Author Topic: WU: relative pressure shown instead of absolute, how can I fix that pls?  (Read 3120 times)

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Offline Ronny St.

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Hi all,
Just set up my brandnew Bresser DMAX 6-IN-1 Weatherstation with built-in wifi :) It all works fine, but in WeatherUnderground the relative air pressure is shown instead of the absolute air pressure as I have on my console. How can I change it so that WU gives the absolute air pressure? All stations in the far neighbourhood give absolute pressure, so my value is a bit weird in comparison with the others.
Anyone here willing to help me out? Thanks in advance!
Ronny

Offline galfert

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #1 on: February 16, 2019, 07:48:04 PM »
If your neighbors are reporting absolute pressure then they are doing the wrong thing. The proper thing to do is to send relative pressure or what is also referred to as sea level pressure.

absolute pressure is also referred to as station pressure. The purpose of WU is to relate weather information. Your absolute (station) pressure is useless to anyone else.

Some people send altimeter pressure. But that is not what WU asks for either.

« Last Edit: February 16, 2019, 07:50:43 PM by galfert »
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Offline nincehelser

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #2 on: February 16, 2019, 07:49:04 PM »
Hi all,
Just set up my brandnew Bresser DMAX 6-IN-1 Weatherstation with built-in wifi :) It all works fine, but in WeatherUnderground the relative air pressure is shown instead of the absolute air pressure as I have on my console. How can I change it so that WU gives the absolute air pressure? All stations in the far neighbourhood give absolute pressure, so my value is a bit weird in comparison with the others.
Anyone here willing to help me out? Thanks in advance!
Ronny

I've no idea about your particular system, but typically you want to send altimeter pressure to wunderground.   In the past, if you sent absolute or station pressure, wunderground would start auto-adjusting it after several weeks.  I don't know if they're still doing that or not.

Offline Ronny St.

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #3 on: February 16, 2019, 08:24:54 PM »
Wow, thanks for the superfast replies! I have yet learned something more now: that absolute pressure is in fact irrelevant for weather statistics or interpretation.
However, I think wunderground does not auto-adjust absolute pressure after a few weeks, since the stations I found in the vicinity are quite some time online now. Whatever, I am glad to know that the data I am sending have better meaning ;)
Thanks again!
Ronny

Offline ValentineWeather

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #4 on: February 16, 2019, 09:26:37 PM »
I don't think they know the difference or even what pressure they want. Back in 2015 WU said this

" Pressure: (range check) Here are two common issues:
 1) The pressure data needs to be adjusted to mean sea level pressure, not absolute pressure. The easiest way to do this is to calibrate your pressure data to match the nearest airport. Check your manual for instructions on calibrating your sensors."


But this statement in itself makes little sense, most airports don't give MSLP only ASOS stations have the ability or weather office and its done once an hour. Altimeter is what almost everyone uses.

This is why its only done hourly:

 Procedure for adjusting station pressure IE “absolute pressure.” to sea level pressure. Most PWS don't have this ability.


1. Determine (a) the current station temperature and (b) the station temperature from 12 hours ago.

2. Compute the average of the above two temperatures.

3. Convert the average temperature to Kelvin by adding 273.1 to the Celsius value.

4. Compute the scale height H = RdT/g, where Rd = 287.1 J/(kg K) and g = 9.807 m/s2. Be sure to record H to at least 4 significant figures (unless you are using MathCad, in which case full precision is maintained automatically).

5. Compute the sea level pressure psl from
psl = p0 exp(z0/H)
where p0 is the observed pressure and z0 is the altitude above sea level where you made your pressure observation.

« Last Edit: February 16, 2019, 09:29:05 PM by ValentineWeather »
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Offline galfert

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #5 on: February 16, 2019, 10:08:23 PM »
I go to Mesowest and I can get MSLP from any station, not just ASOS stations. And you can get it every couple minutes from all these stations including the ASOS stations whose's METAR only reports hourly.

