Relative / Absolute Pressure and resetting records

[steve]

The difference between QNH and MSLP is that the QNH adjustment to sea level only uses altitude (it's what Cumulus calls 'altimeter pressure') whereas MSLP uses the current temperature and humidity (it's how Davis stations calculate sea-level pressure). They tend to produce similar results except at high altitudes.

[SUBYDAZZ]

This calculator from NOAA is helpful with station and altimeter settings.

You just enter your altitude and the absolute pressure reading from your weather station and it will give the relative figure you need.

http://www.srh.noaa.gov/epz/?n=wxcalc_altimetersetting

Yeah, but see this is what I can't work out. My absolute reading right now is 999.9mb, with the calculator it gives me 1007.23mb but the actual reading at the nearest station (and similar at others a bit further away) is about 1019mb and that's a big difference. So perhaps my absolute reading is not absolute at all, it's just a relative thing.

[steve]

As I said earlier, you can't just use a calculator and apply the theoretical difference, because to do that would assume that your station is reading the correct absolute pressure in the first place. And as your station and mine (at least) demonstrate, Fine Offset stations typically do not. So you have to apply an offset that attempts to compensate for altitude and an inaccurate base pressure value, by using a reliable reference reading.

[GraemeT]

I see your point Steve. Plus, I made the mistake of assuming SUBYDAZZ had an accurate starting point.

[SUBYDAZZ]

As written here:

http://wiki.sandaysoft.com/a/FAQ#Cumulu ... et_console

I keep getting that pressure garbage error when I restart Cumulus, so how do I find my offset to add that to cumulus.ini, is it just the difference between the absolute and relative pressure readings on my weather station console?

I assume this value is only ever a positive number? Say for instance if the difference between relative and absolute pressure on my console is 23.8 would I simply add: EWpressureoffset=23.8 or did I misunderstand the description?

[steve]

how do I find my offset to add that to cumulus.ini, is it just the difference between the absolute and relative pressure readings on my weather station console?

Yes.

I assume this value is only ever a positive number? Say for instance if the difference between relative and absolute pressure on my console is 23.8 would I simply add: EWpressureoffset=23.8?

That's right.

[mcrossley]

I assume this value is only ever a positive number?

Not always, my FO absolute pressure reads way too high, so my correction is negative (-155.4 in fact). The resulting relative pressure seems pretty accurate, so I suspect it is just an offset value that the station applies that has become corrupted in some way.

[steve]

Yes, sorry, my reply wasn't precise enough. You just set the offset value in Cumulus to (relative-absolute), whatever that happens to be. In most cases, that's a positive number.