Author Topic: Backing Station Pressure numbers out of VP2 Sea Level Pressure numbers?  (Read 3872 times)

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Offline Old Tele man

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OK, here's my sob-story: I am tired of beating my head against the "Davis-induced SLP Wall" and need an accurate method to "back" the 'raw' barometer sensor Station Pressure "out" of the VP2-calculated Sea Level Pressure (SLP) pressure data that WL-ISP sends to CWOP / MADIS / MesoWest, et.al..

I read the Barometer Pressure section of Davis Application Note #28, Rev. A, dated 5/11/2006, with absolutely NO success because the humidity correction coefficient "C" isn't calculated, but rather comes from a "lookup table." The text wording says the lookup table is  "...(provided in the attached table)..."; however, there is NO attached table! (I've e-mailed Davis about this omission) Most likely they meant use the "lookup table" found in one of the footnote referenced documents, but they forgot to specify *which* footnote document!

So, what I did was collect the last seven days of weather data for weather stations KTUS(NWS @ Tucson airport), KDMA(USAF @ DMAFB), CW5619, and DW6988(my station), into an Excel spreadsheet. Data from each station was collected into columnar rows: SLP, ALT, Sta.P, SLP/ALT-ratio, etc.. I then compared the columns of data against each other and found two sets of data pairs with very high correlation coefficients:

• PART I - What's Related to What?

1) Pstn vs. Palt (both in "Hg):

KTUS(2546'):  Palt = 1.0764*(Pstn) + 0.5599; R2 = 0.9999
KDMA(2703'): Palt = 1.0850*(Pstn) + 0.4952; R2 = 0.9999


2) K vs. SLP/ALT-ratio (K is mb/meter; SLP/ALT is dimensionless):

KTUS(776m):  (Pslp/Palt) = 1.0724 - 0.0085*(K); R2 = 0.9782
KDMA(824m): (Pslp/Palt) = 1.0743 - 0.0087*(K); R2 = 0.9625


NOTE - "scale height" coefficient K, in mb/meter, is found in the following common equation:

Pslp(mb) = Pstn(mb) + (height(m) / K ) ...where K-values range between 8.5 and 9.2

So, what did I learn so far from this exercise? (1) First, the NWS and USAF conversion of Sta.P into ALT is almost perfectly LINEAR! (2) Secondly, the "scale height" coefficient K is LINEARLY related to the ratio of the SLP and ALT values.


• PART II - Putting the Numbers to Work (...TBD...)
« Last Edit: August 09, 2013, 09:32:08 PM by Old Tele man »
• SYS: Davis VP2/WL-IP & Envoy8X/WL-USB
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside


Offline Old Tele man

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Re: Backing Station Pressure numbers out of VP2 SLP numbers?
« Reply #2 on: August 08, 2013, 03:48:35 PM »
Have you read this?
http://server.gladstonefamily.net/pipermail/wxqc/2013-August/018190.html

Yes, that is part of the reason I started this exercise, the other part is that *I* need true 'raw' barometric sensor 'station pressure' (Pstn) to calculate wetbulb temperatures...and I can NOT get it from either one of my Davis units (VP2 with WL-IP; Envoy8X with WL-USB)!

So, I am forced to "roll" my own, hence this mathematical exercise of trying to 'pull' a station pressure RABBIT 'out' of a Davis sea level pressure HAT!

Backing 'raw' station pressure (Pstn) out of barometric altimeter pressure (Palt) is relatively easy because it is a single-function reduction factor that is based solely upon height and typical pressure "lapse-rate" (pressure change vs. height change). There are SIMPLE "lapse-rate" approximations, such as these:

Pstn ~ Palt × (hm/773) ..."lapse-rate" of "Hg-per-meter.
Pstn ~ Palt × (hft/2536) ..."lapse-rate" of "Hg-per-foot.
Pstn ~ Pslp - hft x (0.001"Hg/ft) ..."lapse-rate" of "-0.001 in.Hg-per-foot[/b].

When starting from sea level pressure (Pslp), there are more precise "lapse-rate" approximations, such as these:

Pstn = Pslp - (hm / 8.5) ...Docbee calculation.
Pstn = Pslp - (hm / 9.2) ...Sandhurst Weather, UK calculation.
Pstn = Pslp × [(288-0.0065×hm)/288]5.2561 ...NOAA calculation (assumes 15ºC).
Pstn = Pslp × [(293-0.0065×hm)/293]5.255876 ...WMO calculation (assumes 20ºC).
Pstn = Pslp × EXP[ -0.119*(hm) - 0.0013*(hm)2 ] ...where hm = meters/1000; GLOBE approximation.

However, none of the above approximations account for temperature, humidity, gravity, or latitude compensations typically applied in SLP calculations.