Not sure what you are referring to with, "most PWS don't have this ability," in reference to calculating SLP. Just about any station I see reports SLP. In fact it is Altimeter that none other than the Davis Vue can calculate. So they all do SLP. But what the console does is irrelevant because every online service asks for stations to send SLP. It is only CWOP that asks for Altimeter. And any software that sends to CWOP has been designed to take station pressure and calculate Altimeter for use in CWOP only.

For consoles to keep track of the last 12 hours temperature is no big deal. The user does not need to know all that the station is doing to properly calculate SLP.

From what I've read probably many years ago stations were not able to do this SLP calculation and that is why CWOP being probably the oldest online weather service for PWS decided that Altimeter was the way to go because it was a simple way for most people to consistently and uniformly report pressure with that eras equipment. So CWOP uses Altimeter only because of legacy procedural circumstances. I got this information from the CWOP manual.

I'm not advocating SLP instead of Altimeter. I'm advocating for for both. I send SLP to everywhere except to CWOP where I send Altimeter. I can compare my station to other stations and I have data with both SLP and Altimeter. If I want to compare my station to the local media, no problem, I pull out my CWOP data. If I want to compare to other WU stations I know that I'm using SLP like the rest of them.
« Last Edit: March 01, 2020, 05:30:32 PM by galfert »
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Offline ValentineWeather

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #6 on: February 16, 2019, 10:44:54 PM »
I go to Mesowest and I can get MSLP from any station, not just ASOS stations. And you can get it every couple minutes from all these stations including the ASOS stations whose's METAR only reports hourly.

Not sure what you are referring to with, "most PWS don't have this ability," in reference to calculating SLP. Just about any station I see reports SLP. In fact it is Altimeter that none other than the Davis Vue can calculate. So they all do SLP. But what the console does is irrelevant because every online service asks for stations to send SLP. It is only CWOP that asks for Altimeter. And any software that sends to CWOP has been designed to take station pressure and calculate Altimeter for use in CWOP only.

For consoles to keep track of the last 12 hours temperature is no big deal. The user does not need to know all that the ststion is doing to properly calculate SLP.

From what I've read probably many years ago stations were not able to do this SLP calculation and that is why CWOP being probably the oldest online weather service for PWS decided that Altimeter was the way to go because it was a simple way for most people to consistently and uniformly report pressure with that eras equipment. So CWOP uses Altimeter only because of legacy procedural circumstances. I got this information from the CWOP manual.

I'm not advocating SLP instead of Altimeter. I'm advocating for for both. I send SLP to everywhere except to CWOP where I send Altimeter. I can compare my station to other stations and I have data with both SLP and Altimeter. If I want to compare my station to the local media, no problem, I pull out my CWOP data. If I want to compare to other WU stations I know that I'm using SLP like the rest of them.


Nothing has changed you are looking at station pressure I believe and calling it SLP.
Most PWS still don't have the ability to calculate correctly SLP.
Mesowest doesn't calculate SLP for anyone only station pressure. the only PWS I know of that calculates SLP are the Davis units and rather poorly at that. 
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Offline ValentineWeather

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #7 on: February 16, 2019, 10:51:02 PM »
Here is a non-ASOS airport with AWOS, they don't calculate SLP either.
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Offline nincehelser

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #8 on: February 16, 2019, 10:53:04 PM »
Why does this argument keep popping up?

Everyone can do altimeter.  That's a big reason CWOP went with it.

SLP is *NOT* common.

Offline galfert

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #9 on: February 16, 2019, 10:53:20 PM »
ValentineWeather,
You are correct about Mesowest not showing SLP on the data tables. I should have clarified how I get SLP from Mesowest. I do it using the map. The map has a drop down and you can see all three; pressure (station), Altimeter, and Sea Level Pressure. I like to set the two drop downs to show Altimeter and the other Sea Level Pressure.

