"The instrument for measuring atmospheric pressure is the barometer. Meteorologists apply standards defined by the World Meteorological Organization to compare pressure measurements among themselves. They are thus brought back to sea level and the barometers are calibrated to indicate the pressure at sea level. This correction amounts to adding the weight of the missing air column between the altitude of the measuring point and the sea level (or a rare case, to subtract it for a measurement point located under the surface of the sea)."

source: meteofrance.