QNH and altimeter settings

The last main group of a METAR is the pressure. It is not a weather detail for its own sake: it is the number you set on your altimeter so that the instrument shows the right altitude.

Q or A: hPa and inHg

To convert, multiply hPa by 0.02953 to get inHg, or multiply inHg by 33.864 to get hPa. Standard pressure is 1013.25 hPa, which is 29.92 inHg.

QNH

QNH is the altimeter setting that makes your altimeter read altitude above mean sea level. On the ground at the airport it shows the airport's elevation. It is the setting used for take-off, landing and flying below the transition altitude.

QFE

QFE is the pressure at airport elevation. With it set, the altimeter reads zero on the runway and shows height above the aerodrome. Some countries and some clubs use it for circuit flying, but QNH is far more common, and METARs report QNH.

Standard pressure

Above the transition altitude, pilots set 1013 hPa (29.92 inHg) so that all aircraft share the same reference and separate by flight level. The transition altitude differs from country to country.

Why a few hectopascals matter

• Near sea level, 1 hPa is about 30 ft of altitude, and 1 inHg is about 1,000 ft.
• If you fly into an area of lower pressure and do not update your setting, the aircraft is lower than the altimeter indicates. Pilots remember this as "from high to low, look out below".
• Always set the latest QNH you can get from the METAR, ATIS or air traffic control.

Back to the METAR guide.

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Educational material. It does not replace flight training or an official weather briefing.