Aviation what is qnh

QNH = The pressure measured at station then reduced down to mean sea level pressure

When set on your altimeter it will read your ALTITUDE

Sat on the tarmac at your airfield the altimeter will display the airfields elevation above mean sea level

This is the most commonly used pressure setting in the commercial world

QNH - The pressure set on the subscale of the altimeter so that the instrument indicates its height above sea level

The altimeter will read runway elevation when the aircraft is on the runway

Airfield QNH is obtained by correcting a measured QFE to sea level using ISA regardless of the temperature structure of the atmosphere

The QNH in aviation is the Pressure at the Mean Sea Level, it varies quite often as the pressure in the Atmosphere changes constantly

It is important to know what is the QNH of the area where you are flying because you need to set it on your Altimeter in order to read and fly Altitudes