Author Topic: WS-2902A Barometric Pressure Readings  (Read 819 times)

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Offline xon432246

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WS-2902A Barometric Pressure Readings
« on: July 21, 2018, 12:28:14 PM »
Hello,

I set up my weather station last week and noticed that the absolute pressure readings are always higher than the relative. Right now I'm getting relative 29.96 and absolute 30.04 I thought the reverse was supposed to be true? The FAQ page on Ambient Weather says "Relative pressure is larger than absolute pressure unless you live at or below sea-level." My elevation is 30' above sea level so both readings should be pretty close. I have calibrated the pressure readings but both absolute and relative increase by the same amount. Any ideas?

Thanks

Online galfert

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Re: WS-2902A Barometric Pressure Readings
« Reply #1 on: July 21, 2018, 05:07:29 PM »
You can not correct Absolute Pressure. You can only correct Relative Pressure. (see next post for update) This is because Absolute Pressure is what is measured by the device as if you were at sea level. When you correct Relative Pressure what you are doing is correcting for change in elevation if you are not at sea level. At your altitude of 30' that is practically the same as sea level for all intensive purposes. So you are correct that the pressure should be very close to the same as Absolute Pressure. What I think has happened in your case is that you have over corrected for elevation in the wrong direction and you've basically told the device you are below sea level. You should correct and recalibrate your Relative Pressure to match another local source (weather report) or other known calibrated barometer. And your observation that the Absolute and Relative Pressure go up and down by the same amount over time is a proper observation.

« Last Edit: July 23, 2018, 05:22:46 PM by galfert »
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Online galfert

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Re: WS-2902A Barometric Pressure Readings
« Reply #2 on: July 22, 2018, 12:58:56 PM »
Here is a better answer. I previously said that Absolute Pressure was not able to be adjusted. I was wrong. I didn't read the manual far enough and only saw the first part about adjusting Relative Pressure. So I figured that only Relative Pressure was able to be adjusted and the Absolute Pressure was factory set, which it its. And they really don't want you messing with it unless you really know what you are doing and you think it is necessary.

So it is possible to adjust the Absolute Pressure that was factory set. To get into this menu  you hold down TEMP and MAX/MIN buttons down for 5 seconds. You proceed to the next calibration setting by pressing SET. You adjust each setting with the normal WIND+ and PRESSURE- buttons. Press ALARM to restore given setting to factory default.

Although you are allowed through this advanced calibration menu to adjust Absolute Pressure the addendum mentions that it still recommends you don't mess with this setting and instead just calibrate Relative Pressure only via instructions in section 6.3. (Figure 23 in the manual has this Note in the Absolute Pressure setting section).

With that said I found another weather station manual made by Ecowitt which is very similar in capability to the WS-2902A and is just another similar Fine Offset clone with a different console. In Ecowitt's manual there is more details in understanding Absolute and Relative Pressure calibrations. I found it well laid out, easy to follow so I will copy paste that section below from the HP3500B manual. Shame the WS-2902A manual doesn't explain things this well.
Follow these steps and your Absolute Pressure and Relative Pressure should work properly.

*** This is actually very good information that anyone can use with any weather station of any brand that lets them adjust both Absolute Pressure and Relative Pressure offset.

7.3.1 Calibration of barometric pressure settings.

Calibration of barometric pressure requires some additional
understanding, which we will provide here. Also provided is a step by
step procedure for calibrating correctly.

Absolute barometric pressure, can be calibrated at manufacturing time
by comparing with a precise instrument that measures pressure at the
same location. In practice, sometimes small adjustments of a few hPa
may be needed. The relative pressure represents what the air
pressure would indicate if your station was at sea level and depends
on the altitude of your console and cannot be known in advance. This
is why it needs an adjustment.

There are different manners in which to handle this adjustment. We will
outline a reliable procedure below, which requires adjusting both
pressures. The method assumes that you have an official airport
sufficiently nearby to act as a reliable reference. Usually distances of
up to 25 miles work reliably, but this is not always true and depends on
geography. We start by assuming that your station’s absolute pressure
reading is correct and needs no offset correction.

