What is "Barometer pressure?"

[David Gardner]

Hi,
 I'm getting to grips with my new (watson) station and weewx, and  I'm somewhat confused by how
"Barometer pressure" is calculated and how it relates to what we
might laughingly call reality.  With a name like "Barometer pressure" I'd have assumed it
might show up on a "real" barometer one day, but I guess that's just nomenclature.
"Altitude" is QNH, is "Barometer" supposed to be QFF?

I'd check against the  national weather service here, but they seem to have
given up publishing pressure charts (!).

with wee_config_fousb I get:

altitude: 309.9816 meters (that's what you get for 1st converting it to feet, I suppose)
pressure_offset: 0.0
station pressure (sensor): 985.1
absolute pressure (fixed_block): 985.1
relative pressure (fixed_block): 1021.9
altimeter pressure (davis algorithm): 1022.13885485
altimeter pressure (noaa algorithm): 1021.75359823

barometer pressure (wview algorithm): 1024.02426751

O/S temp is currently 0.2 °C , 12 hours ago it was 8.4 °C, humidity is 85%
I see in weewx.uwxutils that with the right data it's possible to calculate all sorts of pressures,
and so according to StationToSeaLevelPressure, with the default paManBar algorithm I get that
Sea level pressure (QFF) should be 1023.4107 mbar
and without the historical value, I  get 1023.7704, mbar

with paUnivie algorithm I get 1022.581 and 1023.7856 mbar with/without historical values

and with paDavisVp I get 1023.432 and 1023.792 mbar

So, urm, is there a good reason that "Barometer pressure" shows 0.3mbar above what the others generate without history,
and does anyone have any idea why the "paUnivie" algorithm is so much lower when I include the  temperature from 12hours ago?

I'd love to see a graph of  these things... can the graphs be set up to use these functions,
especially with a 12 hour "trend" value? or is that only in the report?

David

[Thomas Keffer]

Weewx generally deals with three different kinds of pressure. 

1. Station or Gauge Pressure (key 'pressure'): This is the pressure as measured by your instrument. I think pilots call this QFE?
2. Altimeter Pressure ('altimeter') : This is the pressure corrected for altitude, using a standard temperature profile. Pilots call this QNH.
3. Sea-level Pressure ('barometer'): This is the pressure corrected for altitude, using a profile from the measured temperature and (frequently) humidity. Pilots: QFF.

(I'm not a pilot, so I'm not familiar with the Q codes, but I think I got them right.)

Naturally, how these relate to each other will depend on what is used for the standard temperature profile, as well as the measured temperature. So, it's not surprising that you can get different answers depending on what formula you use. In an attempt to correct for diurnal effects, some formulas use the average of the current temperature and the temperature 12 hours ago.  Naturally, this will also cause differences.

The Fine Offset instruments have some quirks that make them especially challenging to set up and use accurately. There's a brief guide in the latest User's Guide.

If you want to fiddle with different formulas and graph them, it would not be hard. First, you'd have to make a slot for them in the database, by adjusting the schema. Then, write a custom weewx service that intercepts the LOOP and archive events, calculates any derived quantities you want, then inserts them into the packet. They will then automatically be inserted into the database.

Hope this helps.

-tk

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[David Gardner]

Hi all,

 I've implemented the custom service that Thomas suggested. Results so far are ... interesting. Basically the results from the DaviesVP algorithm are within a pixel (on an image of 640x480) of the results from the ManBar algorithm. The wview algorithm reads a higher pressure midnight to noon, (current temperature < 12 hour average temperature),
Based on today's (foggy) data, the wview algorithm is reading about 0.5 mBar higher for a temperature difference of 2 degrees. It was reporting the same pressure at 8-10pm or so, (12Hr avg temp 1 degree colder than current).

The UniVien algorithm reads a little lower (maybe 0.1mBar) than the ManBar when the wview  algorithm was reading higher.

Would the code be useful for anyone?

David