Wrong Pressure in Cumulus 1.8.4?

[DanielF]

I've just purchased a 'no-name' WH1081PC weather station, which comes with EasyWeather software (which has some minor annoyances). I've only just set them up and alpha-tested them today (the weather transmitter is not yet installed outside!). I discovered and downloaded Cumulus, and while it looks better than EasyWeather, it's currently not working correctly...

I configured it for direct download from the WH1081PC (not using the .dat file), but the air pressure it displays is the 'reference' pressure, not the current actual (absolute) pressure.
(I have my 'base' station set for absolute pressure, not relative.) Is this a known bug?

Another problem I noticed is that the Cumulus wind speed indicator only goes up to 50kph. I live in a Force 1 area, where winds can (potentially) get up to 160kph (we've experienced over 100kph in recent years). Can this be changed??

Daniel

[harrym1byt]

Why would you need to display the absolute pressure? The reference pressure is supposed to be adjusted by you to take into account the difference in pressure between your location's height and sea level. Everyones pressure should then read the same irrespective of their ASL.

The wind gauge rescales itself (I think to 100) if ithe wind speed rises higher than default 50, though I must admit I have yet to see it happen. In kph, 100 may not be enough for your location.

[steve]

Yes, I'm afraid I forgot about km/h when I made the gauge rescale up to 100 but no further Actually, I imagine that one day I'll exceed it in mph, my record gust is close to 90 mph. It's actually quite easy to change, so I'll do that when I get a minute. The 'real time' SWF wind gauge does the same, but I won't be able to fix that that any time soon.

Steve

[steve]

I'll do that when I get a minute

I got a minute.

In the next build, it goes up to eleven. Sorry, I mean 200.

Steve

[Shawn]

the air pressure it displays is the 'reference' pressure, not the current actual (absolute) pressure.
(I have my 'base' station set for absolute pressure, not relative.)

I'm the same is there away of changing this?

Shawn

[steve]

Cumulus only displays sea-level pressure, because that is what is used in meteorology. What is the use that you want to put your local pressure to?

I don't want to seem unhelpful, I normally end up putting in most options that people ask for even if I don't necessarily agree with them. But I don't want to give newcomers the opportunity to start quoting their local pressure because they misguidedly think that's what they should be doing.

Steve

[DanielF]

Steve,

Thanks for your quick action on the wind speed issue!

I'm a complete weather novice, and misunderstood the term 'relative pressure'. After researching it a bit on the 'net, I now see that this is a misnomer - it's not *relative* pressure at all, but *normalised* pressure. And yes, I now understand that's what I should be displaying. The question is how?

It's easy to change my AWS to show 'relative' pressure, but how do I determine the 'reference' pressure to enter, and what exactly does it mean? All the reading I've done so far has not led me to a clear answer.

Is the 'reference' pressure the 'average' sea-level pressure (1013.25 hPa) converted to my altitude? If not, then what? Some articles talked about getting a reading from a nearby official weather station, and using that, but it wasn't clear (to me) *how* to use that reading. And my nearest official station is about 50km away - would that matter?

Incidentally, living on the top of a small cliff that drops into the Tasman sea (a bay thereof), my AWS altitude above sea level (when eventually mounted on the roof) will be about 24 metres. I wouldn't have thought the normalisation would make a significant difference at this altitude.

Can someone please clarify my muddy understanding of how to set the 'referemce' pressure, or point me to a relevant 'how-to' article?

Thanks for your patience!

Daniel

[RichardKB]

I had the same problem as you, and after reading up on it I found that the easiest way is to use local weather stations as a guide, I am lucky in this respect as I have one quite close less than 5 miles and a few others in a 15 mile radius.

Their are also some charts which show predicted air pressure which also compared quite well with the local stations.

So from what I can see correct me if I am wrong people, the relative pressure on these machines will always be set higher than the absolute pressure unless your equipment is mounted below sea level.

I also found a site that quoted the change in pressure against height.

Rich

[steve]

I don't know the details of setting the 'relative' pressure (I don't like the terms 'relative' and 'absolute' myself, the first time I came across them was with these Fine Offset stations) on your station (but others here do), but as Richard says, the way to do it is to get a trusted local reading, preferably when the pressure isn't changing much (e.g. a nice big stationary anticyclone).

You might be surprised to learn that even your 24m 'altitude' will cause a difference of about 3mb in pressure!

Steve

[DanielF]

OK, I can get a reading from a weather bureau station 50km away, as there's a high approaching the east coast in the next few days. But I still don't know what to do with it!

Hopefully someone else who's done this before on a Fine Offset AWS can enlighten me. Otherwise it's going to be a matter of trial and error, changing the 'reference' pressure until my 'relative' reading agrees with the official 'MSLP'.

But I wouldn't feel comfortable doing that, as the 'official' weather readings (temperature most obviously) often differ noticeably between the two official stations 50 and 25km either side of my location, and I'd worry that the pressure might too. Neither of those official stations is as close to the coast as me (the one 25km away is much further inland), which might also lead to weather discrepancies.

Daniel

[steve]

I think you just put the current sea-level pressure into the station. I can't see how it would work any other way. Anyway, it's pointless me speculating, someone who actually knows will be along in a minute

Steve

[Ric Shortman]

I had the same dilema with the REL and ABS pressures....
The easyweather software gave the option the choose REL or ABS, and when I checked against the local(ish) main stations I found they were displaying in ABS...

So i just used Steve's calibration page to adjust the pressure and monitored it... worked fine.
So.... my Cumulus now gives an Absolute pressure in line with the local weather station.....

[beteljuice]

Keep looking at Pressure Charts until there is a large, stable area over most of your part of the country. (isobar contour lines on a map).

You may have to wait a while

[Shawn]

The reason i ask is my local station say 1029 hPa so that's rel or abs?
http://www.metoffice.gov.uk/weather/uk/ ... ather.html

My station says 1021.9hPa abs and 1004.9 hPa rel so my station is set wrong?

I work with natural gas and use abs when pressure testing, that's why i asked.

[RichardKB]

I would assume relative, as the I guess the absolute is the pressure at the sensor. So if most stations show sea level values the you will have to set the relative value to the 1029 or what ever it is at the time you set it.

You are just the other end of the railway line to me in Cheltenham.

http://www.richkb.pwp.blueyonder.co.uk/ ... /index.htm

Rich