Summary

Same sign, above mean sea level and warmer than ISA (+,+) or below mean sea level and colder than ISA (-,-) then QNH is greater than QFF. Otherwise, the QFF is greater than the QNH. 

Stations AT MSL Regardless of temperature QNH = QFF (=QFE) 

The normal range of mean sea level pressure (QFF) extends from 950 to 1050 hPa. The lowest recorded mean sea level pressure is 870 hPa in the eye of Typhoon Tip in the Western Pacific in 1979. The lowest recorded in the North Atlantic is 882 hPa in the eye of Hurricane Wilma in 2005. The highest mean sea level pressure was 1085.7 hPa recorded in winter in Siberia in 2001. 

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