So far, I've found this equation incorporating temperature compensation (in degrees Kelvin) for converting SLP back to station pressure:

Pstn = Pslp × EXP[ -hm/(Kº × 29.263) ] ...where: Kº = Cº+273.15º

Source: Sandhurst Weather, UK, document: http://www.sandhurstweather.org.uk/barometric.pdf

This equation is *very close*...for example, when Steve Hatchett's Vantage Pro Pressure Calculator gives a station pressure of 933.9mb (for conditions: Tnow = 100ºF; T12hr = 72ºF; RH = 14%; H = 2330 ft; VP2 SLP = 29.85"Hg), the equation gives a station pressure of 935.0mb -- a difference of just 1.1mb!

An 'expanded' version of the above NOAA equation incorporates "two-point 12-hour average temperature" for converting SLP back to station pressure and yields a station pressure of 933.7mb using the same setup conditions:

Kº = 273.15º + (Tnow + T12hr)/2 ...both T's in ºC.
Z = 0.0065×hm ...height in meters.

Pstn = Pslp × (Kº / (Kº + Z))5.2561
« Last Edit: August 09, 2013, 10:58:13 PM by Old Tele man »
• SYS: Davis VP2/WL-IP & Envoy8X/WL-USB
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside


Offline Old Tele man

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Re: Backing Station Pressure numbers out of VP2 SLP numbers?
« Reply #4 on: August 08, 2013, 04:38:01 PM »
Have you played with this thing

http://www.softwx.com/weather/vppressurecalc.html

Yes, I have a copy, but that is a 'stand-alone' applet and I need an equation to work with archived spreadsheet data.

However, I *DO* use that applet to verify numbers I derive from calculations...ie: my "second opinion" (ha,ha)!
« Last Edit: August 08, 2013, 06:59:03 PM by Old Tele man »
• SYS: Davis VP2/WL-IP & Envoy8X/WL-USB
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

Offline aweatherguy

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Re: Backing Station Pressure numbers out of VP2 Sea Level Pressure numbers?
« Reply #5 on: September 03, 2013, 03:23:19 AM »
Have you seen this page? http://wiki.wunderground.com/index.php/Educational_-_Sea_level

I think this is closer to the calculations being used at TUS and DMA. To verify your reverse engineering it might be useful to check these values again in winter when temps are a lot colder. One of the values you need to reverse engineer is the "mean annual temperature" which I think is also sometimes called the "normal temperatue".

I think there's an error in that web page however -- in (4) the table should be applied to stations BELOW 1000ft not above. I have a formula (don't recall where I got it so beware) that I think should be used above 1000ft:

f = -3593.722 + 0.34265*mt + 0.15098*md - 0.00060001*elevft - 1.92586*tr + 730.26934*tl

where mt = mean annual temp (F), md = mean annual dew point (F), tr = current temp in Rankin, and tl = loge(tr)


Offline Old Tele man

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Re: Backing Station Pressure numbers out of VP2 Sea Level Pressure numbers?
« Reply #6 on: September 03, 2013, 10:46:08 AM »
No, I had not seen that WunderWiki page before, so definitely "Thank, You!" for posting it!

FWIW, I e-mailed both MesoWest and MADIS telling them that *MY* Davis VP2 pressure data is SLP, not ALT, as CWOP expects, but neither seem to care that it's incorrect. One would think that they would be more interested in receiving correct data.
« Last Edit: September 03, 2013, 10:58:58 AM by Old Tele man »
• SYS: Davis VP2/WL-IP & Envoy8X/WL-USB
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

Offline aweatherguy

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Re: Backing Station Pressure numbers out of VP2 Sea Level Pressure numbers?
« Reply #7 on: September 03, 2013, 02:35:42 PM »
Ditto. I've come to realize that once the data crosses the boundary into the gov't-run part of the network it is a whole different world. While I'm sure there are folks in there doing good work, they are isolated and insulated from the public. I suspect the folks who do interface w/the outside world are connecting only to people in the CWOP and APRS organizations and don't make their contact info public.

I was told by one knowledgable person that every NWS office has a person assigned as a CWOP laison...but I have not tried calling my local NWS office yet...another one of those tasks on my plate that I have not gotten around to yet. Might be worth a phone call just to see what happens.

The CWOP folks are quite accessible on their mailing list and are helpful as far as they can go. They are clearly concerned about this whole Davis SLP vs Altimeter issue. The post by "tlum" linked below is from one of the CWOP folks. Since he's watching the list, perhaps you could post there and inquire about the Davis "calibration hack". Or you can temporarily joing the CWOP mailing list and ask about it there.

http://www.wxforum.net/index.php?topic=19445.0


Offline Old Tele man

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Re: Backing Station Pressure numbers out of VP2 Sea Level Pressure numbers?
« Reply #8 on: September 03, 2013, 02:56:32 PM »
Actually, I have already been to that CWOP discussion page (Phillip Gladstone, et.al.) with this problem. Unfortunately, (in my best Shakespearean voice) it's all "...much ado, about nothing...", because, while everybody there KNOWS about the PROBLEM, nobody there can DO anything about it, because it is a DAVIS™ problem...their WLIP only sends SLP, with no provision whatsoever for sending ALT instead!

