atmosphere

One atmosphere (atm) is the standard atmospheric pressure at sea level on Earth. 1 atmosphere = 1.013 bars = 101.325 kilopascals (kPa) = 14.7 pounds per square inch (psi or lb/sq.in).

 

bar

The bar (b) is a unit of pressure used especially, for example by meteorologists, in measuring the local pressure of the atmosphere. One bar is the pressure created by a column of mercury 75.006 centimeters high at 0°C. It is approximately the average pressure at sea-level on the Earth. 1 bar = 0.987 atmospheres = 1.02 kg/cm² = 100 kilopascals = 14.5 lb/sq. in. Atmospheric pressures are often quoted in millibars (mb), or thousandths of a bar. Standard atmospheric pressure (at sea level) is 1.01325 bars, or 1,013.25 millibars.

 

pascal

The pascal (Pa) is the derived unit of pressure in the SI system of units. 1 pascal is equal to 1 newton per square meter (1.45 × 10^–4 pound per square inch). It is named after the French mathematician and philosopher Blaise Pascal.