Mean sea-level pressure (MSL)

Some definitions

• equipotential surface: A surface defined by the function where Φ is the potential function associated with any field that can be written Examples are gravitational, electrostatic, and magnetostatic equipotential surfaces.
• sea level—The level of the sea after averaging out the short-term variations due to wind waves.
• mean sea level—(Abbreviated MSL; popularly called sea level.) The arithmetic mean of hourly heights observed over some specified period. In the United States, mean sea level is defined as the mean height of the surface of the sea for all stages of the tide over a 19-year period. Selected values of mean sea level serve as the sea level datum for all elevation surveys in the United States. In meteorology, mean sea level is used as the reference surface for all altitudes in upper-atmospheric work; in aviation it is the level above which altitude is measured by a pressure altimeter.
• geopotential —The potential energy of a unit mass relative to sea level, numerically equal to the work that would be done in lifting the unit mass from sea level to the height at which the mass is located; commonly expressed in terms of dynamic height or geopotential height.

  The geopotential Φ at height z is given mathematically by the expression where g is the acceleration of gravity.

  geopotential surface—(Also called equipotential surface, level surface.) A surface of constant geopotential, that is, a surface along which a parcel of air could move without undergoing any changes in its potential energy.
• geoid—The particular geopotential surface that most nearly coincides with the mean level of the oceans of the earth. For mapping purposes it is customary to use an ellipsoid of revolution as an adequate and convenient approximation to the geoid. The actual geoid can depart from a best-fitting sphere in places by as much as 100 m.
• atmospheric tide— (Also called atmospheric oscillation.) An atmospheric motion of the scale of the earth, in which vertical accelerations are neglected (but compressibility is taken into account). Both the sun and moon produce atmospheric tides. The 12-hour or semidiurnal solar atmospheric tide is both gravitational and thermal and has the greatest amplitude. It is defined as

Source:

• AMS, cited 2009. Glossary of Meteorology.

Articles:

• Fraczek, Witold/ESRI- Arcuser online , cited 2009. Mean Sea Level, GPS, and the Geoid, 2003.