The atmospheric pressure at mean sea level, either directly measured or, most commonly, empirically determined from the observed station pressure. In regions where the earth's surface is above sea level, it is standard observational practice to reduce the observed surface pressure to the value that would exist at a point at sea level directly below if air of a temperature corresponding to that actually present at the surface were present all the way down to sea level. In actual practice, the mean temperature for the preceding 12 hours is employed, rather than the current temperature. This “reduction of pressure to sea level” is responsible for many anomalies in the pressure field in mountainous areas on the surface synoptic chart.