Based on the recommendations of the World Meteorological Organization (WMO), air-pressure measurements are reduced to sea-level at stations with an elevation of less than 600 m.a.s.l. Stations at higher elevations are reduced to the meteorological standard pressure areas of 850 or 700 hPa.
When reducing to sea-level, assumptions must be made about the density of the atmosphere between the station elevation and sea-level. It is standard meteorological practice to automatically factor in the temperature measured at the station. The resulting atmospheric pressure value at sea-level is called the QFF. The prevailing humidity is not included in the calculations for Switzerland.
In aviation, the station pressure is reduced to sea level independently of the current weather conditions. The vertical density distribution is uniform according to the ICAO standard atmosphere. The resulting atmospheric pressure value at sea-level is called the QNH.