Strange pressure readings in Cumulus

[niglov]

Hi Steve - I have an odd problem. I have a Fine Offset WD1081, it is reading what I think is the correct barometric pressure (I checked with other local stations). It is currently 1013mB but Cumulus is showing 993mB and that is also what it is sending to the internet (my website and WeatherUnderground) as well. All other readings seem fine. Have you come across this before?

Cheers

Nigel

[steve]

If you've definitely set your station to the correct sea-level pressure, then the most likely explanation is that Cumulus got a duff value when it read the pressure offset from the station at startup - see other thread hereabouts. If you attach the latest file from the Diags folder that will show the values that it read.

Steve

[niglov]

Hi Steve - I think I have got myself in a pickle over abs and rel pressure! I'm sorry, you must get more than your fair share of idiots asking silly questions but I (like the rest of them) am new to this and finding my way.

I have set the relative pressure (which seems to be the value read by Cumulus and sent to the internet) to the same as absolute pressure, which gives no offset for pressure at sea level, but I don't know what the difference should be. If you have a moment to point me in the right direction I would be most grateful as I don't want to be sending incorrect data out to other internet weather sites.

Cheers

Nigel

[harrym1byt]

Nigel, assuming you are in the UK then check the accurate ministry pressure figures at http://freemeteo.com/

All of these are corrected figures for altitude in other words 'relative' figures offset from the 'absolute' pressure reading. It is the relative pressure which you need to be sending to your website and it is the relative pressure which is displayed in Cumulus.

The easy way to correct the relative figure is to do it in EasyWeather, which can write the correction back to your weather station. In EW click System > Setup then if you look in the bottom left corner there is a box to type in or change the relative pressure. After adjusting it you need to click the Save button, at which time it will be written back to your station. That is the easy bit!

So far as what figure to enter in the box goes...

What I did was checked the figures for my nearest weather station on Meto, noted the time of the reading they provided then compared it with my pressure graphs figure at that precise time. From that I was able to work out whether my indicated relative pressure was higher or lower than the official figures, by how much and then make the appropriate adjustment in EW. The stations reports can be a bit hit and miss time wise, but they were accurate at the time indicated - usually 20 and 40 past the hour.

It might take several goes to get it spot on.

Always remember - the higher you go, the less the air pressure becomes which is why your ears pop when you climb a hill. Your station needs to be corrected to show what the pressure would be were you to dig a hole in the ground down to sea level and place your station down there.

[niglov]

Harry

Thanks, that was a very clear and simple answer - I will get it sorted today.

Cheers

Nigel