Barometric Pressure - Imperfect Science?

[akasonny]

I've always wondered how to properly set the barometer on my weather station.

Even if I duplicate the numbers of a known station (NOAA) a couple of miles (or km) away, there can be a meaningful difference between the two geographic areas.

Plus, the placement of my weather station console seems to cause fluctuations in the pressure depending on whether its summer and the A/C is running or winter when the heat is on. The mean error can be as much as 2 mb which, of course, affects Cumulus forecasts.

So my question to those of you out there who have stability in your barometer is "how did you do it?". Placement of the weather console seems crucial and yet there aren't really any readily-available guidelines for it. And how did you go about establishing the "correct" value to begin with?

I know this makes me sound a little naive...and I guess I am....but I bet I'm not alone on this.

[Repairman77]

You're certainly not alone sonny.

I regularly check my WX station one against an aircraft altimeter which is next to the computer and I'm sure is well temperature compensated. In any case it's in a room which only varies by a couple of degrees around 20C.

The difference between the WX station and the Altimeter can be as much as 2 degrees, sometimes in cold weather and sometimes in warm weather, and sometimes the two readings agree.

I also check with two military sites quite close to me, around 10 miles away, which are normally quite near to the WX station readings.

I find it hard to believe that the Altimeter is faulty, more likely to be variations in the WX station sensor.

I have a very accurate scientific vernier mercury stick barometer on the wall but haven't tried the readings compared to that yet.

To be honest I've more of less given up.

Mike.

[callum91]

I calibrated mine to the nearest SYNOP station (03876 - Shoreham Airport) which gives you the pressure to the nearest tenth of a millibar (e.g. 1031.3 mb).

You can also look at surrounding SYNOP stations and roughly work out what the pressure should be where you are.

Basically, I on a day when there is high pressure and little or no wind, I set my barometer to the same as the one at Shoreham airport by looking at my pressure graph for that particular time when the SYNOP report was done.

There are various websites where you can get SYNOP reports and there is a map on the Met Office showing all the SYNOP stations in blue at http://www.metoffice.gov.uk/climate/uk/networks/.

Regards

Callum

[akasonny]

I calibrated mine to the nearest SYNOP station (03876 - Shoreham Airport) which gives you the pressure to the nearest tenth of a millibar (e.g. 1031.3 mb).

You can also look at surrounding SYNOP stations and roughly work out what the pressure should be where you are.

Basically, I on a day when there is high pressure and little or no wind, I set my barometer to the same as the one at Shoreham airport by looking at my pressure graph for that particular time when the SYNOP report was done.

There are various websites where you can get SYNOP reports and there is a map on the Met Office showing all the SYNOP stations in blue at http://www.metoffice.gov.uk/climate/uk/networks/.

Regards

Callum

Thanks Callum but I'm not located in the UK.

[callum91]

Oh right - you will find SYNOP stations all over the world, but if your location is Sahuarita, AZ, I think your nearest SYNOP station is Tucson Airport which is number 72274.

If you use http://www.ogimet.com and click SYNOP reports, then type in 72274, you will get all the SYNOP reports for the day.

The last I can see was 17:53, which was:

Code: Select all

AAXX 13184 72274 32966 21811 10172 20006 39257 40148 58004 91753 333 10172 20061 555 91318

You can either paste that into a decoder (like this http://metaf2xml.sourceforge.net/gui.html) or if you see where it says 40148, that means the pressure at Sea Level was 1014.8 millibars (29.97 inHg).

If you look at various websites you may be able to find other SYNOP stations in your area and also instructions on how to decode them.

Regards

Callum

[akasonny]

I don't really want to embark on a learning curve of SYNOP's coding/decoding just to set a fairly accurate barometer reading, Callum. Frankly I don't have that kind of time....or really any interest in SYNOP or any of the other cryptic agencies.

I just simply want to set my barometer as accurately as possible. The Tucson SYNOP is almost 25km north of here which I don't think would accurately reflect the barometer here.

Thanks, anyway, for trying to help. I appreciate your intentions.

Oh right - you will find SYNOP stations all over the world, but if your location is Sahuarita, AZ, I think your nearest SYNOP station is Tucson Airport which is number 72274.

