Relative / Absolute Pressure and resetting records

[SUBYDAZZ]

For some reason, when my computer restarts, Cumulus quite often comes up with the Error about dubious pressure reading and uses zero offset. I have just realised it has been using what I assume to be absolute pressure reading from my console, but isn't Cumulus supposed to be showing relative pressure, which is normally a higher reading?

Can I assume that my all time high pressure reading is likely to be correct (if it was set when Cumulus was using the relative pressure reading) but that my low pressure record is likely to be quite a bit lower that what it should be?

I tried to reset the all time and yearly records regarding low pressure but clicking the reset icon next to the value doesn't do anything. What's going on here?

[steve]

For some reason, when my computer restarts, Cumulus quite often comes up with the Error about dubious pressure reading and uses zero offset. I have just realised it has been using what I assume to be absolute pressure reading from my console, but isn't Cumulus supposed to be showing relative pressure, which is normally a higher reading?

Yes, Cumulus displays relative pressure. The console supplies absolute pressure, so Cumulus applies the calculated 'offset' to determine the relative pressure. If it's having to use zero because the calculated offset is garbage, then obviously relative pressure = absolute pressure.

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

Can I assume that my all time high pressure reading is likely to be correct (if it was set when Cumulus was using the relative pressure reading) but that my low pressure record is likely to be quite a bit lower that what it should be?

Quite possibly, yes.

I tried to reset the all time and yearly records regarding low pressure but clicking the reset icon next to the value doesn't do anything. What's going on here?

The reset button resets the value to what it was when you opened the editor window. The copy button copies the value calculated from the data files into the current record. This is explained in the help.

I'm not sure, though, what you hope to achieve. If Cumulus has on occasion been forced to log absolute pressure then the values in the logs will also be low, so you are likely to end up with approximately the same value.

[SUBYDAZZ]

Thanks Steve,

For some reason, when my computer restarts, Cumulus quite often comes up with the Error about dubious pressure reading and uses zero offset. I have just realised it has been using what I assume to be absolute pressure reading from my console, but isn't Cumulus supposed to be showing relative pressure, which is normally a higher reading?

Yes, Cumulus displays relative pressure. The console supplies absolute pressure, so Cumulus applies the calculated 'offset' to determine the relative pressure. If it's having to use zero because the calculated offset is garbage, then obviously relative pressure = absolute pressure.

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

"Make sure that you have set your console's relative pressure correctly.

So how do I set my console's relative pressure correctly? The destructions say how to set it but not how I find out what the value should be. Is it simply a function of the altitude of the station, and is it always a constant value above the absolute pressure? I can't recall actually ever setting the relative pressure on my console originally so I don't even know if it is right anyway. All it allows me to do is increase or decrease the value that is shown.

I tried to reset the all time and yearly records regarding low pressure but clicking the reset icon next to the value doesn't do anything. What's going on here?

The reset button resets the value to what it was when you opened the editor window. The copy button copies the value calculated from the data files into the current record. This is explained in the help.

Oh, I thought "reset" reset the record to the current value on Cumulus, as in resetting the low record would allow it to be set again for each future lowest value recorded. To "reset" it the way I want I guess I have to put in an arbitrary value somewhere above the current recorded value so it can create new record lows from now on.

I'm not sure, though, what you hope to achieve. If Cumulus has on occasion been forced to log absolute pressure then the values in the logs will also be low, so you are likely to end up with approximately the same value.

Simply to fix it so it now gives relative pressure and so that the new low record will actually be the new low relative pressure, not the low absolute pressure.

[steve]

So how do I set my console's relative pressure correctly?

You get a known reliable reading from a nearby station (e.g. from the BOM, if you're in Aus) and set it to that.

[SUBYDAZZ]

You get a known reliable reading from a nearby station (e.g. from the BOM, if you're in Aus) and set it to that.

I just searched for a calculator to convert my absolute pressure to relative pressure corrected to my station's altitude, will this achieve the same or am I better off taking it near a BOM sensor and setting it there? It said my pressure correction should be +2.4hpa for my altitude of 64m.

How close does a nearby station need to be? Not sure if any near me transmit near enough realtime data or not.

[SUBYDAZZ]

Hmmm, just discovered my nearest real-time barometer reading is around 30km away and it's reading is 1017.4 (and another even further away is the same), I have 1001.2 (rel) and on mine 998.8 (abs) on mine so I guess mine is just way out then?

[SUBYDAZZ]

So I'm guessing my console's absolute pressure reading wasn't calibrated at sea level?

[steve]

This is one reason it's better to use a reference value rather than simply a calculated offset; it compensates to a certain extent for inaccuracies in your station. For example, I'm very close to sea level, and my Fine Offset is reading 1016 hPa, whereas my Davis and the Met Office station 20 miles away are reading 1028 hPa. So even though I'm close to sea level, I need a 12 hPa offset in the Fine Offset, with the current readings.

[SUBYDAZZ]

OK, so I think I am finally getting to understand all this. No wonder Cumulus was pretty much always forecasting stormy weather...

[SUBYDAZZ]

Now all I need to do is work out whether the pressure reading shown from my BOM station of reference is a MSL or a QNH value.

[steve]

Now all I need to do is work out whether the pressure reading shown from my BOM station of reference is a MSL or a QNH value.

QNH is pressure at mean sea level.

[SUBYDAZZ]

Oh, this page threw me a bit then: http://www.bom.gov.au/products/IDN60801 ... 5771.shtml

[steve]

Fair enough, I guess I was wrong; I'm not an aviator. But a number of places on the web (including the Weather FAQs site which I maintain, whose contents were mostly written by UK Met Office staff) say that QNH is "Pressure at mean sea level, reduced from station level pressure according to International Standard Atmosphere profile".

[GraemeT]

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

[Matt.j5b]

According to here http://www.bom.gov.au/catalogue/observa ... ions.shtml bom readings about the MSL and QNH. Bom readings are reduced to sea level so if you near a station set it to that reading and there will be less of a difference from other station when we got a strong high pressure system.