Air pressure (mean sea level, QNH)
Air pressure mean sea-level pressure sea-level pressure MSLP
Pressure adjusted to sea level makes weather stations at different elevations comparable. QNH is a related aviation value calculated using different assumptions.
First, take out the elevation effect
A barometer on a mountain reads lower than one in a valley. That mainly reflects the shorter column of air above it, not worse weather. Weather charts remove much of this effect by estimating the pressure that would occur at sea level.
Making sense of the numbers
The unit is the hectopascal (hPa). Standard pressure, 1013.25 hPa, is a reference rather than a magic dividing line between good and bad weather. A high or low is defined relative to its surroundings. Lines of equal pressure are called isobars.
QNH and weather-chart pressure
- Meteorological sea-level pressure: The adjustment accounts for observed or modelled temperature conditions.
- QNH: Uses the standard atmosphere. An aircraft altimeter set accordingly reads approximately the airfield’s elevation above sea level when on the ground.
The two values are often close but are not interchangeable. This website displays modelled mean sea-level pressure, not an official QNH setting for flight.
One reading is not the whole forecast
A reading of 1020 hPa cannot tell you whether tomorrow will be sunny. The trend, wider pressure pattern and air masses reveal more. A barometer does not have a view of the clouds.