Author Topic: Altitude setting on an OS WMR80A  (Read 1565 times)

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Offline stormys

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Altitude setting on an OS WMR80A
« on: March 13, 2013, 07:00:01 PM »
I have a WMR80A system, to make the adjustment for altitude atmospheric pressure reading I see that only goes up to 2,500  meters above sea level, I live to 2.760 meters. Can there be errors in the readings of atmospheric pressure?.
Thanks

Offline aweatherguy

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Re: Altitude setting on an OS WMR80A
« Reply #1 on: March 14, 2013, 03:00:33 AM »
Errors you will get will be in sea-level pressure -- the station pressure should still be valid. I used this document to compute station pressure for 2760m elevation:

http://www.srh.noaa.gov/images/epz/wxcalc/stationPressure.pdf

Using an altimiter setting of 29.92 inHg (1013.25 mb), I get this:

Altitude 2760m, station pressure = 722.6mb

So, let's say you have a station pressure as indicated (722.6mb) and are trying to compute SLP but are using an elevation of 2500m in the formula instead of 2760m. That corresponds to the situation where you are only able to set the elevation to 2500m in the WMR80A.

But wait...computation of SLP (especially at high elevations) gets very complicated and for our purposes, those complications are somewhat irrelevant. So, instead, we'll compute altimiter setting and that will give you a rough idea of how much error you can expect to see.

For altimiter setting, I used this document:

http://www.srh.noaa.gov/images/epz/wxcalc/altimeterSetting.pdf

Applying this big formula yields an altimeter setting of 982.0mb and we already know the correct answer is 1013.25mb because we used that number above to compute station pressure (the altimeter formulas assume a "standard" atmosphere).

So, this suggests your SLP readings could be off by as much as 31mb or so if the WMR80A is set to an elevation of 2500m but you are actually at 2760m.

(There's an offset of 0.3mb in the formula for altimeter setting which is meant to deal with airplane cockpits being about 10 feet above ground level on average -- (at least I think that what its there for) -- so there is a small offset in these formulas which again is irrelevant for our purposes as we are concerned with much larger differences)

Since the WMR80 does not have a computer data link, there is no option to perform an adjusment in weather display/logging software.

And here's another factor to consider. At very high elevations (like yours), the outside temperature has a large effect on the computed value of SLP. I have an unproven suspicion that OS consoles use indoor temperature when computing SLP instead of outdoor temperature. This (if true) can cause large errors in SLP values when the outdoor and indoor temperatures are far apart (e.g. during winter).

One other thing to be aware of is that the WMR100 console only goes down to 700mb on station pressure (check your WMR80 manual to see what its limit is). During low pressure events, the station pressure at your elevation may be below 700mb and the WMR80 console might do something funny with the pressure reading.

As an aside, computation of SLP (as mentioned above) is a fairly complex topic and several different formulas are in popular use today. If you want to learn more, try this web page:

http://wiki.wunderground.com/index.php/Educational_-_Sea_level

or try an internet search -- there are some good documents out there which I don't have the links for right now.