With regards to gobberment people, does the phrase "...close enough for government work..." too often come to mind?
« Last Edit: September 03, 2013, 07:04:11 PM by Old Tele man »
• SYS: Davis VP2/WL-IP & Envoy8X/WL-USB
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

Offline aweatherguy

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Re: Backing Station Pressure numbers out of VP2 Sea Level Pressure numbers?
« Reply #9 on: September 04, 2013, 01:06:57 AM »
Okay...here's something else. I just pulled down two documents from Davis, "Reporting Quality Observations to NOAA" and "Derived Variables in Davis Weather Products". I see where the lookup table for "C" is missing, but this is a humidity correction and I suspect it is fairly small in magnitude...and making that statement pretty much guarantees that I'm wrong  :grin:.

Both documents, however claim that Weather Link 5.7 and later properly computes altimeter and send that to CWOP... what's up with that??? Can you get that version for your purposes?


Offline Beaudog

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Re: Backing Station Pressure numbers out of VP2 Sea Level Pressure numbers?
« Reply #10 on: September 04, 2013, 11:14:46 AM »
I don't know where the idea that WL reports SLP to CWOP came from. It does not.

Right now my SLP is 29.828   and the altimeter reported to CWOP is 30.176.

Now is that correct don't know for sure but it always looks high to me.

I use VPLive to report to CWOP and it seems to give a more reliable altimeter.

Part of the problem is that there is about 6 ways to calculate altimeter.

The screen shot is from yesterday so don't go trying to match it to todays numbers.

Offline Old Tele man

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Re: Backing Station Pressure numbers out of VP2 Sea Level Pressure numbers?
« Reply #11 on: September 04, 2013, 01:57:37 PM »
Well, at 10:24 a.m. this morning, my VP2 console (running WLIP Ver. 6.0.2) displayed a pressure of 29.97"Hg (logged: 29.965"Hg), but below are what the NWS-TUC, MesoWest, and Wunderground sites displayed for my site (DW6988) at that same time:

Code: [Select]
_NWS-TUC:  29.97(ALT), 27.537(Station Pressure)
MesoWest:  29.97(ALT), 29.76(SLP), 27.52(Press)
__Wunder:  29.97(SLP)

Also, nominally, ALT values are SMALLER than and very close (~96%) to SLP values, but notice how the MesoWest SLP value is SMALLER than the ALT value, which makes no sense.

Sorta looks to me as though ONLY Wunderground correctly recognizes that it was sent SLP data.
« Last Edit: September 05, 2013, 05:39:41 PM by Old Tele man »
• SYS: Davis VP2/WL-IP & Envoy8X/WL-USB
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

Offline aweatherguy

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Re: Backing Station Pressure numbers out of VP2 Sea Level Pressure numbers?
« Reply #12 on: September 04, 2013, 09:25:12 PM »
Not related to the Davis issue directly...but AFAIK there is only one formula to convert from station pressure to altimeter. The only possible variation I can think of is that sometimes a 10-foot offset is added to the result (perhaps because the static pressure port on many airplanes is a ways off the ground?).

Anyway -- if you can give me pointers to two or more different formulas for converting altimeter to station pressure I'd be interested to learn about the other methods.

Offline Old Tele man

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Re: Backing Station Pressure numbers out of VP2 Sea Level Pressure numbers?
« Reply #13 on: September 05, 2013, 05:38:11 PM »
Unfortunately, the two of us are in the same boat--we know what the problem is, but we don't know WHICH answer(s) to believe because there are so many to chose from...sorta like most 'other' Government "Standards" (wink,wink).

The real problem is that we--the home weather people--are trying to replicate the NWS process in reverse; we're, basically, reverse-engineering Davis's screw-up (*)!


(*) actually, their non-conformance to the established CWOP, MADIS, NOAA protocol.



« Last Edit: September 05, 2013, 05:45:10 PM by Old Tele man »
• SYS: Davis VP2/WL-IP & Envoy8X/WL-USB
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

Offline George Richardson

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Re: Backing Station Pressure numbers out of VP2 Sea Level Pressure numbers?
« Reply #14 on: September 05, 2013, 06:05:56 PM »
Davis' "Screw up" is that they are a WEATHER station and not an airfield altimeter! CWOP uses altimeter readings to give everyone their little green checks because they can get them free via telephone from airfields around the country by pretending they are pilots. SLP is of no use to an airplane and altimeter is of no value to a weather station. I think you will find CWOP "established" their protocol long after Davis was reporting Home Weather SLP.

