The force per unit area that the air exerts on any surface in contact with it, arising from the collisions of the air molecules with the surface. It is equal and opposite to the pressure of the surface against the air, which for atmospheric air in normal motion approximately balances the weight of the atmosphere above, about 15 lb/in.² at sea level. It is the same in all directions and is the force that balances the weight of the column of mercury in the torricellian barometer, commonly used for its precise measurement.