Barometer deviation problems and CWOP QC

I have had my station up and running almost a year. During that time I never worried about any barometer adjustment. Recently I decided to see if I could calibrate my barometer to be more precise. It was as far off as 15-20 millibars. Initially I had my elevation about 60’ to high and that is now corrected. The problem is I tried calibrating the baro over the last few weeks and always end up with the same problem. I wait for a high pressure over head and calm winds. Then I adjust the baro to match 2 airport readings which are both about 30 miles away, one north of me, the other south of me and I ended up with an error of about -0.3 millibars. Everything is fine until the high pressure moves on and a low comes in. Then my baro will drop considerably more than the other stations around me. This last low came in and I dropped as much as 8 millibars lower than the airports. There are also 2 other airports 75 miles and 120 miles away to my SW that track quite closely to the local airports. Also if I leave it alone, when a high comes back it will usually come close again to other stations, but if the high pressure is higher than the one I calibrated with, then my baro will climb higher than the other stations by more than 8 millibars depending on the strength of the high. I seem to have a wider “swing” in baro readings. Other CWOP stations around me are also close to the airports and do not seem to have this swing in readings. I think all the info you would need is on my website from type of station to the CWOP QC link. I would like to get this calibrated to be with in +/- 2-3 millibars if possible. Any help would be appreciated very much.

You are taking measurements in your own microclimate. Why do you want to match anybody else? How much variance in altitude is there from your precise location to the precise location of other stations you are trying to match? Also CWOP has a disclaimer in the report you receive that states that a new station that has come on line may be throwing the average readings off or even an older station that is having problems could be doing the same thing. If you can get your hands on a calibrated analog barometer check the readings on it and see what the variation is between the barometer and what your station barometer is posting.

The reason I am trying to match the other stations is that they all report about the same, even 120 miles away due to the valley we live in and elevations are close. Elevation difference is approximately 200’ to the airport to the north and 100’ for the airport to the south. These stations have also been around for 50+ years. When I look at the local NWS observation map, most stations within a 50 mile radius of me are within a few millibars of each other in the valley, but mine will show to be off the most other than a few PWS that have obviously not been calibrated due to there extreme readings. The mountain stations are usually different which is expected. I have no idea where I could get a calibrated analog barometer. I do not believe that the pressure in my area is really changing as much as my baro shows compared to the surrounding areas.

I agree that the pressure would not be off by such a large amount I was just talking about the calibration through WD. I have an old genuine brass analog barometer that I had recalibrated a year or so ago and compared to WD it is spot on. :smiley:

I couldn’t find a cwop link on your site, is this you?

It’s been a long time since I had a vp2+ but there is a place to set to use an internal WD calculation for barometer that is supposed to be more accurate than the vp2 console, and you can tell WD to use that baro reading for cwop. It might be in the offsets section I can’t remember. I believe Steve (VVP) came up with the routine and Brian added it to WD.

That is me. The link on my site is under site reports/site quality. I tried using the internal WD calculation and send that info, but it was way off compared to not using it. I read in one of the links here that it had something to do with CWOP did the correction after recieving the data, but VP2 did it before sending the info so it was getting corrected twice. I may have to try the WD calculation again and see what happens.

EDIT: I removed the offset I had in WD. So far with this low coming through right now it is tracking very close with the other stations. I will have to wait and see if it decides to swing again when a high comes in.

Well, so far so good. For the last 3 days the QC page shows: 2 Thumbs up

Average barometer error: 0.9 milliBars
Error standard deviation: 0.6 milliBars

That is quite acceptable to me, now if it stays this way everything will be good.

Edit: Looks like things are staying as they should. Another 3 days later and this is what is showing.

Average barometer error: 0.0 milliBars
Error standard deviation: 1.1 milliBars

I wouldn’t get too worked up over any of the CWOP/MADIS QC Stuff. They have a LONG WAY TO GO before they are able to truly judge the accuracy of most private stations. Especially if you live in a mountain community like I do. Then they are pretty much CLUELESS about judging the accuracy of what my station reports on any given day. Some days I am spot on with them on all counts, between 96% and 99%. Other days they say that I need to resite my sensors! :roll:

I will trust my VP2’s readings over the NWS/NOAA guesses for our region any day of the week. One of the reasons I installed my own station was the LAME forecasting and temp data for our area. Getting red Xes and question marks because my data does not jive with their hilariously WRONG analysis is not something I lose sleep over. :wink:

Our station is located in a shallow valley surrounded by high ledges on all but one side. This causes our temperature readings to read much cooler than many of the surrounding stations on a sunny slope or on the top of a hill etc.

One morning for example MADIS might say that I SHOULD have read 36 degrees F as my overnight low, but the “ICE” on my cars, the FROZEN Fish Pond, and the FROST on all the roofs tends to make my 31 degree reading a bit more believable! 8)

As for barometric pressure accuracy goes… Your deviation readings look good to me. I try to keep mine within about .3 millibars of the analysis reading. Mine varies with theirs between -0.4 and 0.00 over a 30 day period.