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Offline galfert

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #10 on: February 16, 2019, 11:01:57 PM »
Why does this argument keep popping up?

Everyone can do altimeter.  That's a big reason CWOP went with it.

SLP is *NOT* common.

I disagree. And every online PWS reporting service except CWOP also disagrees. Every weather software if you enter in your proper elevation into will only send Altimeter to CWOP. So the Meteobridge, WiFi Logger, WeeWx, Weather-Display... etc all disagree with you too.

So here you are advocating Altimeter to WU when their documentation clearly states Sea Level Pressure. Yet you pronounce that rain rate is defined by WU as past 60 minutes rainfall. So in one case you stand behind the documentation and in another case you ignore it.
« Last Edit: February 16, 2019, 11:12:07 PM by galfert »
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Offline CW2274

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #11 on: February 16, 2019, 11:17:28 PM »
Mesowest not showing SLP on the data tables.
If this is what you mean by "data table", looks fine here with SLP, altimeter, and station pressure.

https://mesowest.utah.edu/cgi-bin/droman/meso_base.cgi?stn=C2274&unit=0&time=LOCAL&product=&year1=2018&month1=2&day1=15&hour1=12&hours=24&graph=0&past=0

Offline nincehelser

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #12 on: February 16, 2019, 11:26:11 PM »
Why does this argument keep popping up?

Everyone can do altimeter.  That's a big reason CWOP went with it.

SLP is *NOT* common.

I disagree. And every online PWS reporting service except CWOP also disagrees. Every weather software if you enter in your proper elevation into will only send Altimeter to CWOP. So the Meteobridge, WiFi Logger, WeeWx, Weather-Display... etc all disagree with you too.

So here you are advocating Altimeter to WU when their documentation clearly states Sea Level Pressure. Yet you pronounce that rain rate is defined by WU as past 60 minutes rainfall. So in one case you stand behind the documentation and in another case you ignore it.

(*sigh*)  As ValentineWeather pointed out earlier in this thread, wunderground doesn't seem to know exactly what they want, which is why the pressure readings there have been a mess for years.  The last I heard talking to the developers is that they want ALTIMETER because of its popularity and that it matches what the general population expects for pressure.

Here's your word for the day: Ultracrepidarian

Offline galfert

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #13 on: February 16, 2019, 11:26:43 PM »
Mesowest not showing SLP on the data tables.
If this is what you mean by "data table", looks fine here with SLP, altimeter, and station pressure.

https://mesowest.utah.edu/cgi-bin/droman/meso_base.cgi?stn=C2274&unit=0&time=LOCAL&product=&year1=2018&month1=2&day1=15&hour1=12&hours=24&graph=0&past=0

Cool. Good to know. Thanks!  So it is there.
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Offline galfert

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #14 on: February 16, 2019, 11:29:38 PM »
Why does this argument keep popping up?

Everyone can do altimeter.  That's a big reason CWOP went with it.

SLP is *NOT* common.

I disagree. And every online PWS reporting service except CWOP also disagrees. Every weather software if you enter in your proper elevation into will only send Altimeter to CWOP. So the Meteobridge, WiFi Logger, WeeWx, Weather-Display... etc all disagree with you too.

So here you are advocating Altimeter to WU when their documentation clearly states Sea Level Pressure. Yet you pronounce that rain rate is defined by WU as past 60 minutes rainfall. So in one case you stand behind the documentation and in another case you ignore it.

(*sigh*)  As ValentineWeather pointed out earlier in this thread, wunderground doesn't seem to know exactly what they want, which is why the pressure readings there have been a mess for years.  The last I heard talking to the developers is that they want ALTIMETER because of its popularity and that it matches what the general population expects for pressure.

Here's your word for the day: Ultracrepidarian

Good word. And you are entitled to your opinion. So are you asking me for more Latin lessons?
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Offline nincehelser

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #15 on: February 16, 2019, 11:30:59 PM »
Good word. And you are entitled to your opinion. So are you asking me for more Latin lessons?

