Cumulus Support

I'm not sure if my baromoetric sensor is off, or I'm failing to allow for altitude etc

The figures I'm seeing are always lower than the isobars on the met office website.

We're at about 700 feet asl.

If the difference ISN'T just down to the altitude, can I insert a correcting factor (presuming I can work it out) in the config? I don't fancy my chances of getting Oregon to replace it with one working any better.

And if I can correct it should I correct to the sea-level pressure or the local altitude pressure?

ccomley wrote:I'm not sure if my baromoetric sensor is off, or I'm failing to allow for altitude etc

The figures I'm seeing are always lower than the isobars on the met office website.

We're at about 700 feet asl.

If the difference ISN'T just down to the altitude, can I insert a correcting factor (presuming I can work it out) in the config? I don't fancy my chances of getting Oregon to replace it with one working any better.

And if I can correct it should I correct to the sea-level pressure or the local altitude pressure?

Yes it is due to altitude (mostly ), if your close enough to a Met office to consider that their data should be the same as yours, then you can use it. Wait for the pressure to even out, find the difference between Met and you and apply the difference in the Configer/Calibration in Cumulus.
While I'm at it, my (Fine Offset) keeps dropping roughly 2hPa every couple of months, and I have to keep calibrating. I feel one day it's going to die (thanks for correcting my spelling Dylan **13yo son**)

Hi,

Each 30 feet of altitude reduces the pressure by about 1 mb/hpa, so you should be seeing a pressure of almost 25 mb/hpa less that the Met office reports. Barometric pressure should always be corrected to sea level to be of any value to other users.

Cheers, Alan.

ccomley wrote:I'm not sure if my baromoetric sensor is off, or I'm failing to allow for altitude etc

The figures I'm seeing are always lower than the isobars on the met office website.

We're at about 700 feet asl.

If the difference ISN'T just down to the altitude, can I insert a correcting factor (presuming I can work it out) in the config? I don't fancy my chances of getting Oregon to replace it with one working any better.

And if I can correct it should I correct to the sea-level pressure or the local altitude pressure?

You need to enter your altitude into the station which will then show the sea-level equivalent pressure. No further corrections should be necessary and it's very unlikely your station is faulty.

I found the manual here http://uk.oregonscientific.com/ulimages ... /WMR88.pdf

Go to Page 8 and at the bottom it tells you how to enter your altitude. 700ft is 214 meters. You should then see the same pressure as nearby Met Office stations.

If you're uploading to a website a link is often useful.

Hi

Its best to enter your height above sea level or your relative pressure value on your station which adjusts the absolute station level pressure to sea level, as sea level is standard on the weather charts and with other stations. All weather stations allow for your air pressure setting to be adjusted and much easier as no conversion is needed because it is sea level. Both my LaCrosse barometers stay on track with the local offical readings but I do own a basic Oregon Scientific and the reading on that are not as accurate as my La Crosse's.

The bottom line with any corrections is NOT to make assumptions and do it on your own.

In particular with pressure, contact your nearest Met office and have THEM provide you with current pressures and they will, if you are talking to anyolne competent, then be able to provide you with a corrected pressure for YOUR site (providing of course you know your exact altitude and not simply a guess).

Remember that Met Offices don;t only take pressure from statically placed instruments - aerial soundings are still done around the world and these, together with ground based instruments, allow any Met Office to provide corrected pressures anywhere between ground level and the stratosphere (well, maybe not quite that high...)

But - either way - contact your nearest met office and get them to help you. From that point on you shouldn;t have to touch your instrument again

There is a link (the last one) on my site that will show you your elevation. It uses Google maps and works very well. Also there is a webtag, <#altimeterpressure> Altimeter pressure. Pressure corrected to sea level using altitude only. Same as sea-level pressure for non-Davis stations. Once you've established your altitude and put this into Cumulus, you can work out your Sea Level pressure. I've never used it though.

Thanks for the input guys

I *do* know the asl for the location.

I have told Cumulus this - I assumed it would correct for it given the datum.

ccomley wrote:I have told Cumulus this - I assumed it would correct for it given the datum.

Cumulus does very little with the supplied altitude. It assumes the station has been set up correctly for SLP and reads it directly from the station.

ccomley wrote:Thanks for the input guys

I *do* know the asl for the location.

I have told Cumulus this - I assumed it would correct for it given the datum.

You don't tell Cumulus. You enter it into the console using the instructions I provided. Don't make any changes in Cumulus.

steve wrote:Cumulus does very little with the supplied altitude. It assumes the station has been set up correctly for SLP and reads it directly from the station.

What's this do then? <#altimeterpressure>

MickinMoulden wrote:What's this do then? <#altimeterpressure>

It gives the station pressure corrected to sea level using the altitude of the station only (rather than taking other factors such as temperature into account).

On non-Davis stations, it's exactly the same as sea-level pressure, because those stations only do the simple correction anyway. On Davis stations, which do a more complicated calculation for SLP, it's calculated from various bits of data supplied by the station.

Just a small problem, maybe someone has an idea?
I installed a Fine Offset station at a local small airfield, so barometric pressure is a good thing to have!
When I look at the station's screen it tells me QFE (local pressure). Field is at 1033 feet, so the offset in pressure (at 27.6 feet per hPa) should be some 37 hPA to give QNH (pressure at see level).
If I take the QFE and add the 37hPa I get exactly the same value as in an aircraft's altimeter so things match up (and I know the altimeters are correct).
However, Cumulus consistently displays the the QNH 12 hPa low. I don't mind calibrating it up with the 12hPa, but maybe there is something more to it?
The sensors are some 8 meters above the ground so that should account for 1 hPa.
Any thoughts appreciated.

Have you set the 'relative' pressure on the console to match your QNH value? That's the value which Cumulus displays. Instructions are in the manual- if you have one, if not there's a 'generic' one in the wiki.

Brilliant thinking! That was it. Working fine now.
Thanks!