Pressure is really the only reading I ever concern myself with in regards to the MADIS QC reports. That is something that can’t get messed up by geographic anomolies.

Like I said… Until they come up with a better way of comparing data, it is more or less just a form of geek entertainment! :lol: Phillip Gladstone will be the first one to tell you that! :wink:

I seem to have come to that conclusion myself. The reason I got the Davis is like yours. I live 10 miles from the nearest reporting town and out here we can be as much as 15F cooler in winter or 15F warmer in the summer. Also the wind blows much harder out here. I also live in a large valley, but it is a high desert. The town is located next to the river but I live out in the actual desert past the end of the lava flows, so the weather is much different out here. The lava out here changes temps big time. I tried to calibrate the baro to get rid of the red x’s. Things will look good for a few days until we have a pressure change from high to low or vise verse, and then everything is off again. I feel pretty good since I did the calibration during a period of nice stable high pressure. I watch the CWOP QC report daily, but have not made any changes for a few weeks now due to the fact that nothing I do will ever give me two thumbs up, other that constant fine tuning every few hours, and to me that is not being accurate or reasonable. I also know the way they collect and process the local data to come out with an average does not work well when there are other local stations that are way out of calibration. I do appreciate the feedback as it confirms what I have been learning and seeing.

No problem! Glad I could help you accept your own observation! :wink:

I think there is a lot of confusion over exactly what the whole MADIS QC program is supposed to be about.

MADIS is not some kind of accuracy competition!

Seems that a lot of station owners approach it like some kind of contest instead of the “science” it is supposed to be contributing to. In their mad rush for geek bragging rights, they end up polluting the data pool by constantly screwing around with their sensor offsets if they don’t get a “gold star” on any given day. This kind of thing feeds on itself and in no time an entire area can become so messed up by the “Twiddlers” that it is literally impossible for ANYONE in that region to get a decent QC report from MADIS.

IMHO, the guys at CWOP do not spend enough time hammering a simple point home to new members! CWOP and MADIS DO NOT WANT STATIONS CHANGING THEIR SENSOR SETTINGS SIMPLY TO GET A BETTER SCORE! #-o

A lot more attention should be paid to making sure that new station owners do not CHASE their sensor settings all over the place.

As long as you are using a high quality weather station like a Davis VP2, taken the time to site it in as best as possible and made sure that all the sensors are functioning properly, then you are doing GREAT. You should LEAVE the station ALONE and allow your data to be gathered by NOAA and complied with all the other PWS data out there.

If there was a way to magically remove all the bogus readings that are posted to MADIS by stations that have had their settings skewed for a “perceived” better rating, I am sure that many stations that are currently getting erroneous QC error reports would see that their data was actually very accurate for their location.

Not sure if this issue is ever going to be solved, but as long as there are enough of us out here with decent gear and a reasonably solid grasp of the reality of data gathering, then the smart weather bugs out in the “audience” will soon separate the stations with a clue from the wannbe/also-rans. lol! :lol:

Cheers! :slight_smile:

Tinplate would know more, but if the Altimeter option in the CWOP setup is used, then dont have any altitude set on the console?
(and for normal baro readings in WD, use the the baro offset in WD (but you can also set to use the CWOP altimeter value in WD too)

The console should have the elevation set to the true elevation, and the elevation provided to CWOP at signup should be true as well. Altimeter should be sent for pressure to CWOP, using the feature for doing that in WeatherDisplay. If this is not done, and you are not near sea level, then you can expect to see your barometer drift when compared to reliable stations (i.e. those operated by the NWS & FAA).

The quality checks are useful for alerting people to possible problems, but the expected values take into consideration all the nearby stations, including other PWS that may be providing bad data themselves. So it should not be relied upon as something to which you calibrate your instruments. It is a useful in that it indicates you might look into the issue to see if it is in fact your instrument that is off. For barometer, where microclimate is not much of an issue, I like to use the check boxes at the bottom of the graph to select the nearest two NWS stations to include in the graph. Those are what you should be comparing to to see if you have actual drift. If you have set your elevations properly, and are submitting altimeter as you should, then what you may find is that your pressure tracks well with the NWS stations, but it is the quality check expected value is drifting because of other nearby PWS that are NOT sending proper values for pressure.

For other sensors like temperature/humidity/dewpoint you really have to take the quality check numbers with a grain of salt. Being out of sorts according to the quality check might indicate a problem, but it is quite a bit more likely that it’s caused by microclimate variance. For example, during the spring, fall and winter, my dew point quality check is excellent, and I track well to the nearest airport ASOS station (about 8-10 miles away) as well. But in the summer I get a big thumbs down because my humidity reads lower than expected. But I don’t worry about it too much because all the nearby PWSs and the ASOS station are in the middle of the big, wide, heavily irrigated Boise river valley. I am on the edge of the valley on dry land, with miles of unirrigated BLM land to the north. Interestingly, my humidity does track much more closely to the Boise airport 20 miles away, and it, like me, is on dry land at the edge of dry BLM land. So I just let it give me a thumbs down. I looked into the issue, and decided it was microclimate, and I never calibrate my temperature or humidity because of it.


