Barometer deviation problems and CWOP QC

[niko]

That's a great post Goldengoose7 

[prvrt]

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).

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... 

[SoftWx]

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.

Steve
SoftWx

[prvrt]

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.

[D2149]

That's a great post Goldengoose7 

Thanks Niko! 

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! 

Cheers.   

[Storm017]

Don't know if this will help.  What I did was watch the quality reports for a week then adjust the station.  I did this for a couple of weeks and it seemed to work.  Maybe my theory is wrong but...

http://weather.gladstonefamily.net/site/D2176

[D2149]

Don't know if this will help.  What I did was watch the quality reports for a week then adjust the station.  I did this for a couple of weeks and it seemed to work.  Maybe my theory is wrong but...

http://weather.gladstonefamily.net/site/D2176

Your pressure looks a lot like mine does most of the time, but the last few days mine has been reading higher than the ANALYSIS. 

http://weather.gladstonefamily.net/site/D2149

Notice that I am still getting a 99% rating even though I am not lined up right on the RED line?  That's cause the TRACK is still right on with the MADIS changes.

During a typical Spring/Autumn-like  day, my temp graphs look a lot like yours does right now, but when we have these winter storms coming through, all bets are off.  In my case,  those living within a mile of my station would laugh at the MADIS Temp and Dewpoint graph.  The BLUE graph is where our temps were today. We still have a foot of un-melted snow on the ground.    I wish it got up to 49 today!  We might have gotten some melting. 

[Storm017]

Thanks Niko! 

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.

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 a good 8 degrees colder than the other two Bass Lake stations today.

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! 

Cheers.   

Nice write up....My main thing is I would like to know what stations I'm getting analyzed with.  If the MADIS/CWOP system brings every station to a commom denominator for every value then I'll except the error. 

[Storm017]

Your pressure looks a lot like mine does most of the time, but the last few days mine has been reading higher than the ANALYSIS. 

http://weather.gladstonefamily.net/site/D2149

Notice that I am still getting a 99% rating even though I am not lined up right on the RED line?  That's cause the TRACK is still right on with the MADIS changes.

During a typical Spring/Autumn-like  day, my temp graphs look a lot like yours does right now, but when we have these winter storms coming through, all bets are off.  In my case,  those living within a mile of my station would laugh at the MADIS Temp and Dewpoint graph.  The BLUE graph is where our temps were today. We still have a foot of un-melted snow on the ground.    I wish it got up to 49 today!  We might have gotten some melting. 

Depending on who I'm analyzed with, I should have a higher baro reading.  My day and night temp will vary since I'm on a southern side of a mountain facing a valley.

[D2149]

Nice write up....My main thing is I would like to know what stations I'm getting analyzed with.  If the MADIS/CWOP system brings every station to a commom denominator for every value then I'll except the error. 

Thanks!

I agree!

It would be nice if we KNEW WHO we were being compared with!  Still not really sure why NOAA makes that some kind of deep dark secret. 

Based on the swings/spans in temp and dewpoint I see on my ratings, I have to assume that I am getting compared to stations in Oakhurst Ca. which is 8 miles west and more importantly...1800 feet LOWER in elevation! 

It doesn't help that I have two neighboring stations that are consistently TOO WARM in their temp readings.

[D2149]

Depending on who I'm analyzed with, I should have a higher baro reading.  My day and night temp will vary since I'm on a southern side of a mountain facing a valley.

If there is any way you can borrow a high end barometer just to use for a few days, I would highly recommend it.  Once you dial in your VP2 to match, you shouldn't need to mess with it anymore.

My VP2 was VERY close right out of the box.  I adjusted my VP2 pressure a total of 1.0 millibar UP. (Make sure you nail your elevation as close as possible before doing anything.  This can have a HUGE effect on your initial setting and accuracy)

Here is a link to the analog "Fortin Style" mercurial that I have:

http://www.princoinstruments.com/barometers.htm

Model 453X

Unfortunately, it is getting nearly impossible to buy these anymore because of all the restrictions on MERCURY. Princo stopped making all Mercury based instruments back in 2007,  I've had my "Princo" for years.  That is why I said BORROW if possible.

[Storm017]

If there is any way you can borrow a high end barometer just to use for a few days, I would highly recommend it.  Once you dial in your VP2 to match, you shouldn't need to mess with it anymore.

My VP2 was VERY close right out of the box.  I adjusted my VP2 pressure a total of 1.0 millibar UP. (Make sure you nail your elevation as close a possible before doing anything)

I'll try to borrow one.  Right now my station is adjusted UP 2.86 mb.  Elevation is close as possible (Handheld GPS/Google Maps).  Off subject --- What I like to see is a network of PWS's in realtime +- 5 minutes, so you can see what is actually going on at the surface.  I always believed that what is going on in the current time is better than any model.

[D2149]

I'll try to borrow one.  Right now my station is adjusted UP 2.86 mb.  Elevation is close as possible (Handheld GPS/Google Maps).  Off subject --- What I like to see is a network of PWS's in realtime +- 5 minutes, so you can see what is actually going on at the surface.  I always believed that what is going on in the current time is better than any model.

I totally agree!

You might want to check on Weather Underground and find as many local stations sending with RAPID-FIRE as possible. Separate them out from the rest and track them for a few days.  If you are handy with programming, perhaps you could write a little network app that gathers their data and compiles it into a graph like CWOP does with their stations?

I send my Wunderground data packet every 2 seconds using Rapid-Fire, but I am the only station up here at Bass Lake that is currently using this great new Wunderground option!  In fact, I think only two other stations up here in the Central/Southern Sierras out of a few dozen are using Rapid-Fire.  I assume it must be a limitation of their weather software, or they simply don't know they have this new option.

http://www.wunderground.com/weatherstation/WXDailyHistory.asp?ID=ISQLSJSJ2

[SoftWx]

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.

The formula for SLP takes raw sensor pressure, elevation, temperature, and humidity as inputs, and from this it calculates the sea level reduced pressure. The altimeter formula only takes raw sensor pressure and elevation as inputs. So, given a specific elevation and a specific raw sensor pressure, the altimeter formula will always produce the same altimeter value. But given that same elevation and raw pressure, the SLP formula can produce a range of values over the range of possible temperature and humidity inputs. That is where the drift comes from. For the same elevation and pressure, SLP will produce a different result on a cold day than it will on a warm one.

[prvrt]

OK, I understand that now. I think what confused me is I was trying to make the Davis display read correct. After I changed to the WD CWOP setting and use that setting also for WD and my website, everything looks good. The Davis display has a different reading, but I am assuming that is the raw baro not the corrected since WD can not change the Davis display. I will watch this now for a few days and see how things track on the CWOP QC page. My Temp and Hum/Dew on CWOP has always been good and tracks very well. Maybe now I have the Baro setup correct and it will also track correctly. Thanks for the explanation.

EDIT: 72 hours later and everything looks good, tracking very well now.

EDIT: Seems everything is going good now. I am tracking very well with the local airports now. Average deviation is around 0.4 millibars now. I can deal with this much better. Thank you guys for your input, it did help quite a bit.