Legacy Cumulus 1 release 1.9.4 (build 1099) - 28 November 2014
(a patch is available for 1.9.4 build 1099 that extends the date range of drop-down menus to 2030)
Download the Software (Cumulus MX / Cumulus 1 and other related items) from the Wiki
Steve,
I'm reporting to CWOP and had been getting a 2 thumbs up on the MADIS QC check for my barometer data. I now suddenly have a two thumbs down from MADIS, a 26% rating. It took maybe two days and all the while QC rating has been going down. http://weather.gladstonefamily.net/site/C0945
I did some looking around and wonder how you have Cumulus reporting the barometer data. Are you sending SLP or altimeter to CWOP?
For history, please see this link: http://www.wxforum.net/index.php?topic=1995.0
Of course, it could just be a calibration issue on my end.
I'm trying to figure out if that's a problem. My baro is spot on and other times it's really off. Sending SLP would cause this.
Below is a post from Steve with SoftWX from the link on my first post.
I know in the past, VWS just sent SLP to CWOP, which is the wrong thing to send. Prior to about two years ago, ALL the weather programs sent the VP's SLP to CWOP, and so there was a lot of wrong data being sent (over half the CWOP stations are Vantage Pro 1 or 2, if I remember correctly). It was a real problem, especially for those like me above 1000 feet, where the difference between altimeter and SLP can be quite significant.
A push was started to fix this issue, and most of the weather programs have since addressed this. You may want to check if there is a newer version of VWS that has this fix. Otherwise, you'll just have to accept that the QC is going to be unhappy with your data from time to time since it's not getting the right values from you. It's kind of like if you were sending temperature in Celsius instead of Fahrenheit as it expects. Your sensor may be perfectly calibrated, but if you and CWOP aren't on the same page as to what it is you're sending and it's expecting, then there will be a problem.
which is where I was sent to by the CWOP people when I asked for the spec, sea level pressure is correct:
b10245 The barometric pressure in tenths of millbars -- this can be omitted. This is a corrected pressure and not the actual (station) pressure as measured at your weatherstation. The pressure is adjusted according to altimeter rules -- i.e. the adjustment is purely based on station elevation and does not include temperature compensation.
Actually, being a meteorologist (cough) and not an aviator, I think I misunderstood what you meant by 'altimeter'. I thought you meant 'station pressure', but this is actually the pressure corrected for height, but not temperature, as described in that quote above, yes?
So what we're talking about is not the difference between sea-level pressure and station pressure, it's the difference between sea-level pressure calculated using temperature and humidity, and some other pressure used in aviation that is calculated just using station pressure and altitude? And it's the latter that CWOP wants?
Yes. This may clear it up below. I do not know the formula to convert SLP to Altimeter pressure. Apparently CWOP is wanting altimeter pressure, not SLP.
There are several forms of barometric pressure that can be reported, and
different services have different requirements.
Station pressure is the basic output of the pressure sensor at your
station. It's raw, uncorrected for altitude and temperature.
Sea Level Pressure is corrected by outside air temperature and altitude
to show the pressure you'd be at if your station were miraculously
transported to sea-level, but at the same horizontal position with
regard to ambient weather conditions.
Altimeter pressure is station pressure corrected solely by altitude.
CWOP elected to go with Altimeter Pressure. Others (a lot of others;
there was good reason for the CWOP decision, but that's not necessarily
a pertinent topic in this forum) prefer... or mandate... either station
pressure and correct it themselves, or sea level pressure.
Most weather stations and associated software report Sea Level Pressure.
If something else is required, they have to perform another conversion
to get to altimeter pressure.
I hope this is responsive to your question.
gerry
Gerry Creager -- gerry.crea...@tamu.edu
Texas Mesonet -- AATLT, Texas A&M University
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.862.3983
Office: 1700 Research Parkway Ste 160, TAMU, College Station, TX 77843
I find it a bit strange that a weather site wants an aviation measure rather than a meteorological one. And how big a difference is there between the two anyway, taking into account the accuracy of the equipment?
Anyway, to get to altimeter pressure I first have to have station pressure, and the VP doesn't give me that. What I have to do is get the barometer calibration data, use that to convert the MSLP back to station pressure, and then convert that to altimeter pressure. But to do the last bit I need a formula (x mb per metre or whatever). Rules of thumb that I'm aware of are 1 mb per 8 metres and 1 inHg per 1000 feet. But how do I know what to use? There seem to be a number of algorithms out there - MADIS is one I've found.
Maybe it would be interesting to generate a table from Cumulus users showing Relative, Absolute and Height. Just to see what sort of variations people are using.
If I use the (1 mb per 8 metres) then I am out quite a lot compared to the local airport readings. My formula is probably more like (1 mb per 4.2 Metres), assuming the height is correct from Google and that the local Airport is accurate.
The sea is only a couple of K's away....maybe I should take the Desktop unit down to sea level and see what it reads.
k1 := 0.190284; // discrepency with calculated k1 probably because Smithsonian used less precise gas constant and gravity values
k2 := 8.4184960528E-5; // (standardLapseRate / standardTempK) * (Power(standardSLP, k1)
Result := Power(Power(pressureHPa - 0.3, k1) + (k2 * elevationM), 1/k1);
You could try plugging your data into that and see what it comes up with. The 'Power' function is "first parameter raised to power of second parameter".
I've decided this is the algorithm I'm going to use for CWOP; I think it's the one Steve uses in VPLive.
The only problem I have is getting the station pressure from the VP, so I can calculate altimeter pressure. There's a function in the DLL to get the barometer calibration data, which I need to get from SLP to station pressure, but I can't get it work. I've emailed Steve Hatchett to see if he can help. He's a very nice helpful knowledgeable chap
I know there are ways of getting from SLP to station without the calibration data, but it's a bit messy, involving looking up temperature data from an hour ago.
It's been a bit of a struggle, but I seem to have it working now. I'll run it for a while myself to see if it causes any problems, then I'll upload it and you can give it a proper test. I'm pretty much at sea level, so it's not making much difference to mine, about 0.3 hPa.
The difference is: Previously it was reporting the MSLP as calculated by adding the offset (obtained either from the station, or from the Cumulus ini-file over-ride, if you're using that) to the station (absolute) pressure read from the station. Now it reports altimeter pressure calculated from the absolute pressure and the altitude configured in Cumulus, using the algorithm listed a few posts back.
To see whether I've got it right or not, we need your absolute pressure from the console and your altitude as configured in Cumulus.
