QNH

QNH is the atmospheric pressure at mean sea level, calculated from the surface pressure at a given location. When set on an aircraft's altimeter, QNH causes the instrument to read altitude above mean sea level (AMSL). It is the most commonly used altimeter setting worldwide.

Key concepts:

• QNH: Pressure adjusted to sea level — altimeter reads elevation above sea level
• QFE: Actual station pressure — altimeter reads zero (or height above the station) on the ground
• QNE/Standard: Standard pressure 1013.25 hPa (29.92 inHg) — used above the transition altitude for flight level references

QNH is provided in METAR reports and by ATC in the ATIS broadcast. In most of the world, QNH is expressed in hectopascals (hPa). In the United States and Canada, the equivalent value in inches of mercury (inHg) is used. Correct altimeter setting is critical for terrain clearance, approach procedures, and vertical separation between aircraft.