Incorrect pressure in mmHg

  • Hi
    My wether station report 743mm
    Local wether agency 741mm

    Windy 769mm

    Best regards

  • Sailor Moderator

    This is what I see right now at Ekatarinburg weather station on Windy.


    That looks correct.

  • Moderator

    I do not trust weather stations that are private ..only weather stations of the local weather services are reliable. There are so much rules for installing a weather station correct..and i think, most of the private stations do not following these rules

  • Moderator

    Agree! Also I want to mention that some PWS give station's pressure
    (not converted to sea level).

  • windy.jpg

    Koltsovo airport is about 10 km from my location. I think this is insubstantial.
    Unfortunately, all mercury barometers were taken from us. They say it is not environmentally friendly.

  • Moderator

    This is the latest METAR for Koltsovo airport.
    USSS 011600Z 22003MPS 9999 SCT023 M08/M10 Q1023
    R26R/490132 NOSIG RMK QFE746/0995

    Q1023 stands for QNH (atmospheric pressure measured at airport then reduced down to mean sea level pressure).

    On remarks (RMK)part of METAR,
    QFE stands for pressure (corrected for temperature) for a specific site (Koltsovo airport) NOT reduced to msl.

    The observed pressure for Koltsovo (elevation 227 m) is 746 mm Hg or 995 hPa.

    If a pilot sets QNH in aircraft's altimeter, he reads ALTITUDE (above msl).
    If sets QFE, he reads HEIGHT above airfield.

  • Moderator

    ... the same way,
    At Beslan airport (North Osetia), altitude 500 m
    QNH 1014 hPa = 761 mm Hg and
    QFE 956 hPa=717 mm Hg.

    METAR URMO 011630Z VRB01MPS 9000 SCT006 M02/M02 Q1014
    R09/010070 NOSIG RMK QFE717/0956

  • 746 is ok
    But windy gives 767.

  • Moderator

    746 is QFE, 767 is QNH.

    at the picture above,
    Koltsovo Airport measured atm. pressure reduced to mean sea level=767
    ECMWF model forecasts for the same place and time = 769.

  • Yes, I understood it.
    I think this is not very practical for ordinary people.

  • Moderator

    Atmospheric pressure varies greatly with height,
    dropping by about 1 hPa (or 1 millibar) for each 10 m in the vertical.
    To make sense of pressure readings from a network of barometers
    located at different locations
    (each exposed at a different height above sea level)
    all values of station level pressure are converted to an estimate
    of the pressure at mean sea level,
    using a formula that takes into account the air temperature.

  • Thank you for explanation.

    How it is comlex.

    It seems to me it is also an approximation.
    Even we place barometers at same heigth, the reading will vary.

  • Moderator

    The readings will vary if we place barometers at same altitude AT DIFFERENT locations.
    That's how we see barometric lows and highs in the atmosphere.
    If we draw weather maps using pressure readings (not reduced to msl)
    there will be always a LOW over mountains and a HIGH over the oceans!

  • Moderator Mariners - Seafarers Sailor

    Just as an extra note to this discussion, I have a barometer on my phone and a basic small widget showing a pressure graph. Alongside it I have a meteogram simplified to only show a forecast pressure graph, so that forecast vs actual trends are easily comparable.

    The meteogram also has the actual QNH (local pressure converted to mean sea level pressure) from the airport a few km away so that is also useful to calibrate and confirm the phone sensor.

    They all end up being within 1-2 hPa of each other, so that is good, but seeing the trend is helpful of course. Here is a screenshot of when the Low passed over today:

    2019-02-02 22.45.22 - Photos.jpg

    The Barometer graph (actual pressure) on the left is showing the last 72 hours, and the Meteogram (forecast pressure) on the right is showing the next 36 hours.

    Actual from the phone of 1001.99 hPa, actual QNH from the airport of1001.xx hPa (decimals not shown), and forecast of 1002.5 hPa :)

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