Station Pressure: This is the pressure that is observed at a specific elevation and is the true barometric pressure of a location. It is the pressure exerted by the atmosphere at a point as a result of gravity acting upon the “column” of air that lies directly above the point. Consequently, higher elevations above sea level experience lower pressure since there is less atmosphere on which gravity can act. For example, locations near 5,000 feet above mean sea level normally have pressures on the order of 24 in. Hg. This value however, is generally not reported to pilots.
Altimeter Setting: This is the pressure reading most commonly provided to pilots. It is not the true barometric pressure at a station. Instead it is the pressure “reduced” to mean sea level using the temperature profile of the “standard” atmosphere. The altimeter setting is the pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude (above mean sea level) of the aircraft on the ground at the location for which the pressure value was determined. The altimeter setting is an attempt to remove elevation effects from pressure readings using “standard” conditions.
Mean Sea Level Pressure: This is the pressure reading most commonly used by meteorologists to track weather systems at the surface. Like altimeter setting, it is a “reduced” pressure that uses observed conditions rather than “standard” conditions to remove the effects of elevation from pressure readings. This reduction estimates the pressure that would exist at sea level at a point directly below the station using a temperature profile based on temperatures that actually exist at the station.