[wxqc] AP versus SLP with Weatherlink
Dan Ames
dan_ames at msn.com
Thu Mar 1 13:08:51 CST 2007
Steve, you stated that the newest version of Weatherlink does the SLP to AP correction correctly for CWOP? I run a Davis Vantage Pro2 with weatherlink and downloaded the latest version but my readings are still off according to MADIS. Is it just a matter of getting a good calibration now that the program calculates right?
Thanks
Dan CW5688
----- Original Message -----
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Sent: Tuesday, February 27, 2007 2:24 PM
Subject: wxqc Digest, Vol 28, Issue 46
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Today's Topics:
1. Re: what barometer reading is MADIS qc'ing against?
(Steve Hatchett)
2. Re: VWS freezing (Steve Hatchett)
3. Re: what barometer reading is MADIS qc'ing against? (Lance)
----------------------------------------------------------------------
Message: 1
Date: Tue, 27 Feb 2007 13:36:13 -0700
From: "Steve Hatchett" <steve at softwx.com<mailto:steve at softwx.com>>
Subject: Re: [wxqc] what barometer reading is MADIS qc'ing against?
To: "'Discussion of weather data quality issues'"
<wxqc at lists.gladstonefamily.net<mailto:wxqc at lists.gladstonefamily.net>>
Message-ID: <01fd01c75aae$ee812980$900ac80a at SoftWx1<mailto:01fd01c75aae$ee812980$900ac80a at SoftWx1>>
Content-Type: text/plain; charset="us-ascii"
Chris,
CWOP expects altimeter, so I would guess that's what's used for QC. The
Vantage Pro console calculates the sea level pressure (SLP) internally from
the raw sensor pressure. That SLP value is displayed on the console, and is
also sent to connected weather programs via the data logger. So, you will be
sending SLP unless the weather program sending to CWOP calculates the
altimeter from the provided SLP value (this involves reversing the SLP
calculation to find out the raw sensor pressure, and then calculating the
altimeter value from that). The following programs are the only ones I know
of that currently do this:
WeatherLink (latest version)
VPLive
WeatherDisplay (must turn on the option to send altimeter)
PC-WeatherStation (must turn on the option to send altimeter)
I'm not aware of any other programs that correctly calculate and send
altimeter for VP weather stations. Unless you are near sea level elevation,
this can cause problems, because SLP and altimeter are not the same. In
particular, temperature is part of the SLP algorithm, so compared to
altimeter, the SLP will drift up and down with the seasons as the daily avg
temperature changes, and also during weather events with high temperature
swings.
After developing the source code for converting VP's SLP to altimeter, I
wrote to all the weather software authors offering free use of the code
hoping it would help CWOP data quality since a large number of stations are
VP. WeatherDisplay and PC-WeatherStation fairly quickly incorporated the
change. The others apparently weren't interested.
Regarding comparing your barometer to NWS stations, it doesn't really matter
if your elevation and the NWS station are at different elevations because
altimeter already takes elevation into account. What is more important is
that the station be as close as possible as the crow flies.
Steve
SoftWx
_____
From: Chris Topf [mailto:topfcd at yahoo.com]
Sent: Tuesday, February 27, 2007 10:22 AM
To: wxqc
Subject: [wxqc] what barometer reading is MADIS qc'ing against?
I have a problem with the MADIS qc checks on my barometer:
http://weather.gladstonefamily.net/site/C3603?tile=0<http://weather.gladstonefamily.net/site/C3603?tile=0>
As you can see, it shows I'm consistently 1.5 to almost 2 mb higher than the
'analysis'. I've always tried to gauge the accuracy of my station to the
NWS at Pittsburgh, particularly when it comes to pressure and temperature.
Our elevations, oddly enough, are within a meter or two of each other even
though we are some 20-30 miles apart. Since barometer is primarily a
function of temperature and altitude, I can directly compare my bar to it's
bar.
So my main question is this, what pressure (altimeter versus sea level
pressure) does MADIS use in it's qc? Another question is, what pressure is
reported to CWOP? If I look at 0Z on 2/27, the pressure listed in the CWOP
weather data table for my station was 1011.1 mb and the corresponding
pressure for KPIT was 1009.9 for the altimeter and 1011.7 for the sea level
pressure.
If I need to adjust my barometer, I will but I want to understand what I'm
updating. Also, the pressure on the VP2 console at home, is it the same
value reported to CWOP? My station is at home and I'm at work so I can't,
right now. see what's being reported to CWOP and what's displayed on the
console.
Thanks for anyone who can help.
Chris
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Message: 2
Date: Tue, 27 Feb 2007 13:44:15 -0700
From: "Steve Hatchett" <steve at softwx.com<mailto:steve at softwx.com>>
Subject: Re: [wxqc] VWS freezing
To: "'Discussion of weather data quality issues'"
<wxqc at lists.gladstonefamily.net<mailto:wxqc at lists.gladstonefamily.net>>
Message-ID: <020801c75ab0$0d438060$900ac80a at SoftWx1<mailto:020801c75ab0$0d438060$900ac80a at SoftWx1>>
Content-Type: text/plain; charset="us-ascii"
In addition to a loose or disconnected cable, the VP's USB data logger can
also lose connection because of a failure in the USB to Serial device driver
provided by Davis (nobody has firmly pinned down what causes this). This
doesn't happen to everyone with a USB data logger, but it happens to more
than a few. I added an option to VPLive (and VirtualVP) to automatically
reset the device driver when it detects that the data flow has stopped. I
don't know of other programs that go this far to resurrect the connection,
so in those cases when the driver fails, the only other option to fix it is
to unplug and then plug back in the USB cable, or reboot the computer.
