Station Altitude

[Karen K]

Regarding the station altitude the user's guide of WeeWX simly says "station altitude".

The WMO distinguishes between the station altitude and the pressure gauge altitude.

• The station altitude is defined as the elevation of the ground below the air temperature sensor.
• The pressure gauge altitude is the real altitude of the pressure gauge, used to calculate sea level pressure (barometer value).

The altitude in the [Station] section of weewx.conf is directly used to calculate the sea level pressure (barometer value)  in the weewx.wxxtypes.PressureCooker class. So strictly spoken it is not the station altitude but the pressure gauge altitude, what the user has to put there.

Would it make sense to clarify that in the user's guide?

[Greg Troxel]

Karen K <kk44...@gmail.com> writes:

> The WMO distinguishes between the station altitude and the pressure gauge
> altitude.
>

> - The pressure gauge altitude is the real altitude of the pressure

> gauge, used to calculate sea level pressure (barometer value).

Does the WMO specify the datum for this altitude? I suspect it makes at
most 2m of difference, and that a best guess for operational use might
be "WGS84 Orthometric Height" -- but I have not been able to find
anything that clearly states this.

I have been working on finding out the elevation of the desk on which my
VP2 console sits to a fraction of a meter. Then I need to model the
barometer sensor vs the desk surface to get fully silly. However I
think getting it right to 1m matters.

[michael.k...@gmx.at]

I think it makes sense to clarify this. I have set my station's altitude to the elevation of the ground below the air temp sensor. My pressure sensor is 5m above. The impact may vary from case to case, in my case I guess the impact on the calculated sea level equivalent barometer value is greater, than on the almanac values, and I should set the station altitude to the elevation of the pressure sensor.

[Tom Keffer]

The stated accuracy of the Davis pressure gauge is 1 mbar, which, at sea-level, represents about 8m of altitude. And, it's one of the better pressure sensors.

The reduction to sea-level also assumes a "standard atmosphere," but in areas with terrain, nighttime temperatures can be considerably colder at the surface than the standard model would suggest, causing reduced SLP to be too low. This can be a 2-4 mbar error, or the equivalent of 16-32 meters.

My point is, unless you're located in a high-rise building, errors caused by the difference between console and ground altitude will be small.

Still, I'll put the change in.

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[Tom Keffer]

https://weewx.com/docs/5.0/usersguide/weewx-config-file/stations-config/#altitude

[gszla...@gmail.com]

Getting a bit confused by all the terms....

For personal weather statons station (non-airports) station elevation is the ASL height of the barometric sensor and sea level pressure (Altimeter and MSLP) is calculated from that elevation.

For airports, station elevation and pressure  are calculated valuse derived from the sensor elevation that is corrected ( removal correction) to field elevation.. The datum (station elevation) is usually the highest point of the runway (station elevation). This would be QFE.

The obscure and elusive "removal correction" has been difficult to track down. Steve Hatchett gives a formula in the WeeWX code but I could not find out where Steve obtained it or verify the source.I don't think a removal correction is utilized for any personal weather stations - only airports.

The term "barometric pressure" is somewhat confusing. U.S weather equipment manufacturers tend to call this sea level pressure however the NOAA/NWS definition is very different. So is Canada's definition. Barometric pressure is a reading from a barometer which could be anything.. In Europe, the term "air pressure" is commonly used although it can also have more than one meaning.

Bottom line - at airports, station elevation, station pressure and QFE are calculated values. So is QNH and QFF.

Sounds like the Tower of Babel doesn't it?

In deference to Tom's use of altitude versus elevation and gauge pressure, my preference for "universal" barometer terms for personal weather stations are restricted to station elevation, station presure, Altimeter and SLP (MSLP). I try to avoid "sea level pressure" because it can refer to either Altimeter or METAR's SLP depending on the context.Both are reduced sea level pressures.

We could also have a long  discussion about absolute pressure and relative pressure but I think I hear "enough already!".

[Greg Troxel]

"gszla...@gmail.com" <gszla...@gmail.com> writes:

> Getting a bit confused by all the terms....

It is confusing!

> For personal weather statons station (non-airports) station elevation is
> the ASL height of the barometric sensor and sea level pressure (Altimeter
> and MSLP) is calculated from that elevation.

I pointed out earlier that "above sea level" is ill-defined formally and
asked about datum, suggesting "WGS84 Orthometric Height", which is
height above an equipotential surface defined to globally approximate
sea level.

> For airports, station elevation and pressure are calculated valuse derived
> from the sensor elevation that is corrected ( removal correction) to field
> elevation.. The datum (station elevation) is usually the highest point of
> the runway (station elevation). This would be QFE.