If you use http://www.ogimet.com and click SYNOP reports, then type in 72274, you will get all the SYNOP reports for the day.

The last I can see was 17:53, which was:

Code: Select all

AAXX 13184 72274 32966 21811 10172 20006 39257 40148 58004 91753 333 10172 20061 555 91318

You can either paste that into a decoder (like this http://metaf2xml.sourceforge.net/gui.html) or if you see where it says 40148, that means the pressure at Sea Level was 1014.8 millibars (29.97 inHg).

If you look at various websites you may be able to find other SYNOP stations in your area and also instructions on how to decode them.

Regards

Callum

[TNETWeather]

Realistically, all you could go by would be the elevation difference between your site and KTUS and adjust the baro based on that. Take a number of different reading especially when there are not a lot of gradients taking place. Today would not be a good day as we have a low pressure cell flowing through the State.

It looks like the "official" pressure for Sahuarita, AZ is coming from KTUS.

[akasonny]

Yes, I know about KTUS. I've been using that as the elevation is almost the same but it lies some 15 miles away as the crow flies and wasn't all that sure it would be indicitive of the baro pressure here. Sometimes the pressure is the same and sometimes its off as much as .02 in.

In lieu of anything better, I guess I'll keep using it.

Thanks

Realistically, all you could go by would be the elevation difference between your site and KTUS and adjust the baro based on that. Take a number of different reading especially when there are not a lot of gradients taking place. Today would not be a good day as we have a low pressure cell flowing through the State.

It looks like the "official" pressure for Sahuarita, AZ is coming from KTUS.

[Repairman77]

Yes, I know about KTUS. I've been using that as the elevation is almost the same but it lies some 15 miles away as the crow flies and wasn't all that sure it would be indicitive of the baro pressure here. Sometimes the pressure is the same and sometimes its off as much as .02 in.

In lieu of anything better, I guess I'll keep using it.

Thanks

Sonny I think it can be assumed with a fair bit of confidence that the WX station sensor is not that linear across the range. It does seem to vary at the high and low ends.

May I be as bold as to suggest that this is the main problem we all have keeping it in step with more accurate instruments?
I'll try and do some measurements across the range, very high and very low pressure.

However, when Steve does the calibration bit of C2, perhaps we could have a multiplier as well as an offset for the Pressure settings; it should be possible to get the tracking more in step then.

Mike.

[akasonny]

Yes I'm sure you're right.

Its just that the actual placement of the console can aggravate that lack of consistency and so thats what has me in a "snit". Apparently no matter where I place the console it is, in some way, aggravated by heat or cold which is affecting the baro readings. Short of placing it in an airtight container, I haven't been able to find a location that is "barometer neutral".

I'll not "stew" over this situation any longer as I've been fighting it for 6 months and haven't made any meaningful progress. I was just hoping somebody had a "magic bullet" when it comes to placement of these silly things. I acknowledge I may be asking too much of these things.

Thanks for your remarks.

[Sonny I think it can be assumed with a fair bit of confidence that the WX station sensor is not that linear across the range. It does seem to vary at the high and low ends.

May I be as bold as to suggest that this is the main problem we all have keeping it in step with more accurate instruments?
I'll try and do some measurements across the range, very high and very low pressure.

However, when Steve does the calibration bit of C2, perhaps we could have a multiplier as well as an offset for the Pressure settings; it should be possible to get the tracking more in step then.

Mike.

[Repairman77]

Yes I'm sure you're right.

I'll not "stew" over this situation any longer as I've been fighting it for 6 months and haven't made any meaningful progress. I was just hoping somebody had a "magic bullet" when it comes to placement of these silly things. I acknowledge I may be asking too much of these things.

Thanks for your remarks.

I rather think you are right sonny; we are just expecting too much of a cheap unit.
Perhaps the only way to get it to read anywhere near right would be to put the sensor itself in a electronically controlled low temp oven (like the old xtal ovens) that was also humidity controlled as well. A bit OTT I think!

I may give up as well; but at the moment just doing some tests with the mercury stick barometer just to see how far out it gets.

Mike.