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  • Altimetry

    Hi Guys,
    I've been going through some MET questions and cam across this one.

    1) The QFF at an airfield located 400m above sea level is 1016 hPa. The air temperature is 10?c lower than standard atmosphere. Will the QNH be higher, lower, or equal to the QFF?

    Thanks in advance!

  • #2
    In cold air, QNH is less than QFF ( in other words, QNH is a smaller number).

    The reason is that the QNH is the QFE (the pressure measured at the airfield) adjusted to sea level using the standard atmospheric conditions (ISA) and, if the airfield is above sea level, it will be a larger number than QFE.

    But QFF is the QFE adjusted using the actual atmospheric conditions pertaining on the day and in cold air, the pressure changes more quickly with a change in altitude, so the QFF will be a larger number than QNH.

    To summarise:

    For Airfields above sea level:

    Warmer than ISA: QNH greater than QFF

    Colder than ISA: QNH less than QFF


    For Airfields below sea level:

    Warmer than ISA: QNH less than QFF

    Colder than ISA: QNH greater than QFF

    For Airfields at sea level QFE equals QNH equals QFFas no adjustment to the measured pressure (QFE) is required.

    Hope this helps

    Comment


    • #3
      So the answer is : Less than 1016 hPa

      Comment


      • #4
        Thanks alot!!!!
        Much appreciated.

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