[wxqc] Barometer Tweaking
Sam Drinkard
sam at wa4phy.net
Sun Feb 5 23:33:21 EST 2006
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.
>>>>
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>>>>
>>>> The contents of this message are the responsibility of the author.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>
>>>
>>
>>
>>
>>
>>
>>
>>
>>
>
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