see Pressure, Barometric.
Pressure is the measurement of a force (weight) on a per unit area. The barometer measures the force our atmosphere exerts on a per-unit area of surface, or the "weight of the air".
is the absolute atmospheric pressure existing at the surface of the earth. It is the weight of a unit column of air above the point of measurement. It varies with altitude and, at any given location, with moisture content and weather.