Offline Old Tele man

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Re: Backing Station Pressure numbers out of VP2 Sea Level Pressure numbers?
« Reply #15 on: September 05, 2013, 07:36:02 PM »
My concerns are:

1) CWOP, MesoWest and MADIS "know" about this 'error' yet seemingly don't care about a resolution (I have e-mailed all of them).

2) Davis "knows" about this 'error' (of protocol) and makes NO provision for users to 'select' ALT vs SLP, especially via their WLIP software & hosting.

3) Why can Wunderground "correctly" identify my VP2 pressure data as SLP when nobody else can get it right?
« Last Edit: September 05, 2013, 07:38:32 PM by Old Tele man »
• SYS: Davis VP2/WL-IP & Envoy8X/WL-USB
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

Offline Beaudog

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Re: Backing Station Pressure numbers out of VP2 Sea Level Pressure numbers?
« Reply #16 on: September 06, 2013, 11:28:02 AM »
Because it is not up to them to identify what pressure you are sending it is up to you to send the correct pressure in the format that they want it.

Offline George Richardson

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Re: Backing Station Pressure numbers out of VP2 Sea Level Pressure numbers?
« Reply #17 on: September 06, 2013, 02:12:24 PM »
My opinion, and I think they are correct:

"1) CWOP, MesoWest and MADIS "know" about this 'error' yet seemingly don't care about a resolution (I have e-mailed all of them)." They DON'T care as long as they have a number of stations "reporting".

"2) Davis "knows" about this 'error' (of protocol) and makes NO provision for users to 'select' ALT vs SLP, especially via their WLIP software & hosting." Regretfully, Davis has recently added the option of reporting Altimeter pressure rather than SLP. Only VPLive is "smart" enough to receive pressure from the Davis console, assume it's SLP, and send the proper pressure to the various recording agencies.

"3) Why can Wunderground "correctly" identify my VP2 pressure data as SLP when nobody else can get it right?"  As far as I can tell, Wunderground only identifies your pressure as "pressure" and would report it as such whether you sent "station", "altimeter", or "SLP".

For further barometer info, try: http://www.starpath.com/barometers/baro_cal.php and the "Barometer Handbook" by David Burch is excellent reading.

Offline SlowModem

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Re: Backing Station Pressure numbers out of VP2 Sea Level Pressure numbers?
« Reply #18 on: September 06, 2013, 08:00:31 PM »
To further complicate matters, here's what the Davis Vue console manual pg. 12 has to say:

Screen 11: Barometric Reduction Setting

The Barometric Reduction Setting screen indicates the method by which barometric pressure
is to be determined and calculated. The factory default is NOAA, but in this screen you may
select a different method.

To change the barometric reduction setting:

1. Press + or - to change the barometer
reduction setting type:

• NOAA (Default Setting) — The
barometer is reduced to sea level using a
technique that factors in the humidity and temperature of the column of air.

• ALT SETTING (Altimeter Setting) — The barometer is reduced to sea level using a
“standard” column of air, often referred to as a “standard atmosphere.”

• NONE — Reports a raw barometric pressure reading unadjusted for elevation/altitude.

2. Press DONE to move to the next screen.

Note: See “Calibrating Barometric Pressure” on page 28 to learn how to fine-tune your barometric
pressure to a local source.
Greg Whitehead
Ten Mile, TN USA


Offline Weather Display

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Re: Backing Station Pressure numbers out of VP2 Sea Level Pressure numbers?
« Reply #19 on: September 07, 2013, 12:40:59 AM »
Quote
Only VPLive is "smart" enough

WD uses the same routines as VPLive
if you select to send that baro type to CWOP
Brian
info@weather-display.com
http://www.weather-display.com

Offline George Richardson

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Re: Backing Station Pressure numbers out of VP2 Sea Level Pressure numbers?
« Reply #20 on: September 07, 2013, 08:31:16 AM »
Sorry Brian. Should have known. Others may also be doing this by now.

George

Offline Old Tele man

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Re: Backing Station Pressure numbers out of VP2 Sea Level Pressure numbers?
« Reply #21 on: March 01, 2017, 07:00:54 PM »
FINAL RESOLUTION = replaced the original VP2 console with a newer VUE console which DOES send ALT data!
• SYS: Davis VP2/WL-IP & Envoy8X/WL-USB
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

Offline CW2274

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Re: Backing Station Pressure numbers out of VP2 Sea Level Pressure numbers?
« Reply #22 on: March 01, 2017, 07:22:39 PM »
FINAL RESOLUTION = replaced the original VP2 console with a newer VUE console which DOES send ALT data!
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