confused regards pressure readings!

[daj]

I have been trying to get my head around pressure readings but I now seem to have found a contradiction which is confusing me.

Firstly, am I correct in saying absolute = sea level pressure; relative takes into account altitude ?

As I understand it from a search of the forum, Cumulus takes the relative pressure reading from my station (WH1080). It ignores the absolute?

However, the <#press> webtag delivers the sea level pressure (as stated in the help) -- which is absolute pressure?

Another forum post also states that Cumulus does not adjust pressure to take into account altitude. So now I am confused as how can Cumulus read my relative pressure but deliver my absolute in a web tag



I seem to be going round in circles!

Any help appreciated

[beteljuice]

However, the <#press> webtag delivers the sea level pressure (as stated in the help) -- which is absolute pressure?

Wrong .....
Absolute is the 'weight' of air above the sensor, and is all it actually knows about.

Relative aka Mean Sea Level Adjusted is the equivalent 'weight' at sea level.
A normallity device we use so that we can compare notes.

So, unless you live in Holland, your Relative pressure should be higher than your Absolute.

[gemini06720]

David, here is what has been written in the Vantage Pro2 console manual:

------------------------------------------------------

Barometric Pressure

The weight of the air that makes up our atmosphere exerts a pressure on the surface of the earth. This pressure is known as atmospheric pressure (air column). Generally, the more air above an area, the higher the atmospheric pressure - this means that the atmospheric pressure changes with altitude. For example, atmospheric pressure is greater at sea level than on a mountaintop.

To compensate for this difference and facilitate comparison between locations with different altitudes, atmospheric pressure is generally adjusted to the equivalent sea level pressure. This adjusted pressure is known as barometric pressure. In reality, the Vantage Pro2 measures the atmospheric pressure. When the location's altitude is entered, the Vantage Pro2 stores the necessary offset value to consistently translate the atmospheric pressure into barometric pressure (sea level pressure).

Barometric pressure also changes with local weather conditions, making the barometric pressure an extremely important and useful weather forecasting tool.

High pressure zones are generally associated with fair weather while low pressure zones are generally associated with poor weather.

For forecasting purposes, however, the absolute barometric pressure value is generally less important than the change in barometric pressure.

In general, rising pressure indicates improving weather conditions while falling pressure indicates deteriorating weather conditions.

------------------------------------------------------

Did the above explanation helped in any way??

[steve]

Another forum post also states that Cumulus does not adjust pressure to take into account altitude.

It doesn't use the altitude that you enter into Cumulus. It uses the absolute pressure and offset that it gets from the station.

[gemini06720]

Steve, just to clear things in my mind (sure, he writes after all that eggnog stuff.. ), and using the information I posted previously from the Vantage Pro2 manual, which pressure type are available to you from the Vantage Pro2 (the 'VantagePro.dll' file), the atmospheric (or air column) pressure or the barometric (sea level) pressure, or both, and which one(s) do you or can you use?

Is the station altitude (as entered/recorded in the Vantage Pro2 console) also available to you?

No, I am not going to ask you to add some new tags/variables to Cumulus...

Steve, I would like to offer you and you dear ones my best wishes for the new year.

[steve]

The DLL gives me easy access to the sea-level corrected pressure. With a bit more work, I can get the station pressure (which I use with the Cumulus altitude to determine altimeter pressure for CWOP). The same call that gives me station pressure also gives me the console altitude, but I don't use it.

Happy New Year to you too.

[callum91]

Has anyone else noticed that a lot (if not all) of these personal weather stations underead pressure by around 2 mb? I found my one reads about 3.7 mb too low, even after correcting to mean sea level so I have had to add a total correction of 5.5 mb (1.8 mb for height above sea level of 53 feet for the barometer).

[steve]

How do you know it's reading too low (i.e. what are you comparing it with)? You're always going to have problems with a station that only allows a fixed offset between station pressure and sea-level pressure, because the difference isn't fixed, it varies with temperature and humidity, but the Fine Offset stations don't take that into account. So even if you set your sea-level pressure to the same as a nearby official station (which is how you should really do it, rather than just applying the altitude calculation), there will always be a bit of variance (as the official station will be doing it "correctly").

