[wxqc] Barometer Tweaking

[Charles Bland]

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.
> >
> >
> >  
> >
> 
>