The procedure below assumes you are starting from the console’s
factory setting. With those settings, ABS and REL should, at this time,
be displaying the same value. We also assume, for the moment, that
ABS pressure is 100% correct.

1. For this procedure we will get the most precise results if our
display for pressure is in hPa units. Even if you do not want to use
those units eventually, set the console to use them for now.

2. Determine the altitude, or elevation above sea level, of your
station’s console. This measurement is necessary to account for
the difference in air pressure caused by the elevation of your
console. Elevation above sea level reduces the absolute pressure
measured by your sensor. Determine this altitude using a GPS, or
look it up using a tool such as this web site:
https://www.freemaptools.com/elevation-finder.htm. You can
input your location’s GPS coordinates, or manipulate the map to
your location. Click on “Estimate Elevation” and observe the result.
For an example we will use a console location at 42 ft. above sea
level.

3. This tool will provide the ground level elevation at your location, so
you will need to add the right amount for how high above ground
level your console is. If you are on a ground floor and have the
console on a desk, you’ll have to add something like 3-4 ft. If you
are using a GPS system that tells you elevation, make sure it is
right next to the console and you’ll be able to read the correct
elevation right from the GPS results without further adjustment.

4. With the correct altitude/elevation in hand you will need to
determine the correct offset. To be added to the absolute pressure
reading in order to compute relative pressure (sea level
equivalent). Correction tables can be found on-line in many places.
One example is the table found at the web site at
https://novalynx.com/manuals/bp-elevation-correction-tables.pdf.
Locate your elevation in the first column and read the correction in
the third column. This table, however is rather coarse, making it
hard to be precise. An alternative is an on-line calculator such as
the one found here:
http://www.csgnetwork.com/barcorrecthcalc.html
For our example of 42 ft. above sea level we input 42 ft. of
elevation and a standard pressure of 1013.25 hPa/mb and press
calculate. We find an “absolute barometer value” that should be
-1.5626061222588443 hPa lower than at sea level. The inverse
(because relative pressure is higher than absolute pressure) of
this number will be our “REL PRESS OFFSET” value. Use the
settings procedure to increase REL by +1.6 (nearest rounded
value we can input).

5. Now we need a reliable reference for pressure at sea level. Locate
the official identifier for the nearest airport. Refer to “World Airport
Codes” at https://www.world-airport-codes.com or a similar
reference. Enter your location or nearby airport name, and press
“Search.” Select the correct airport from your search results and
click on it. For example, search for “Mountain View” and click on
“Moffet Field.”

6. From the resulting page find the ICAO code, if listed. Otherwise
use the IATA code. For the example above, you would find IATA
code “NUQ.”

7. Now go to a web site like AVIATION WEATHER CENTER (for US
locations) at https://www.aviationweather.gov/metar?gis=off and
enter the code you found in step 2, and click “Decoded” (to make
the next step easier) before requesting the METAR information.
For the example we would enter “KNUQ” and find a result output
like: “30.09 inches Hg (1019.0 mb) [Sea level pressure:
1019.1 mb]”

8. Go to the calibration settings page and observe the “REL
Barometer value (this is the value we just adjusted in step 4
above). Compare the REL value with the value from the airport. IN
our example, the REL display was 1022.9 where we expected
1019.1. This then tells us that our displayed REL pressure is
1022.9 – 1019.1 = 3.8 hPa different from the reference source.

9. Since we assumed the absolute pressure measured was correct,
and we presumably calculated the elevation related offset
correctly, we must conclude that the absolute pressure was not
correct after all. It appears to be 3.8 too high, so we’ll now enter a
correction of -3.8 in the settings for “ABS Barometer” until it reads
a value 3.8 hPa lower than before. This kind of correction is
entirely normal as during manufacturing small shifts in the
pressure sensor readings can be introduced.

10. For a more precise procedure, locate a very precise barometer
that you can place right next to the console, you would adjust
“ABS Barometer” until the ABS pressure reads identical. You
would then still adjust “REL barometer” until it displays the value
from the reference airport. This procedure would also produce the
correct relative pressure, but due to a precise calibration of the
absolute pressure, it too is correct.