[callum91]

How do you know it's reading too low (i.e. what are you comparing it with)? You're always going to have problems with a station that only allows a fixed offset between station pressure and sea-level pressure, because the difference isn't fixed, it varies with temperature and humidity, but the Fine Offset stations don't take that into account. So even if you set your sea-level pressure to the same as a nearby official station (which is how you should really do it, rather than just applying the altitude calculation), there will always be a bit of variance (as the official station will be doing it "correctly").

Yes I used the two nearest SYNOP stations on days of high pressure. I think you're right - the problem is my weather station isn't taking into account my indoor temperature so it's possible that in the summer when it's warm in here I might have to recalibrate it again.

[steve]

Yes I used the two nearest SYNOP stations on days of high pressure. I think you're right - the problem is my weather station isn't taking into account my indoor temperature so it's possible that in the summer when it's warm in here I might have to recalibrate it again.

It would be interesting to know to what accuracy these things measure pressure in absolute terms. My three stations differ by a few mb.

[daj]

This chat becomes more interesting (re: accuracy).

I have been having an offline chat with a chap who received an email from AWEKAS, an extract below

> > > Your station is sending invalid values to AWEKAS Weather Network. One of
> > > our operators has disabled the following sensor(s):
> > >
> > > air pressure (wrong value: 1011hPa )
> > >
> > > The blocking of the sensor(s) is for 6 hours and will be automatically
> > > removed at Thu, 31.12.2009 22:04 UTC
> > >
> > > Please check your sensors and transmission unit.
> > > If this mail is unjustified please contact us by mail at mail@awekas.at.
> > >
> > >
> > > The administrator has added the following note: AIR PRESSURE IS TOO LOW.

How do they know it is too low?

[steve]

How do they know it is too low?

I think by comparison with other stations in the area. I think this is how the CWOP issue with 'altimeter pressure' came to light. Cumulus users' pressures were just a few mb out compared to others, in certain situations, because Cumulus was sending MSLP and CWOP wants the (slightly different) altimeter pressure.

[daj]

Hmm..he seems to think the all the stations around him report similar readings to himself.

I will ask Peter to join in the topic so that we can help him further.

[callum91]

I wouldn't be surprised if I get a the same email then because if you take the uncalibrated value from my station, it is about 3 or 4 mb too low!

[peter_randle]

Hmm..he seems to think the all the stations around him report similar readings to himself.

I will ask Peter to join in the topic so that we can help him further.

_____
Hi everyone

I'm Peter from Dorset who only started ca 2-3 weeks ago using Cumulus and a WH1081 PWS.

I registered the main USA weather sites including CWOP but received no real feedback except from CWOP where as I had not specified any elevation, it reported a possible error/inconsistency. I had then registered with AWEKAS without remark. As I corrected the elevation the Admin Operator at AWEKAS contacted me to say my weather data was now 11 hPa too high and would I made suitable corrections. I did so and AWEKAS re-instated my air pressure measurements. However, assuming AWEKAS could not know the air pressure so accurately when I saw that my WH1081 was reporting still a difference of 5 hPa I subtracted yet another 5 hPa via cumulus so that my PWS and site data were exactly the same values. This is when AWEKAS wrote to me again and said now my air preesure was showing 5 hPa too much and so correct again.

AS David correctly states: I check with the Weymouth Council Weather BAraometer and friends who have a weather system ca 1.4 miles away. All 3 of us agreed when just using my monitor for the WH1081 and their published values. As my PWS and monitor were showing the correct air pressure I assumed the weather station and monitor (ie all the hardware) was functioning correctly. The AWEKAS email did suggest investigating the sensors and hardware as first port of call.


Steve: You have said how cumulus works with the Vantage PWS model but can you please say what happens with the WH1080/WH1081 PWS please? Why does the cumulus s/ware report a different value from the WH monitor air pressure?

Best wishes,
Peter