You might wonder why a millibar isn't exactly 1/1000th of an atmosphere.The unit is actually related to the old CGS units because atmospheric pressure is close to this unit.In CGS units 1 bar = 1000 dynes/cm2.Pressure is often incorrectly reported as kg/cm2,which is neither a legitimate pressure unit nor a standard SI unit (pressure is force/unit area,and kg are a mass unit,and SI units never mix kg and cm).So let's straighten out your friends on these units.Standard air pressure in this incorrect unit is 1.034 kg/cm2.The weight of a kg at the earth's surface is 980,000 dynes.So what is standard air pressure in millibars?

Question

Quizplus · Accepted Answer

To convert from kg/cm2 to millibars (mb), we need to first convert kg/cm2 to dynes/cm2 and then convert dynes/cm2 to millibars. 
Standard air pressure is 1.034 kg/cm2 = 1.034 x 980,000 dynes/cm2 = 1,013,320 dynes/cm2. 
1 millibar = 1,000 dynes/cm2, so dividing 1,013,320 by 1,000 gives us 1013.32 mb. Rounding to the nearest millibar gives us the answer of 1013 mb.

You Might Wonder Why a Millibar Isn't Exactly 1/1000th of an Atmosphere.The