Metric

The derived SI unit of pressures and stress is pascal (Pa) named after the French physicist, mathematician, writer and philosopher Blaise Pascal. The unit is defined as the pressure of applying one newton of force for each square metre of area, 1 Pa = 1 N/m2. On the surface of the Earth, the standard atmospheric pressure is 101.325 kPa. This is almost equal to a bar which is roughly the pressure of one atmosphere.

The units below where a mass acts on a surface like is strictly the gravitational force one kilogram of mass exerts at the surface of the Earth.

British (Imperial) and U.S. Customary

The mass units used here are strictly to be interpreted as the gravitational force applied by that mass at the surface of the Earth.

Mercury and Water Pressure Measurements

A mercury barometer can be used to measure the atmospheric pressure. THe principle is that a hollow glass tube, closed at one end and open at the other is filled with mercury. The open end is displaced in an open cistern filled with mercury. The mercury in the tube will fall due to gravity until the pressure (weight) of the air (atmosphere) is equal to the weight of the mercury left in the tube. At standard atmospheric pressure at sea level this equals 760 mm or 29.92 inches of mercury.

Measruing millimetres of mercury has a special name, torr, after the Italian Evangelista Torricelli who invented the barometer in 1643.

The same principle can be applied with other fluids such as water. The water readings below are at at 39.2°F or 4°C.

Atmosphere

A physical atmosphere is the pressure or weight exerted by the column of air against the Earth as sea level by gravity.

A technical atmosphere is almost the same magnitude but is defined as the pressure equal to one kilogram-force per square centimetre.