P; Barometric Pressure
The pressure exerted by the atmosphere as a consequence of gravitational attraction exerted upon the "column" of air directly above the point in question.
The atmospheric pressure decreases as the height above sea level increases. The atmospheric pressure at a station is reported as what it would be if it were actually situated at sea level. Adjustment of atmospheric pressure readings to a common elevation (sea level) removes the influences of the earth's relief (topography) on atmospheric pressure readings.
Under normal conditions, the atmospheric pressure above sea level is less that the atmospheric pressure at sea level which is sometimes referred to as one atmosphere.
1. Weather Prediction
2. Global Climate
Other sources for data