Steve
SoftWx
> -----Original Message-----
> From: Evan Bookbinder [mailto:Evan.Bookbinder at noaa.gov]
> Sent: Tuesday, February 27, 2007 12:20 PM
> To: Discussion of weather data quality issues
> Subject: Re: [wxqc] VWS freezing
>
> RAM is most definitely not your problem. Even if you had insufficient
> RAM, your entire computer would be suffering problems and you would
> receive swap space error messages.
>
> These hangups are likely the result of one of the following:
> 1.) If you have a USB Vantage Pro device, an application can hang if
> the connection is loose or the USB port is disconnected for any other
> reason. I know that with VPLive, the author had to modify the
> application 6-8 months ago to prevent it from intermittently hanging
> when it lost connectivity to the virtual USB/COM Port.
>
> 2.) Bad code in the VWS software causes the application to hang when an
> FTP connection fails or doesn't complete as expected.
>
> Evan
------------------------------
Message: 3
Date: Tue, 27 Feb 2007 13:24:05 -0800
From: Lance <lwood at mountainbase.com<mailto:lwood at mountainbase.com>>
Subject: Re: [wxqc] what barometer reading is MADIS qc'ing against?
To: steve at softwx.com<mailto:steve at softwx.com>, Discussion of weather data quality issues
<wxqc at lists.gladstonefamily.net<mailto:wxqc at lists.gladstonefamily.net>>
Message-ID: <45E4A175.7030407 at mountainbase.com<mailto:45E4A175.7030407 at mountainbase.com>>
Content-Type: text/plain; charset="iso-8859-1"
Hi Steve
You are just a wealth of information :-)
This email brought a question to mind : If I were to switch Weatherlink
versions from 5.6 to 5.7 (Didn't work last time and don't know if I want
to try again), how does that affect VPLive. Do you determine which
version is in use and use a different formula for each? or ?
Just using some unused brain cells here. :-)
Lance
Steve Hatchett wrote:
>
> Chris,
>
> CWOP expects altimeter, so I would guess that's what's used for QC.
> The Vantage Pro console calculates the sea level pressure (SLP)
> internally from the raw sensor pressure. That SLP value is displayed
> on the console, and is also sent to connected weather programs via the
> data logger. So, you will be sending SLP unless the weather program
> sending to CWOP calculates the altimeter from the provided SLP value
> (this involves reversing the SLP calculation to find out the raw
> sensor pressure, and then calculating the altimeter value from that).
> The following programs are the only ones I know of that currently do this:
>
> WeatherLink (latest version)
>
> VPLive
>
> WeatherDisplay (must turn on the option to send altimeter)
>
> PC-WeatherStation (must turn on the option to send altimeter)
>
>
>
> I'm not aware of any other programs that correctly calculate and send
> altimeter for VP weather stations. Unless you are near sea level
> elevation, this can cause problems, because SLP and altimeter are not
> the same. In particular, temperature is part of the SLP algorithm, so
> compared to altimeter, the SLP will drift up and down with the seasons
> as the daily avg temperature changes, and also during weather events
> with high temperature swings.
>
>
>
> After developing the source code for converting VP's SLP to altimeter,
> I wrote to all the weather software authors offering free use of the
> code hoping it would help CWOP data quality since a large number of
> stations are VP. WeatherDisplay and PC-WeatherStation fairly quickly
> incorporated the change. The others apparently weren't interested.
>
>
>
> Regarding comparing your barometer to NWS stations, it doesn't really
> matter if your elevation and the NWS station are at different
> elevations because altimeter already takes elevation into account.
> What is more important is that the station be as close as possible as
> the crow flies.
>
>
>
> Steve
>
> SoftWx
>
> ------------------------------------------------------------------------
>
> *From:* Chris Topf [mailto:topfcd at yahoo.com]
> *Sent:* Tuesday, February 27, 2007 10:22 AM
> *To:* wxqc
> *Subject:* [wxqc] what barometer reading is MADIS qc'ing against?
>
>
>
> I have a problem with the MADIS qc checks on my barometer:
>
> http://weather.gladstonefamily.net/site/C3603?tile=0<http://weather.gladstonefamily.net/site/C3603?tile=0>
>
>
>
> As you can see, it shows I'm consistently 1.5 to almost 2 mb higher
> than the 'analysis'. I've always tried to gauge the accuracy of my
> station to the NWS at Pittsburgh, particularly when it comes to
> pressure and temperature. Our elevations, oddly enough, are within a
> meter or two of each other even though we are some 20-30 miles apart.
> Since barometer is primarily a function of temperature and altitude, I
> can directly compare my bar to it's bar.
>
> So my main question is this, what pressure (altimeter versus sea level
> pressure) does MADIS use in it's qc? Another question is, what
> pressure is reported to CWOP? If I look at 0Z on 2/27, the pressure
> listed in the CWOP weather data table for my station was 1011.1 mb and
> the corresponding pressure for KPIT was 1009.9 for the altimeter and
> 1011.7 for the sea level pressure.
>
> If I need to adjust my barometer, I will but I want to understand what
> I'm updating. Also, the pressure on the VP2 console at home, is it
> the same value reported to CWOP? My station is at home and I'm at
> work so I can't, right now. see what's being reported to CWOP and
> what's displayed on the console.
>
>
>
> Thanks for anyone who can help.
>
>
> Chris
>
> ------------------------------------------------------------------------
>
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