For QFE, I would say that the heights of "reference datum" and of the
pressure sensor are both measured, but that one needs to know the
difference more accurately than either one.

> The obscure and elusive "removal correction" has been difficult to track
> down. Steve Hatchett gives a formula in the WeeWX code but I could not find
> out where Steve obtained it or verify the source.I don't think a removal
> correction is utilized for any personal weather stations - only airports.

It seems obvious (dangerous I know!) that since QFE is an altimeter
pressure concept, the "removal correction" would be solely dependent on
height, applying the altimeter correction from the sensor to 0 height
(MSL) and then inversing the correct from 0 back to the reference
elevation. However, what one really wants to know is what the pressure
sensor would have read had it been placeed on the reference elevation.
Surely the reference elevation and the sensor location are within a few
meters of each other, so 2nd-order effects should be small.

> The term "barometric pressure" is somewhat confusing. U.S weather equipment
> manufacturers tend to call this sea level pressure however the NOAA/NWS
> definition is very different. So is Canada's definition. Barometric
> pressure is a reading from a barometer which could be anything.. In Europe,
> the term "air pressure" is commonly used although it can also have more
> than one meaning.

Agreed it is confusing. I find that US NWS tends to use barometric
pressure to mean that observed pressured reduced to sea level.
Formally, NWS tends to call is:

https://w1.weather.gov/glossary/index.php?word=sea+level+pressure

I thought that this was their formal term for
reduction-with-temperature. On a public-facing forecast page, a value
which has surely been reduced is just labeled "Barometer". In 3-day
history, there is altimeter pressure in inHg and SLP in mb.

They leave "barometric pressure" vague, but I read it as meaning the
reduced-to-MSL pressure from a particular instrument:

https://w1.weather.gov/glossary/index.php?word=barometric+pressure

https://w1.weather.gov/glossary/index.php?word=station+pressure
https://w1.weather.gov/glossary/index.php?word=altimeter+setting

> Bottom line - at airports, station elevation, station pressure and QFE are
> calculated values. So is QNH and QFF.

I don't see that "station elevation" is calculated. And yes, if a
virtual station is reported with an elevation that isn't that of the
sensor, then it makes sense that a faux station pressure would be reported.

> In deference to Tom's use of altitude versus elevation and gauge pressure,
> my preference for "universal" barometer terms for personal weather stations
> are restricted to station elevation, station presure, Altimeter and SLP
> (MSLP). I try to avoid "sea level pressure" because it can refer to either
> Altimeter or METAR's SLP depending on the context.Both are reduced sea
> level pressures.

Probably we should align to WMO standard terms.

If you want to head down the rabbit hole:

https://library.wmo.int/doc_num.php?explnum_id=2849
https://library.wmo.int/doc_num.php?explnum_id=3445

Apparently the 1968 document is still current.

[Karen K]

The German Weather Service DWD provides a formula in their operating documents:

DWD VuB2 BUFR page 28

SP * exp (g / R * H / (TT + VP * Ch + a * H / 2))

• SP  measured pressure
  
• g  earth acceleration 9.80665 m/s^2
  
• R  gas constant 287,05 J/kgK
  
• TT  air temperature in K
  
• VP  actual vapor pressure
  
• a  vertical temperature gradient 0.0065 K/m
  
• H  altitude of the pressure gauge above NN
  
• Ch  average vapor pressure gradient 0.12 K/hPa
  

The formulae of the vapor pressure and saturation vapor pressure are found in that document as well.

Wikipedia says about NN:

"Normalnull ("standard zero") or Normal-Null (short N. N. or NN ) is an outdated official vertical datum used in Germany. Elevations using this reference system were to be marked Meter über Normal-Null (“meters above standard zero”). Normalnull has been replaced by Normalhöhennull (NHN)."

The same DWD document says about the station data to send within synoptic messages according to the WMO BUFR standard:

• 0 07 030 station elevation above NN
  
• 0 07 031 pressure gauge altitude above NN
  

The WMO specifies BUFR in Manual on Codes, volume I.2. They say there:

"Geometric altitude. Vertical distance (Z) of a level, a point or an object considered as a point, measured from mean sea level."

"Height of station ground above mean sea level is defined as the height above mean sea level of the ground on which the raingauge stands or, if there is no raingauge, the ground beneath the thermometer screen. If there is neither raingauge nor screen, it is the average level of terrain in the vicinity of the station (Reference: Guide to Meteorological Instruments and Methods of Observation (WMO-No. 8), 1996)."

"Height of barometer above mean sea level, referring to the location of barometer of a station, does not redefine the descriptor 0 07 030."