[wxqc] Barometer Tweaking

gary.oldham at adelphia.net gary.oldham at adelphia.net
Mon Feb 6 08:45:14 EST 2006 

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.
>>>>>
>>>>> _______________________________________________
>>>>> wxqc mailing list
>>>>> wxqc at lists.gladstonefamily.net
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>>>>>
>>>>> The contents of this message are the responsibility of the
>>>>> author.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>     
>>>>
>>>>
>>>>   
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>  
>>>
>>
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