The first procedure above generally works quite well, but for stations at
fairly high altitudes (e.g. 5,000 ft. or higher) it may be more incorrect
than at lower altitudes. In such cases comparisons with other known
correct, and nearby at similar altitude, stations may help.

Now that calibration is complete, feel free to change the pressure units
to whatever you like.

Note: Airport METAR data is often only updated every 10, 15 or even
30 minutes. If you use the information in the procedure above,
you may be looking at pressure data that is out of date by as
much as the update interval. To get best results observe
several times and figure out the update interval and then use
two values for the procedure: one taken immediately after an
update, another taken about halfway through the interval.

Note: It is also a good idea to observe some more after the calibration
procedure is complete to make sure the numbers are correct.
« Last Edit: July 22, 2018, 03:07:30 PM by galfert »
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Offline jaltman

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Re: WS-2902A Barometric Pressure Readings
« Reply #3 on: July 23, 2018, 08:05:10 AM »
Thank you.

Online galfert

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Re: WS-2902A Barometric Pressure Readings
« Reply #4 on: July 24, 2018, 07:04:43 PM »
*** Barometer Calibration Tips ***

Here are some tips to hopefully remove some ambiguities.

1. When looking up an airport's METAR data ignore first part which is the altimeter reading. Focus only on Sea Level Pressure numbers. Some airports only report altimeter readings. If your airport doesn't provide you with Sea Level Pressure data then try again in a few hours as sometimes the report omits it for some reason. If it is never there then look for the next closest airport to see if that airport reports Sea Level Pressure numbers.

* Attached below is a screenshot of examples of two airports. One that omits the Sea Level Pressure data and one that includes it. Don't use METAR data that omits Sea Level Pressure.

2. After you have calculated your Relative Pressure offset for your altitude and it is set don't further adjust the Relative Pressure offset. But rather to tweak and adjust the final Relative Pressure till it is just right you adjust by actually calibrating the Absolute Pressure up or down. By way of changing the Absolute Pressure the Relative Pressure will move up and down with it as a constant offset to what you configured based on your altitude.

3. Tweak your barometer on clear dry non-windy days. Getting the barometer just right requires maybe tweaking it for a few days. But make the adjustments at close to noon every day (on good days). It is best to do it when there is high pressure (at or above 1013.25 hPa Relative Pressure - (Mean Sea Level)). If you aren't home at noon then do it on the weekend or over a couple weekends. Why is a windy day bad...because windy means change is happening or high pressure or low pressure is moving in. If you live on the coast in an always windy area then just settle for a nice clear dry day.

4. Relative Pressure is the same as saying Mean Sea Level Pressure or sometimes just referred to as Sea Level Pressure. METAR "altimeter pressure" (where Sea Level is omitted) or sometimes referred to as QNH is not useful for anything regarding the barometer in your weather station. The purpose of altimeter pressure (QNH) is for pilots to use in their instruments and relates more to altitude computations for approaching airplanes. I'm not a pilot so you don't need to know more than to know it doesn't apply. So always use METAR data that includes Sea Level Pressure.

5. Indoor and outdoor pressure is the same. The pressure in your house is the same as the pressure outside of your house even with all the windows and doors closed...it equalizes. That is why most weather stations incorporate the barometer in the inside console as it will last longer indoors. If you do have a very tightly sealed house and your central air kicks in while you are making adjustments maybe in theory it could change the pressure. I'm not really sure but it is just a thought I had that maybe someone else can comment on that knows better. So if you think this might be affecting you then open a door or window while making adjustments. Remember your barometer refreshes after a certain amount of time and what is actually displayed is an average of the time between the last refresh.

6. The reason you do all these tweaking calibrations when the device is set to show Hecto Pascals (hPa or same as millibars) is because you can be more precise in the adjustment calibration tuning. In my experience 4 tenths of a hPa equals 1 hundredth of inches of mercury (inHg). So there is more fine tuning possible when you are in hPa. When your calibration is done you can change your device to show inHg and the calibrations you have done are still there but now just showing instead in inHg units. The tenths place in hPa is the last significant digit you can adjust. Likewise the 1 hundredth place is the last significant digit you can adjust. So if you adjust up or down in the inHg scale by 1 hundredth it moves the hPa scale by 4 tenths. Better to adjust by four times the precision if you work in hPa. When you are done and change it to show inHg you'll know there is more precision in there that you aren't really seeing when displaying inHg. Less of a chance of being off by 1 hundredth of a inHg. Your device internally works with hPa and does a conversion to actually show you inHg. You'll get a better CWOP score if that is your goal, or even if not you'll be more accurate.

