Cumulus Support

Sorry for a probably very basic question which i am sure most of you will be able to answer.

I have setup my weather station and all seems fine, but i have had to adjust my air pressure to bring in line with other stations as it was considerably out.

Why should i have to do this? And how would one determine the correct air pressure if there were no other station around to compare?

Thanks

Your station measures the 'absolute' pressure at your location. Simply put, this is the 'weight' of the column of air above you, and hence varies with altitude (amongst other things). In order to be able to compare pressure readings at various locations at differing altitudes, meteorologists use pressure figures which are corrected to sea-level, i.e what your barometer would read if you were at sea-level. This is why you need to 'calibrate' your barometer so that its sea-level ('relative') reading is correct.

If you didn't have a station particularly close, then I guess you could interpolate from stations further away, but in the UK that's not really an issue as there will always be a station close enough, particularly when there is a large area of high pressure over the country.

Aside from all of this, of course, there's always the question of the basic accuracy of cheap 'home' weather equipment.

Many thanks for your response. That makes sense now.

I will assume from what you said in your last statement that the Maplin N96GY is classed as an 'entry level' weather station? I guess like all things you get what you pay for, but to what extent would pressure levels vary if you stood 2 weather stations side by side over a longer period of time?

I don't really know what sort of 'drift' you could expect once you had calibrated. Some people do seem to find that a Fine Offset station can stay reasonably in line with other more expensive stations once set up. It's an experiment I would like to conduct myself at some point, as I have a Fine Offset, a more expensive Oregon Scientific WMR200, and an even more expensive Davis VP2. I just need to find the time!

Thanks Steve, i have just had a look at the Davis VP2....gulp! Fine offset will stay and to be honest if it is not bang on accurate it will at least give me a good indication and good consistancy of what is going on and thats all i'm after really.

Vic, you can compare my Fine Offset against Ray's Davis Vantage Pro station on the WOW website. Ray is a few miles away from me, and my temperatures always seem to be lower - I'm closer to open country side.

Here is the last days readings you can play around with the settings and compare other readings and time periods.

Thanks Mark, that's a good comparison.

Another quick question, the site i am measuring against is 100ft higher than me, would this have a significant bearing on the measurement or not?

Vic wrote:Another quick question, the site i am measuring against is 100ft higher than me, would this have a significant bearing on the measurement or not?

Their station/absolute pressure will be about 4 mb less than yours. That's why you use sea-level/relative pressure.

I had to readjust my pressure offset the other day. Moved 3mb from over 3 years use. Either that or the ground is rising considerably under me

Super-T wrote:I had to readjust my pressure offset the other day. Moved 3mb from over 3 years use. Either that or the ground is rising considerably under me

OK, thats pretty good then!!

I totally understand the whole Barometric Pressure at sea-level argument. However, it beggers the question of why do I need to adjust my Station's readings when I have entered my elevation above sea-level? Surely this is taken into account when working out Barometric Pressure at sea level otherwise what is the point of entering the elevation of the station?
Cumulus asks for Elevation, as does AWEKAS and I just got an email from AWEKAS saying my "Barometer has not been adjusted to sea level".
OK.... I adjust it to sea-level and now I have a Weather station at 292ft asl that is telling me I am down on the beach 1 mile away. My ear drums however are telling me a different thing!!!
I know I am new to this.... but there seems to be a lack of logic here somewhere!

kingstonweather wrote:I totally understand the whole Barometric Pressure at sea-level argument. However, it beggers the question of why do I need to adjust my Station's readings when I have entered my elevation above sea-level? Surely this is taken into account when working out Barometric Pressure at sea level otherwise what is the point of entering the elevation of the station?

Cumulus doesn't use the elevation you supply to do the conversion to sea-level, it uses the figures supplied by the station. This is so that your console and Cumulus are displaying the same figure.

Cumulus uses the elevation you supply mainly for display purposes (e.g. so it can be automatically displayed on a web site, in NOAA-style reports etc) and for a few calculations such as theoretical solar radiation.

Thanks for that

Vic wrote:Thanks Steve, i have just had a look at the Davis VP2....gulp! Fine offset will stay and to be honest if it is not bang on accurate it will at least give me a good indication and good consistancy of what is going on and thats all i'm after really.

Vic

I set up my Fine Offset in January 2011. I calibrated it to the Met Office station in Bedford which is about 25 miles from my location. Their readings are only published as whole numbers, and my station is still within 1 digit after two years.

The local Look East tv news on the BBC has a 'barometer night' on a Thursday when they report barometer readings for the region, when the weather is stable my station is always with 1 digit of their reading, which I guess is all you can expect from a sub £100 system. Annoys the wife no end when I insist on getting my iphone out to check my station while they broadcast the reading!