Topic: CWOP altimeter VS SLP?

[Skywatch]

A few weeks ago I started noticing my air pressure readings were getting locked up on my station when I had Weather Display set to send altimeter pressure. But when the option was taken off the pressure readings were fine. I assumed this was a problem in weather display. but today I tried to experiment with WD by reselecting the altimeter setting in the CWOP setup and watched my pressure for a day and noticed WD was still sending sea level pressure. But then I took a look at other pressure readings from stations near by and my pressure readings seem to correlate correctly to those around me.

So that being said does anyone know if CWOP changed their air pressure input? Comparing my pressure readings manually against others in the area looks like all is good but in the MADIS QC the blue line representing my station is below others in the area. I know it's said those MADIS checks are not accurate but I always hear this for stations in rural/farm regions. But my area has such a density of stations I'm guessing my MADIS quality checks are more accurate although I don't know this for sure. Before my pressure line was close to the analysis. but visually comparing the readings my self looks like nothing is wrong.

Here's my QC page http://weather.gladstonefamily.net/site/D4088
Here are the stations near my station http://www.findu.com/cgi-bin/wxnear.cgi?call=DW4088

I've unsubscribed from the WXQC list after my inbox was getting cluttered. So I don't know what's going on if anything.

[Weather Display]

you should only be wanting to use the altimeter reading if you live at say over 5000 ft altitude

[Skywatch]

That makes sense. Can't remember who originally told me to use that setting. Sounds good.

[tlum]

you should only be wanting to use the altimeter reading if you live at say over 5000 ft altitude

You should be sending Altimeter [QNH] to CWOP regardless of your altitude. Altimeter is the standard that is expected.

Unfortunately, non-compliance is significant. So anyone trying to use actual data to determine the specification is likely to arrive at the wrong conclusion. There are areas where non-compliance is so bad that the QC is actually failing compliant stations, much to the ire of people who go out of their way to become knowledgeable about adhering to the specifications.

Sending something other than Altimeter is but one cause of inaccuracy. There are cases where an Altimeter value from an airport unwittingly is used to calibrate the stations MSLP value. And there are those that will calibrate their station to obtain the least errors from the MADIS QC.

The keys to correct pressure are:

• Make sure your station/software is accurately calculating and sending Altimeter to CWOP. Some software does this intrinsically. Some requires correct activation. And others - like WeatherLink-IP - are incapable of producing correct Altimeter data and require an alternate calibration hack to get it right.
• Calibrate the station to a known, accurate, reliable, local reference
  
  • Make sure it really is MSLP - or is at least what you think it is
  • Don't try calibration when unstable air is prevailing
  • Don't use the QC as a calibration reference - a big old red X might actually be the correct answer as much as that bothers people
• Evangelize - many pressure problems are caused by ignorance and the spread of misinformation. Pressure is a particular problem because its far from simple and there is a lot of misinformation running around.

-Ted-