« Last Edit: July 24, 2018, 07:41:46 PM by galfert »
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Offline CW2274

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Re: WS-2902A Barometric Pressure Readings
« Reply #5 on: July 24, 2018, 07:47:08 PM »
*** Barometer Calibration Tips ***

4.  METAR "altimeter pressure" (where Sea Level is omitted) or sometimes referred to as QNH is not useful for anything regarding the barometer in your weather station.
Except for the fact that practically every media outlet in the country uses the altimeter setting, as their data almost always comes from a nearby airport ASOS, which is usually the "official data" for that area. If you want to compare apples to apples in the "big picture", set for the altimeter.

Online galfert

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Re: WS-2902A Barometric Pressure Readings
« Reply #6 on: July 24, 2018, 08:10:35 PM »
*** Barometer Calibration Tips ***

4.  METAR "altimeter pressure" (where Sea Level is omitted) or sometimes referred to as QNH is not useful for anything regarding the barometer in your weather station.
Except for the fact that practically every media outlet in the country uses the altimeter setting, as their data almost always comes from a nearby airport ASOS, which is usually the "official data" for that area. If you want to compare apples to apples in the "big picture", set for the altimeter.

I accept the part of your comment where you mention that every media outlet uses altimeter setting. But just because that is what they use doesn't mean you should for your equipment. You can't compare altimeter pressure to sea level pressure so why bother to compare your station to what a media outlet reports? If you try and set your barometer to altimeter pressure you'll never dial it in. You'll be at it constantly.  This is because altimeter pressure is dependent on other constantly changing atmospheric conditions that your weather station can't account for. So by setting your barometer to altimeter pressure you are always going to be off. Your weather stations sea level pressure (relative pressure) is the same as the airport's sea level pressure. So I would call that an apples to apples comparison.

If you observe over time an airport's altimeter pressure compared to their sea level pressure you will notice that sometimes they differ by different amounts at different times. There is no constant offset there. Your weather station is not capable of working with altimeter pressure.

I don't understand or agree with your comment of trying to calibrate to altimeter pressure and then have a weather station that is never right. Just because the media reports that number. You mean the fake news media? When do they get anything right?

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Offline CW2274

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Re: WS-2902A Barometric Pressure Readings
« Reply #7 on: July 24, 2018, 08:35:49 PM »
*** Barometer Calibration Tips ***

4.  METAR "altimeter pressure" (where Sea Level is omitted) or sometimes referred to as QNH is not useful for anything regarding the barometer in your weather station.
Except for the fact that practically every media outlet in the country uses the altimeter setting, as their data almost always comes from a nearby airport ASOS, which is usually the "official data" for that area. If you want to compare apples to apples in the "big picture", set for the altimeter.
Your weather station is not capable of working with altimeter pressure.
Really? Got news for ya, that's exactly what it does.

I merely posted this as an alternative for the OP should he/she want to try.

Online galfert

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Re: WS-2902A Barometric Pressure Readings
« Reply #8 on: July 24, 2018, 08:59:28 PM »
Ok. I'm cool with that. Let people choose how they want to run their station.
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Offline xon432246

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Re: WS-2902A Barometric Pressure Readings
« Reply #9 on: July 25, 2018, 07:01:16 AM »
Thanks for all the great info. I do have one question. Is this right:

Altimeter pressure=relative pressure
Sea Level pressure=absolute pressure

Online galfert

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Re: WS-2902A Barometric Pressure Readings
« Reply #10 on: July 25, 2018, 08:26:20 AM »
Thanks for all the great info. I do have one question. Is this right:

Altimeter pressure=relative pressure
Sea Level pressure=absolute pressure

No no, Nope nope. There are three different types of pressure (at least just 3 as it relates to what we are talking about). You are mixing them all up.

