Mean Sea Level
The average height of the sea surface water level. For the United States, it is computed by averaging the levels of all tide stages over a nineteen year period, determined from hourly height readings measured from a fix, predetermined reference level. It is used as a basis for determining elevations, as the reference for all altitudes in upper air measurements, and as the level above which altitude is measured by a pressure altimeter for aviation. Often referred to as MSL.
Tide
The periodic rising and falling of the earth’s oceans and atmosphere. It is the result of the tide-producing forces of the moon and the sun acting on the rotating earth. This propagates a wave through the atmosphere and along the surface of the earth’s waters.
Elevation
The measure of height with respect to a point on the earth’s surface above mean sea level. Sometimes referred to as station elevation.
Altitude
In meteorology, the measure of a height of an airborne object in respect to a constant pressure surface or above mean sea level.
Pressure Altimeter
An aneroid barometer calibrated to indicate altitude in feet instead of units of pressure. It is read accurately only in a standard atmosphere and when the correct altimeter setting is used.