Which Pressure Is Sent To Wunderground?

[PDC]

Evening all,

I took the plunge last weekend and purchased (am awaiting the refund of the cost as it was an early birthday present!) a weather station, having had a desire to own one for a few years now. It's a Maplin branded WH-1080. I've had it reporting to wunderground since last Sunday, and today I got a message from another station owner pointing out that my pressure readings are way out compared to others

http://www.wunderground.com/global/stations/03334.html (mine is Whalley Range).

Upon investigating I've realised that it appears that I am reporting relative pressure whilst most of the others are reporting absolute.

The FAQ at http://wiki.sandaysoft.com/a/FAQ#Cumulu ... et_console suggests that my station supplies the absolute pressure to Cumulus, and that Cumulus converts it to relative. I guess it then passes this calculated value to wunderground.

Is there anyway to force the absolute to be passed?

Thanks

Will

[steve]

If the other stations really are reporting absolute pressure, then they're doing it wrong. Absolute pressure is of no use in meteorology. I think a more likely explanation is that you haven't set the relative pressure correctly in the station. You're currently reporting about 1007 and the (sea-level/relative) pressure in your area is actually about 1029, so it looks like you need to add about 22 mb to your relative pressure setting.

[RCE]

To follow on from what Steve said, to setup correctly.

Close Cumulus.

Open Easyweather, once it finishes updating itself go to system/setup bottom left you will see absolute which is fixed and relative which you can adjust.

Enter the new relative value and click OK your station should now be giving the correct relative value.

Close Easyweather (spit at it if you like) then open Cumulus again.

[PDC]

Thanks for the information. I couldn't fathom how the station was calculating the relative pressure, as I couldn't see a setting on it to set altitude, or any correction.

I didn't realise that you could configure the station using the Easy Weather software. I had read around the topic of Easy Weather before buying the station last week, and got the message that you should stay away, and use other packages instead, so I hadn't even installed it!

I was really confused that the station's relative pressure was lower than the absolute. and was wondering if there was something wrong with it. It's not a case of RTFM in this case, as it doesn't tell you in the so called 'manual' how to adjust the settings.

It's corrected now. Thanks.

[RCE]

Looks a lot better see you are tweaking it a little.

When we get a nice high pressure sat over us will be the best time to do a little more tweaking.

http://magicseaweed.com/UK-Ireland-MSW- ... essure/in/ has a good looking pressure chart which may help with your checking.

On wunderground the MADIS stations "should" be the best ones to use for reference.

[EvilV]

You probably know this, but it hasn't been mentioned in the thread so just in case, the difference between absolute and relative pressure is that our weather stations are not set up on a level playing field. Some of us live in higher places than others so absolute pressure is what the sensor measures, and relative pressure is what the pressure would be at sea level.

This javascript pressure calculator will work out the exact offset you should put into Easyweather to correct for your exact altitude above sea level. If you don't know that, Google Earth will show you what that is. Simply find your location on google earth and float the mouse over the position of your weather station, then look at the altitude figure displayed at the bottom of the screen. If you want to be picky, correct for the height the barometric sensor is above the ground surface, and 'BINGO' you have the perfect corrected value. If you are a bit OCD, you can even correct for air temperature too, which has a tiny effect.

Take a look here ->

http://www.csgnetwork.com/barcorrecthca ... m=990.2990

[RCE]

That javascript is very confusing if I am using it right ?!?

It actually looks to give the opposite result, the corrected value gets lower the higher you get which is wrong.

The beauty of using comparisons to good instruments is that you are also correcting for any inaccuracies in the actual instrument.

[steve]

I didn't realise that you could configure the station using the Easy Weather software. I had read around the topic of Easy Weather before buying the station last week, and got the message that you should stay away, and use other packages instead, so I hadn't even installed it!

Cumulus can't make any changes to the station, it can only read data, so there are some things that you have to use EW for - actually, I think that the logger interval is the only thing you need EW for. You can set the relative pressure from the console - press the pressure 'button' three times, then use the + and - buttons to set the pressure.

[steve]

By the way, you can download a manual here: http://www.maplin.co.uk/Media/PDFs/inst ... _n96fy.pdf

It's not great, but it sounds better than what you have, because it does at least tell you how to set the relative pressure (page 10).

[Gina]

I had to reset my rel pressure the other day after removing the console batteries to attempt a fix for a loose USB connection. I found using the console touch pad the easiest way (easier than running EW which I no longer have installed anywhere). I set mine up to match an official Met Office station just a couple of miles away. I have the Maplin PDF on my computer, having lost the original skimpy instructions.

[EvilV]

That javascript is very confusing if I am using it right ?!?

It actually looks to give the opposite result, the corrected value gets lower the higher you get which is wrong.

The beauty of using comparisons to good instruments is that you are also correcting for any inaccuracies in the actual instrument.

Maybe I am misunderstanding you, but for my money, as you get higher in the atmosphere the pressure decreases. That's how generations of altimeters worked.

[Gina]

I have not used the javascript personally but it seems to work for others. But yes, as your altitude increases your instrument will read lower absolute pressure and therefore the difference (and therefore correction) between absolute and relative pressure will increase the higher up you are. They should be the same if you are at nominal sea level. Absolute pressure will always be lower than relative pressure if you are above sea level and your pressure sensor is reading correctly. I hope that spells it out - I realise that it can be rather confusing. I was confused when I first came across it

[RCE]

I have not used the javascript personally but it seems to work for others. But yes, as your altitude increases your instrument will read lower absolute pressure and therefore the difference (and therefore correction) between absolute and relative pressure will increase the higher up you are. They should be the same if you are at nominal sea level. Absolute pressure will always be lower than relative pressure if you are above sea level and your pressure sensor is reading correctly. I hope that spells it out - I realise that it can be rather confusing. I was confused when I first came across it

Which is why it is odd.

You enter the observed reading and height, and it gives you the Actual Barometer Corrected. "The barometer is calibrated at sea level; the higher the elevation, the lower the pressure"

Yet when you enter

1000 millibars at 100ft = 996mb Actual Barometer Corrected
1000 millibars at 200ft = 992mb Actual Barometer Corrected

The Actual Barometer Corrected should surely be higher than the observed reading on an instrument calibrated at sea level.

[Gina]

Yes, certainly sounds wrong to me. Something isn't right!

[PaulMy]

RCE Wrote:
On wunderground the MADIS stations "should" be the best ones to use for reference.

In my area the MADIS station is always waay lower than the others, including the airport.

Paul