house to the airport.
>>>
>>> Note Dave Helms's stunning brilliance and Chuck's apt reply (gack!)
>>>
>>> Date sent:          Sun, 05 Feb 2006 12:16:47 -0500
>>> From:               Dave Helms <dshelms at comcast.net>
>>> To:                 BackForty at BlandRanch.net,
>>>     Discussion of weather data quality issues 
>>> <wxqc at lists.gladstonefamily.net>
>>> Subject:            Re: [wxqc] Barometer Tweaking
>>>
>>>  
>>>
>>>> Hi Chuck,
>>>>
>>>> Altimeter corrects pressue to sea level so you do not have to 
>>>> account for elevation differences between your station and the 
>>>> reference station.  Just match the reported airport METAR altimeter 
>>>> pressure ("QNH" XX.XX inches of Mercury (Hg) or "INS") to your 
>>>> corrected altimeter pressure.  Listen to the Amador METAR  report 
>>>> from its AWOS station by calling 209-257-1292 (this  is  on the 
>>>> airnav.com page).
>>>> Dave
>>>> CW0351
>>>>
>>>> P.S.  Near sea-level, pressure decreases about 0.001 inch of 
>>>> Mercury for every foot in the vertical.  Station pressure is only 
>>>> used when comparing two station's raw, uncorrected, station 
>>>> pressures.  Station pressure is not generally available from 
>>>> ASOS/AWOS stations.  In addition to requiring both (raw) station 
>>>> pressures in inches of Mercury, you also need both station's 
>>>> elevations in feet to do a comparison.
>>>> Charles Bland wrote:
>>>>
>>>>  
>>>>
>>>>> Me and my Peet Bros Weather Station live in the Sierra Nevada 
>>>>> mountains at 3500 ft AMSL.
>>>>>
>>>>> My nearest airport with AWOS is at approximately 1700 ft AMSL. 
>>>>> http://www.airnav.com/airport/O70
>>>>>
>>>>> So, when I calibrate my barometer AT THE AIRPORT, then take it 
>>>>> home, the reading always falls because of the elevation/air 
>>>>> pressure change.
>>>>>
>>>>> My question is, how do I adjust the AWOS reading at the airport to 
>>>>> compensate for the elevation change to my house? Is there a 
>>>>> ballpark number for air pressure change as you change elevation?
>>>>>
>>>>> Chuck
>>>>>
>>>>> p.s. a recent power outage here really goofed my station, so if 
>>>>> you look the data quality page for me (CW4289) it is going to look 
>>>>> REAL BAD. So, since my station battery died and I lost a bunch of 
>>>>> data, I took the opportunity to solve my calibration problem on 
>>>>> the barometer.
>>>>>
>>>>> _______________________________________________
>>>>> wxqc mailing list
>>>>> wxqc at lists.gladstonefamily.net
>>>>> http://pond1.gladstonefamily.net:8080/mailman/listinfo/wxqc
>>>>>
>>>>> The contents of this message are the responsibility of the author.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>     
>>>>
>>>>
>>>>   
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>  
>>>
>>
>> _______________________________________________
>> wxqc mailing list
>> wxqc at lists.gladstonefamily.net
>> http://pond1.gladstonefamily.net:8080/mailman/listinfo/wxqc
>>
>> The contents of this message are the responsibility of the author.
>>
>>
>
>
> _______________________________________________
> wxqc mailing list
> wxqc at lists.gladstonefamily.net
> http://pond1.gladstonefamily.net:8080/mailman/listinfo/wxqc
>
> The contents of this message are the responsibility of the author.
>
>

More information about the wxqc mailing list