Absolute Pressure = QFE = STA: Local barometric raw real sensor data without any correction for elevation nor atmospheric conditions. (I think STA stands for Station as in local station pressure)

Relative Pressure = QFF = SLP: Also referred to as Sea Level Pressure or Mean Sea Level Pressure. Which is arrived at by taking Absolute Pressure (QFE) and just doing elevation correction and maybe also current temperature.

Altimeter Pressure = QNH = ALT: A different but close to Sea Level calculation which is arrived at by taking Absolute Pressure (QFE) and doing some elevation correction AND using ISA atmospheric temperature conditions.

UPDATE - I don't know if these statements are 100% correct.
With that said lets take a look at your weather station. It only deals with two of these. There is Absolute Pressure (QFE) and Relative Pressure (QFF Sea Level). That is why I say ignore Altimeter Pressure (QNH). Because you can only enter in a static fixed offset between Absolute and Relative in your weather station. Conversely Altimeter Pressure changes in relation to Absolute Pressure constantly depending on atmospheric and temperature conditions so it is never by a fixed offset amount. Your weather station does not have a way to deal or compute Altimeter Pressure. If you try and set your Relative Pressure to equal Altimeter Pressure then you will notice that it is impossible to dial it in and you'll be constantly fiddling with it trying to play catch up or something.

Your weather station only really has one simple barometric sensor. Its job is to just sense Absolute Pressure and that is it. Then there is a basic formula in your weather station that it uses to then take this Absolute Pressure and display Relative Pressure (Sea Level). When you adjust Relative Pressure (the fixed offset from Absolute Pressure) in your weather station what you are actually doing is changing the value in the formula to account for your altitude, that is it, without taking atmospheric temperature and whatever conditions into account.

UPDATE - This paragraph too should be taken lightly for acuracy
If you really want to use Altimeter Pressure instead of Relative Pressure (Sea Level) in your weather station you might find the spot where it just tends to vary more or less by a value that you are comfortable with and you'll notice that sometimes it is closer and sometimes further from real Altimeter Pressure. But that is your choice. I just want people to understand the difference. I also understand CW2274 differing opinion that he wants to try and use Altimeter Pressure instead of Relative Pressure (Sea Level) because media stations report on Altimeter Pressure.

The purpose of weather stations reporting Sea Level Pressure instead of Absolute Pressure is so that we can compare one station to another regardless of elevation. It is a way to compare different locations weather conditions to one another. I do agree that Altimeter Pressure is a more precise way of comparing different locations because it takes not just altitude into account but it also takes temperature and maybe more into account (I don't know formula specifics). But our weather stations just have not been designed to really use Altimeter Pressure. If you want to see the accuracy of your weather station then set your Relative Pressure to be Sea Level and then compare your station to local airports Sea Level and then you'll know if your barometer needs replacing or not or you'll at least know how accurate it is and if that ever changes.

Note 1: I've read that the Davis Vue console does display Altimeter Pressure. I have no experience with this but it seems then that the Vue console does deal with all 3 of these types of pressures. The Davis Vue is a newer model than the Davis VP2, but the Vue is cheaper the VP2. So I suppose that the VP2 being that it is older just never got Altimeter pressure when it was designed. Perhaps when we see a VP3 released it might have Altimeter in its console. Some people use the Vue console with their VP2 as it seems they are compatible.

Note 2: Some other brands of weather stations get calibrated a bit differently. Instead of setting the fixed offset between Absolute Pressure and Relative Pressure (Sea Level) you only enter in your altitude. The weather station then uses the altitude you tell it to compute the fixed offset between Absolute and Relative. It is really doing the same thing as the Ambient Weather station, just making you work less to arrive at the same required data for the formula to then display Relative Pressure. I think I read that Oregon Scientific uses this method. Still it is just dealing with Absolute and Relative (Sea Level) only. And in some stations like Oregon Scientific I think this offset (via altitude setting) is the only thing you can adjust. So if the Raw Absolute Pressure is off then there is no way to adjust it. People then resort to faking their altitude in order to correct Relative Pressure because of not being able to adjust Absolute Pressure. And that kinda works but I don't think it is as elegant a solution (as then they are misreporting their elevation)...but they have no other choice. Well I suppose they could send it back for replacement with a better calibrated Absolute Pressure sensor.

