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QNH Observations

QNH is the pressure setting applied to the altimeter (cockpit instrument for indicating aircraft's altitude) so that it will read the altitude above mean sea-level within a certain region. The Hong Kong Observatory has been providing QNH observations at the Hong Kong International Airport (HKIA) to airline users and air traffic control staff since its opening in 1998.

A slight change of 1hPa of QNH value will cause a difference of about 10 meters in the altitude reading.  Regional difference of QNH over Hong Kong is generally small under stable weather situations.  However, when there is passage of a front or a tropical cyclone over the territory, the regional difference of QNH could be significant.
As an enhancement to the weather service for general aviation and search and rescue operation in Hong Kong, a map showing QNH observations at the Observatory's Headquarters, HKIA and an airfield for general aviation at Shek Kong with an update frequency of half-an-hour are now available on the AMIDS.

 Display of QNH observations over Hong Kong

Figure 1  A sample display of QNH observations over Hong Kong during the passage of Typhoon Koppu on 15 September 2009