Adjusting barometric pressure

[TheBridge]

I notice now that my Website forecast ( https://climate.tommueller.com/gauges-ss-crows.htm ) is constantly saying 'rain' or 'heavy rain' is coming when nothing of the sort is true. I notice my Davis monitor device (not the PC but actual standalone device in the house) reads a barometric pressure of .08 Hg higher than the website is showing. This would put the Cumulus barometric more into the 'unsettled' or 'rain' prediction. IS there a setting to normalize the Cumulus setting (which I had assumed calc'ed in the elevation of 850') to self adjust the barometer?

[sfws]

I don't use a Davis, but I have found this on Wiki

This parameter (attribute=value) is added directly to the file, it does not appear on a configuration screen. Davis stations calculate Sea Level Pressure from Station Pressure using a formula based on several parameters such as temperature, humidity, etc, rather than just using altitude as most other stations. CWOP require 'Altimeter Pressure' to be uploaded, i.e. a value calculated simply using altitude.

Note for MX, use of the LOOP2 (see here) means s.l.p. can be obtained from the Davis station.

on this page https://cumuluswiki.org/a/Cumulus.ini#D ... _specific:


I have also found some posts on this forum that might help you.

I notice my Davis monitor device

viewtopic.php?f=11&t=15358&p=123801&hil ... re#p123801 This post explains

The Davis console applies all sorts of "corrections" to the pressure reading, not sure how does it. But it does use temperature and humidity

which I had assumed calc'ed in the elevation of 850'

I can't find a reference to check that, but this is what comes closest viewtopic.php?f=39&t=15745&p=120880&hil ... re#p120880

Cumulus uses the absolute pressure read from the LOOP2 packet and converts it to altimeter pressure using the altitude that you have set in Cumulus.

That suggests there is a sea level pressure and an altimeter pressure, not sure which feeds your forecast.

[freddie]

I notice now that my Website forecast ( https://climate.tommueller.com/gauges-ss-crows.htm ) is constantly saying 'rain' or 'heavy rain' is coming when nothing of the sort is true. I notice my Davis monitor device (not the PC but actual standalone device in the house) reads a barometric pressure of .08 Hg higher than the website is showing. This would put the Cumulus barometric more into the 'unsettled' or 'rain' prediction. IS there a setting to normalize the Cumulus setting (which I had assumed calc'ed in the elevation of 850') to self adjust the barometer?

Just as an aside, your current site position given on your gauges page (Latitude N 34° 19' 34" Longitude W 119° 27' 50") is a point about 2.5 miles off the Pacific coast (Mussel shoals is the nearest point to it on the coast). The position on your main page (Latitude N 34° 10' 30" Longitude W 118° 53' 42") looks okay.
KLAX was reporting 30.14 ins at 0150 your time, which compares to 30.10 on your station website. Correcting for the .08 in difference you mention, you get 30.18 which looks okay to me.

If your Monitor pressure is okay, then the offset is being applied in the Cumulus software (it is up to 1.9.4, build 1099, so you may wish to upgrade). The Wiki gives advice on where these are set.

I think the root of the problem is that your pressure extreme values have been incorrectly set - see the FAQ for details of what to do.

[TheBridge]

SFWS and Freddie,

3 changes appear good so far...
1.Added this: UseDavisLoop2=0
2.Revised the gauges upper range setting in the admin panel.

At this point the forecast was no reading something reasonable: "exception weather. settled fine" (in all caps of course). Well it is a nice day but I though the 'exceptional weather' it a wee bit over top so the final change was...

3. I tweaked the PressOffset=Offset_Value with the value at the time I checked NOAA pressure here versus my monitor. It was as discussed earlier in this thread.
Now the Cumulus pressure looks more accurate and the forecast only says 'settled fine' which I think is appropriate. I will continue to monitor and revise items 2 & 3 to best match actual conditions over time and then leave it at that.

Thank you both very much!!

Best,
Bridge

[beteljuice]

2.Revised the gauges upper range setting in the admin panel.

Having got the reading corrected you really shouldn't need to change the default settings other than to fine tune.

Don't forget this is a range, you may need to 'shift' your lower setting by a similar amount.

[TheBridge]

I am looking forward to checking the low range. Low pressures are usually is when it rains but here it may not rain again until September. Hope I remember all of this

[beteljuice]

It's not just rain - It's everything in the middle as well.

All (26) decisions are now being made in a compressed range.

[freddie]

All (26) decisions are now being made in a compressed range

A quick Google gave me a page on the Weather Underground site that shows historic pressure measurement ranges for various locations. It suggests (for Los Angeles) a maximum of 30.59 and a minimum of 29.07 - which I would probably round to max 30.60 and min 29.05. That should give you a good range based on the history of air pressure readings in your area.

The "forecasts" are only a rough guide, though. Remember that air pressure is just the weight of atmosphere pressing down on your pressure sensor. The atmosphere (especially the lowest part - the troposphere) is a complex three dimensional fluid that is forever in motion, and there are other elements (water vapour especially, and temperature) that have a far bigger influence on the weather at any location.