The pressure exerted by the air is said to be the atmospheric pressure and it acts as force over the surface of the earth. As pulls exerted by the gravity of the earth.
Commonly measured by the barometer, a device used to measure that atmosphere when values are measured or taken from the average mean sea levels.
Thus its defined be as force per unit area as air has weight. The development of high and low pressure is measured at the sea level which is 1013 pascal.
West of the Mississippi River has many more mountains due to more recent orogenic processes. Blackhills, Rocky Mountains, Cascades, Sierras.... List goes on.