Note 3:
In aviation and radio communications there are tons of Q codes that deal with more than just pressure. The ones we discussed are below. Q stands for Question.
QFE stands for: Question Field Elevation
QFF stands for: Question Field as in airfield? (or maybe it is one of those that doesn't translate)
QNH stands for: Question Nil Height

...some other non-pressure related Q codes and the letters don't always translate to specific words:
QRC: what is your bearing
QRB: what is your distance
QUA: Where may I jettison fuel, or I'm about to jettison fuel
« Last Edit: July 25, 2018, 04:54:29 PM by galfert »
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Offline CW2274

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Re: WS-2902A Barometric Pressure Readings
« Reply #11 on: July 25, 2018, 03:29:48 PM »
Thanks for all the great info. I do have one question. Is this right:

Altimeter pressure=relative pressure
Sea Level pressure=absolute pressure
I also understand CW2274 differing opinion that he wants to try and use Altimeter Pressure instead of Relative Pressure (Sea Level) because media stations report on Altimeter Pressure.

Just some minor clarifications:

I don't try, I do.

Note 1: I've read that the Davis Vue console does display Altimeter Pressure. I have no experience with this but it seems then that the Vue console does deal with all 3 of these types of pressures. The Davis Vue is a newer model than the Davis VP2, but the Vue is cheaper the VP2. So I suppose that the VP2 being that it is older just never got Altimeter pressure when it was designed. Perhaps when we see a VP3 released it might have Altimeter in its console. Some people use the Vue console with their VP2 as it seems they are compatible.

This is all correct except they don't seem compatible, they are compatible. Many folks like myself use a VP2 ISS and a Vue console.

Note 3:
In aviation and radio communications there are tons of Q codes that deal with more than just pressure. The ones we discussed are below. Q stands for Question.
QFE stands for: Question Field Elevation
QFF stands for: Question Field as in airfield? (or maybe it is one of those that doesn't translate)
QNH stands for: Question Nil Height
...some other non-pressure related Q codes and the letters don't always translate to specific words:
QRC: what is your bearing
QRB: what is your distance
QUA: Where may I jettison fuel, or I'm about to jettison fuel

The only time I every used a spoken "Q" was QNH (we called it Queen's Nautical Height), is when the Brits would come over to play in my back yard and we'd give the altimeter in millibars. The other "Q's" are spoken in plain language phraseology.

Online galfert

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Re: WS-2902A Barometric Pressure Readings
« Reply #12 on: July 25, 2018, 04:32:23 PM »
Sorry to bring this up. But I think I may have my descriptions of Relative Pressure (QFF) and Altimeter Pressure (QNH) wrong. I've some some minor editing to my post above, with an attempt at keeping most of what I originally wrote intact for clarity.

Ahhhhhhh! I really thought I got this but the more I researched and read I'm now getting confused.

Turns out I've always ignored Altimeter Pressure (QNH). This is because it wasn't Sea Level. And in my weather station manual and in my understanding of weather reporting the reference made is to compute Sea Level Pressure so that you can compare one station location weather to another. This is what you are taught in grade school too. Nobody mentions Altimeter Pressure in these circumstances regarding weather.

But now thanks to me thinking I was qualified to answer these questions, I'm now second guessing myself. It is as it regards to how to arrive at Altimeter Pressure versus Sea Level Pressure. From what I gather is now correct that Altimeter Pressure (QNH) does take atmospherics into account but uses ISA for temperature which is a constant. Conversely it turns out that Sea Level Pressure also takes atmospherics into account and uses the current temperature. And I believe humidity is also a factor in one or both of these.

So I think some of my comments are incorrect. Perhaps it is Sea Level Pressure (QFF) that is more accurate given current weather conditions not Altimeter Pressure (QNH).

Also QNH and QFF can if the temperature and humidity are ideal or equal to ISA then in that case QFF = QNH.
And so depending on temperature some times also QFF is greater > than QNH but if the temperature is at the other range of ISA then the opposite will be true where QFF will be less than < QNH. And also if you are below sea level then I both these cases the opposite will be true.