I am having the same discussion via email with Russ Chadwick and David Helms. Since I now move around I am noticing that I can’t get my QC checks to pass. I explained to Russ that I think the pws stations are offsetting their data to match the metar or airport, then when I move into the area I am seen as the odd man out since I don’t have the same offsets. Then when I arrived here in Deming I am about 1 mile from the airport and I found their dew point does not seem to be calculated correctly. Using an online calculator and plugging in the metar numbers I found their dew point is out by over 4 degrees. See attached dew point and humidity charts.

I agree with Goldengoose7 but would add that the station should be calibrated to ensure the readings are correct. My old station, a vp2+, I calibrated with an Extech Digital Sling Psychrometer that had been NIST certified. I think this ensures the data is correct and the sensors are giving good readings. A much better solution than chasing the closest metar.

Of course now in the RV and using the cheap WMR100 I just do the best I can…



regarding the airport not calculating the dew point correctly…

It’s very possible that the official station is measuring the dew point directly, as automated stations often have this feature. The ASOS stations for example do. So they would be calculating the humidity value from the measured dew point, not the other way around.

regarding ASOS, see
On page 11 it describes the dew point sensor and adds that “Relative humidity is calculated using the 5-minute average ambient (dry-bulb) temperature and dew point temperature.”

(It looks like KDMN is an ASOS station)


Thanks Steve, interesting read. Learn something new everyday :slight_smile: I would guess I’ll get the same answer from Dave. It is ridiculous to question the ASOS using my WMR100 for reference. I only questioned their readings since the OAT and humidity did not compute to the expected DP using the online calculators. Now that I know they are using a different calculation method I should be able to verify the readings they show.

add: Doing the same checks today the ASOS calculations are within 1 degree. Strange. Maybe I caught the readings yesterday before it did the daily calibration.

That’s a great post Goldengoose7 =D>

I did correct my elevation about 2 months ago since I did a little more research into my area and discovered I was about 60’ to high in my first atempt to setup the station. I have been sending the Current Raw Baro to CWOP Since April 08’. I have tried using the WD Altimeter setup, but it seems way off compared to with out it. At this moment I turned it on and I get these readings.

Current Raw Baro: 1013.0 hpa 29.914in
Altituse 4534ft
Virtual Temp 32
C 1.6
Station Pressure 854.7 hpa 25.238in
Altimeter Pressure 1009.2 hpa 29.802in

The other stations in my area are around 1019.0hpa at this moment or 30.08 in.
WD will send the Altimeter pressure which is showing 29.802in right now which seems low compared to the other stations and what the Current Raw Baro reads which is a much closer reading at 29.914in.
Right now CWOP shows I am about 6 millibars lower than the other stations in the area, but if I leave the WD CWOP setting checked, on the next update it will show me 10 millibars lower than the other stations. I do not have an offset in WD or the Davis console. I guess I do not understand the concept behind the WD CWOP info that is being set. To me the Current Raw Baro has always been the closest reading. I am confused as what to do here. Should I offset the Davis so the CWOP Altimiter pressure is closer to the surrounding areas? Or, should I use the Current Raw Baro? Confused… :?

If you’ve been sending sea level pressure to CWOP, and you switch to sending altimeter, it usually requires a calibration adjustment to back out previous adjustments that artificially tried to make the SLP look like altimeter. But the adjustment made after switching to altimeter is a one time thing. After that you should track quite well. What you will stop seeing once you start sending altimeter is the temperature induced drift that happens as the seasons progress, and which can sometimes be seen on a single day’s graph when there is a big temperature swing. This temperature drift happens because SLP is calculated using temperature, but altimeter is not. So as the temperature changes, the altimeter and SLP values diverge.


I will send the WD CWOP pressure and see what happens. The “Drift” is the problem I have been having. During high pressure everything in right on, but when a low comes in, my pressure drops much lower “Drift” than other stations. If a high pressure comes in that is higher than the one I used to calibrate with, then I get the high “Drift” compared to other stations. I think I understand the difference now with the SLP being calculated by temp, but the Altimeter is calculated by pressure.

Thanks Niko! :slight_smile:

I just hope that one of these days some of what I said is taken more seriously by the MADIS/CWOP folks so that we can get a more useable result not only for us as private station owners but for the forecasts that are derrived from these compiled reports.

It does go without saying that you need to make sure that your station hardware in properly calibrated to something reliable and not fudged to line up with another station in the area or the analysis “RED” line.

Personally, I use some very high quality ANALOG instruments used by weather professionals. I was pleased to discover that the Davis VP2 was impressingly accurate right out of the box. My current VP2 readings which are being sent to CWOP as we speak are dead nuts ON with those analog sensors, yet I was a good 8 degrees colder than the other two Bass Lake stations all day today. These stations are less than 2 miles away. I suspect that some offsetting has been going on with those stations plus the effect of their proximity to the lake.

Microclimate is a HUGE part of all of this, as was mentioned by several other folks in this thread. Owners need to give their gear the benefit of the doubt and put more weight towards the microclimate phenominon before they start going under the hood and messing with all of their offset values! :wink:

Cheers. 8)