I don't take lessons from people who don't know the subject matter.


Offline ValentineWeather

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #16 on: February 16, 2019, 11:31:27 PM »
Mesowest not showing SLP on the data tables.
If this is what you mean by "data table", looks fine here with SLP, altimeter, and station pressure.

https://mesowest.utah.edu/cgi-bin/droman/meso_base.cgi?stn=C2274&unit=0&time=LOCAL&product=&year1=2018&month1=2&day1=15&hour1=12&hours=24&graph=0&past=0

I didn't realize that data table was there. Is it accurate? Looks like mine is running way above what the airport reports +.10 only 1 mile away.  Are they calculating the 12 hour mean? Etc. 
Randy

Offline ValentineWeather

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #17 on: February 16, 2019, 11:33:48 PM »
No way its accurate our altimeters are the same elevations are within a few feet. 
Randy

Offline CW2274

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #18 on: February 16, 2019, 11:52:23 PM »
No way its accurate our altimeters are the same elevations are within a few feet.
Not sure what you mean, but mine is. My link above, my WL straight from the console and this link are all exactly the same. Always.

https://www.wrh.noaa.gov/total_forecast/other_obs.php?wfo=twc&zone=AZZ504

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Offline ValentineWeather

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #20 on: February 17, 2019, 12:13:28 AM »
No way its accurate our altimeters are the same elevations are within a few feet.
Not sure what you mean, but mine is. My link above, my WL straight from the console and this link are all exactly the same. Always.

https://www.wrh.noaa.gov/total_forecast/other_obs.php?wfo=twc&zone=AZZ504

What I mean is the SLP accurate for our stations? Are they doing all the calculations needed including the last 12 hour mean temperature or are they just throwing some numbers around?

If I use their numbers SLP is (.11") off from the airport only 1 mile away. 
My console shows SLP at 29.81 and they are saying 29.94 which is wrong. Not really close.
The Airport is close to what I show on console 29.84 only (.03) off.  Most likely the temperature difference.

Randy

Offline CW2274

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #21 on: February 17, 2019, 12:20:33 AM »
No way its accurate our altimeters are the same elevations are within a few feet.
Not sure what you mean, but mine is. My link above, my WL straight from the console and this link are all exactly the same. Always.

https://www.wrh.noaa.gov/total_forecast/other_obs.php?wfo=twc&zone=AZZ504

What I mean is the SLP accurate for our stations?   Most likely the temperature difference.
Well, I'd like to think so...I mean, after all, this isn't exactly WU....

Offline Ronny St.

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #22 on: February 17, 2019, 08:20:10 AM »
Geeh, I seem to have started some discussion here   8-)
Being a newbie in this field, I tried to follow it but there are a lot of abbreviations used that I will have to learn, so I know what to do  ;)
Thank you all for your contributions to this topic!
Ronny

Offline ocala

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #23 on: February 17, 2019, 08:36:25 AM »
Geeh, I seem to have started some discussion here   8-)
Being a newbie in this field, I tried to follow it but there are a lot of abbreviations used that I will have to learn, so I know what to do  ;)
Thank you all for your contributions to this topic!
Ronny
No worry's.
This "discussion" has been going on for a while. :-)
The blues had a baby and they named it Rock & Roll

Offline ValentineWeather

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Re: WU: relative pressure shown instead of absolute, how can I fix that pls?
« Reply #24 on: February 17, 2019, 09:27:41 AM »
Geeh, I seem to have started some discussion here   8-)
Being a newbie in this field, I tried to follow it but there are a lot of abbreviations used that I will have to learn, so I know what to do  ;)
Thank you all for your contributions to this topic!
Ronny
No worry's.
This "discussion" has been going on for a while. :-)

And I'm always right.  :roll:
I did learn something new. Mesowest attempts to calculate SLP.
Randy