In summary:
*ISA =59F always!

If the temp = ISA conditions
QNH = QFF

If above Sea Level
If the temp > ISA then QNH > QFF
If the temp < ISA then QNH < QFF

If below Sea Level then this is reversed
If the temp > ISA then QNH < QFF
If the temp < ISA then QNH > QFF

Maybe that explains why sometimes METAR data omits Sea Level Pressure... Like when temperture equals 59F which is ISA.

Also Ive read that Weather Underground wants Sea Level Pressure but CWOP wants Altimeter Pressure. I'm not doing CWOP yet and this will be yet something else to deal with if I get into that. I have a Meteobridge and hopefully that will somehow convert to Altimeter Pressure for me rather than it sending Sea Level Pressure.

Well I like to research, read and learn and share. My brain is now fried. Time for someone else to correct me if needed.

Bottom line though is at least with what I've written most people should be able to at least come to understand how to calibrate their barometer. Whether you choose to calibrate to Altimeter (QNH) or Sea Level Pressure (QFF) is up to you.

« Last Edit: July 27, 2018, 06:42:20 PM by galfert »
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Online galfert

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Re: WS-2902A Barometric Pressure Readings
« Reply #13 on: July 25, 2018, 05:18:14 PM »
This gets even better /sarcasm

According to VP Pressure Calc software there are 6 different formulas for Altimeter Pressure. And 3 different formulas for Sea Level Pressure. No wonder I'm loosing my head.

http://www.softwx.com/weather/vppressurecalc.html
WS-2902A | ObserverIP | WeatherBridge (Meteobridge)
WU: KFLWINTE111  |  PWSweather: KFLWINTE111
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Offline nincehelser

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Re: WS-2902A Barometric Pressure Readings
« Reply #14 on: July 25, 2018, 05:25:14 PM »
https://www.weather.gov/bou/pressure_definitions


STATION PRESSURE: This is the pressure that is observed at a specific elevation and is the true barometric pressure of a location. It is the pressure exerted by the atmosphere at a point as a result of gravity acting upon the "column" of air that lies directly above the point. Consequently, higher elevations above sea level experience lower pressure since there is less atmosphere on which gravity can act. Put another way, the weight of the atmosphere decreases as one increases in elevation. Consequently then, in general, for every thousand feet of elevation gain, the pressure drops about 1 inch of mercury. For example, locations near 5000 feet (about 1500 meters) above mean sea level normally have pressures on the order of 24 inches of mercury.

ALTIMETER SETTING: This is the pressure reading most commonly heard in radio and television broadcasts. It is not the true barometric pressure at a station. Instead it is the pressure "reduced" to mean sea level using the temperature profile of the "standard" atmosphere, which is representative of average conditions over the United States at 40 degrees north latitude. The altimeter setting is the pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude (above mean sea level) of the aircraft on the ground at the location for which the pressure value was determined. The altimeter setting is an attempt to remove elevation effects from pressure readings using "standard" conditions.

MEAN SEA LEVEL PRESSURE: This is the pressure reading most commonly used by meteorologists to track weather systems at the surface. Like altimeter setting, it is a "reduced" pressure which uses observed conditions rather than "standard" conditions to remove the effects of elevation from pressure readings. This reduction estimates the pressure that would exist at sea level at a point directly below the station using a temperature profile based on temperatures that actually exist at the station. In practice the temperature used in the reduction is a mean temperature for the preceding twelve hours. Mean sea level pressure should be used with caution at high elevations as temperatures can have a very profound effect on the reduced pressures, sometimes giving rise to fictitious pressure patterns and anomalous mean sea level pressure values.

Offline CW2274

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Re: WS-2902A Barometric Pressure Readings
« Reply #15 on: July 25, 2018, 06:20:14 PM »
So I think some of my comments are incorrect. Perhaps it is Sea Level Pressure (QFF) that is more accurate given current weather conditions not Altimeter Pressure (QNH).
Neither is more accurate than the other. They're merely different ways of measuring atmospheric pressure for different applications.