[wxqc] Barometer Tweaking
dshelms at comcast.net
dshelms at comcast.net
Mon Feb 6 12:30:11 EST 2006
FYI... The CWOP Guide has just about every metric to US unit conversion table you will ever need for winds, pressure, and temperature observations.
Dave
CW0351
-------------- Original message ----------------------
From: gary.oldham at adelphia.net
> Airports use inches of mercury as that's how all aircraft altimeters are
> configured.
>
> gary.oldham(at)adelphia.net
>
> ---- Don Palmer <don at kansasphotos.com> wrote:
>
> =============
> I started using that local phone number for my area (Topeka, KS) that Dave
> brought
> to everyones attention for the local altimeter reading several months ago, but
> what
> gets me is that about a year ago they would always give it in millibars along
> with
> everything else in metric, but now they give everything in metric, but the
> altimeter
> is in inches. Oregon Scientific barometers are more accurate using MB instead
> of
> inches and the inches from the ASOS or airport don't necessarily line up with
> the
> inches displayed on the OS barometer.
>
> don
> CW2327
>
>
>
>
> On 5 Feb 2006 at 23:37, Dave Helms wrote:
>
> 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.
> >>>>>
> >>>>> _______________________________________________
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> >>>>>
> >>>>> The contents of this message are the responsibility of the
> >>>>> author.
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>
> >>>>
> >>>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>
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